Design Guide for 12V Systems – Dual Battery Systems, Solar Panels and Inverters

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last updated 9/03/2021

Need help deciding how you’re going to power your beer fridge? This is a design guide for 12V systems or dual battery systems used in vehicle setups for touring and camping. It is not a guide on how to actually wire up equipment, terminate cables, use a soldering iron etc. It is not recommending which setups are good or bad. The purpose is to explain different solutions to this design problem (good and bad) and identify what equipment would suit your application and how to arrange them in your design. The drawings are not wiring diagrams, they are schematics showing topology only. All wiring should be done according to equipment datasheets and manuals. All designs should be checked and installed by a qualified auto-electrician.

Every design is a compromise. This guide gives you the info you need to pick your compromise.

Dual Battery Systems

A dual battery system is where one or more auxiliary batteries are installed in addition to the standard starter battery of a car, 4WD or motorhome. The arrangement provides additional battery capacity to allow appliances like fridges and lighting to be powered, and prevents these appliances from loading the starter battery, ensuring your engine will start even after lengthy use of the appliances. Dual battery systems are common in touring vehicles set up for camping, fishing and travelling around the outback.

One or More Batteries

Whether there are one or more auxiliary batteries, and whether they are installed in the vehicle, under the bonnet, in the passenger cabin, in the tray, in a trailer or in a caravan, does not effect the design arrangements outlined in this article. For the purpose of simplicity, this article will refer to only one auxiliary battery. A bank of batteries in parallel behaves the same as one large battery. Wherever you see “second battery” or “auxiliary battery” you can replace it with a bank of batteries. Batteries arranged in banks work best if they are matched, so that they have matched voltage profiles and charge / discharge at the same rates, but this is not absolutely necessary. All lead acid batteries behave fairly similarly. When connected in parallel, batteries do not need to be exactly matched, as charge and discharge current will naturally distribute between them according to their capacity. A larger battery or a battery with less internal resistance will sink more current when charging as it pulls down the charging voltage. That battery will supply more current under discharge as its voltage will not drop as much.

Types of Batteries

An auxiliary battery is most likely going to be cycled. That is, discharged at slow to moderate currents for long periods of time and then recharged. This is different to a starting battery, which needs to deliver high currents for short periods, but does not get depleted or cycled significantly. Cycling lead acid batteries wears them out, causing sulfation of the negative electrodes. Deeper discharges cause a disproportionate increase in wear. Double the discharge depth, and the damage to the battery is more than doubled.

Batteries used for cycling applications are specially designed to minimize wear caused by deep discharges, at the expense of reduced high current ability, increased weight and increased cost. They are called deep cycle batteries and are the type of batteries that should be used as the second battery in dual battery systems.

Putting your deep cycle battery in parallel with your starter battery is not a perfect arrangement, as they are of two different battery types. However the charging profiles and voltages of standard flooded batteries vs deep cycle batteries are very similar, practically identical for most batteries. So it can almost always be done without any issues. Check the datasheets for any significant difference between specified float charge and cycle charge voltages. Most batteries are 13.8V float and 14.5V cycle, or pretty close to those values.

A particular design of deep cycle batteries, called Absorbent Glass Mat (AGM) batteries, are especially suited to dual battery systems and should be considered in any design. The advantages of AGM batteries over standard deep cycle batteries include:

  • Much higher charge rate (up to 5 times higher). This gives you the ability to perform quick top up charges from the alternator by running your vehicle. A half hour charge from the alternator could add say 25Ah, compared to only 5Ah with a standard deep cycle battery.
  • Higher specific power (can provide higher short term current, useful if you have a problem with your starting battery)
  • Greater tolerance to vibration due to glass mat sandwich construction
  • Spill-proof

Disadvantages of AGM batteries are:

  • More expensive
  • Fewer number of charge / discharge cycles compared to standard deep cycle batteries
  • Lower specific energy (heavier battery)
  • Capacity has gradual decline as the battery wears. Standard deep cycle lead acid batteries have a flatter decline to start with, maintaining a greater proportion of original capacity for longer, before losing capacity quickly towards the end of its life.

Batteries degrade with elevated temperatures. Higher temperatures reduce battery life. Try to install your battery in a cool location, away from hot parts of the motor or out of the engine bay completely. Sealed batteries are a good choice, as they can be stored inside a vehicle without the risk of spillage or dangerous gas venting. As a guide, every 8 degrees Celsius rise in temperature halves the life of a typical lead acid battery. That’s pretty significant when considering how hot it gets under the bonnet of a car with the engine running, especially in the sun on a hot day.

For more information on battery chemistry, visit

Electrical Protection – Fuses and Circuit Breakers

Fuses and circuit breakers protect cables and equipment from electrical faults. They work in the same manner – interrupting the path of electrical current during fault conditions. They are available in a range of current ratings to suit various applications. They should be rated such that they will not trigger during any normal loading (including inrush from starting appliances), but need to be rated as small as possible to ensure they are activated in a timely manner during a fault. The biggest risk in a 12V system in the event of an electrical fault is fire. A fault can occur during a car accident, equipment failure (short circuit in an appliance), cable insulation deterioration, etc. Fuses and circuit breakers should be used to reduce the risk of fire and minimize damage in these circumstances.

Fuses have a wire element that melts due to the heat generated from high current, creating an open circuit in the event of a fault. The advantages of fuses over circuit breakers are lower cost and faster acting (minimizes damage).

Circuit breakers work through thermal and / or magnetic pickups that are triggered by high current. Circuit breakers are resettable whereas fuses need to be replaced once they have blown.

To avoid cluttering drawings and repetition, fuses and circuit breakers will not be covered in each section. Instead apply general principles to your design:

  • Fuses and circuit breakers should be used in any 12V system. Omitting fuses could lead to fire and denial of insurance claim. Ideally every cable feed should be individually protected, but this introduces complexity. The design is a trade-off between complexity and fault discrimination.
  • Place fuses and circuit breakers on the positive side of cable runs
  • Fuses and circuit breakers should be placed as close to the battery as possible to minimize the unprotected portion of the cable run between the battery and fuse / circuit breaker.
  • The highest risk occurs where there are bigger cables, as these cables will carry a higher fault current. For example the cables that link the starting battery with the auxiliary battery, or the cables that feed an inverter.
  • A cable linking two energy sources needs to be protected from both ends. So the cable linking the positive terminals of the starting battery to the auxiliary battery should have a fuse or circuit breaker at both ends.
  • If you are distributing power to many different locations via many cables, it may be impractical to individually fuse each run. In this case you can use one common fuse or circuit breaker for the main feed, then distribute to the other cables after that point.
  • Protecting every cable run individually provides discrimination between faults. When a fault occurs, only the fuse feeding that run will blow, automatically pointing you to the location of the fault and leaving your other cable feeds energized.

Earthing of an Auxiliary Battery

The negative terminal of the auxiliary battery can either be connected to a metallic part of the vehicle, or connected directly back to the starter battery via a dedicated cable. Both ways work. Running a dedicated cable ensures you will get a low resistance connection, but is more expensive and could be considered wasteful. Earthing to the nearest metallic part of the vehicle reduces the length of heavy and expensive cable used in the design, but could introduce problems from high resistance joins in the chassis, engine etc, for example where two sections of chassis are bolted together, or between the chassis and the engine. You may need to add short cable links across any high resistance joins. It can be difficult to prove whether the resistance through the chassis is acceptable. The least risk option is to run a dedicated cable back to the starter battery.

I don’t have a dedicated negative return cable. I connected my aux battery to the chassis and tried to crank the engine from it with the starter battery disconnected. Cranking the engine is a good test for resistance and voltage drop. On the first test the engine could not start. I added a short cable link from the chassis to the engine and the engine started successfully.

Sizing Your Auxiliary Battery

Battery sizing is determined by the load and how long the loads need to run without charging. Calculate the amp hours (Ah) consumed over that period, double it, and that gives you the Ah capacity of the battery required to meet that criteria. Doubling is so that you deplete the battery to 50% and no lower. The deeper you discharge a lead acid battery, the greater the damage you cause, and the damage grows disproportionately with depth of discharge. Around 50% is a good compromise. If you want the batteries to last longer you’d size them so they will discharge to say 75% of full capacity before being recharged. The tradeoff is the expense of needing more batteries and the extra space / weight.

As an example, let’s assume our loads are a fridge that consumes an average of 2A, and an incandescent light rated at 240VAC 60W that you want to run for 2 hours a day. Let’s say you would like the system to run for a whole day – 24 hours.

To convert the AC power of the light to DC current, divide power by 12 and multiply by 1.1 to take into account the energy wasted converting from AC to DC. Multiply the currents by the run times and add them together.

Fridge Ah: 2 x 24 = 48Ah

Light Ah: 60 / 12 x 1.1 x 2 = 11Ah

Total Ah = 48 + 11 = 59Ah

Battery capacity = 59 x 2 = 118Ah

That’s a good sized battery. What if you run your alternator for 30 mins / day? This might be to drive down to the fishing spot and back to camp or might be idling the engine specifically to charge the battery. The charge rate is difficult to predict. Let’s assume you have an AGM battery that can accept a pretty good charge rate and take a guess of 30A. That’s 15Ah if it runs for 30 mins. The calculations change as follows:

Total Ah = 48 + 11 – 15 = 44Ah

Battery Capacity = 44 x 2 = 88Ah

What if you have solar panels that are large enough to fully charge the batteries every day? Then the batteries need to last say 16 hours between charges. The lights run at night so the full 2 hours are still used during the discharge phase.

Fridge Ah: 2 x 16 = 32Ah

Light Ah: 60 / 12 x 1.1 x 2 = 11Ah

Total Ah = 32 + 11 = 43Ah

Battery capacity = 43 x 2 = 86Ah

The Simplest System – Direct Connection of a Second Battery

A second battery can be connected in parallel with the starter battery with no other additional hardware as shown in the circuit diagram below.

battery in parallel

This system is simple to install and minimizes costs. The alternator will charge both batteries and all loads will be shared between both batteries. Both batteries will contribute to cranking the starter and both batteries will provide capacity to run the auxiliary loads. The main problem with this arrangement is that the starter battery will be depleted by the auxiliary loads, after which the vehicle will be unable to start. An easy solution is to manually disconnect the starter battery as required. This is the arrangement I used in my early years of camping. The starter battery in my vehicle had round terminals that were tapered in shape. This meant I could leave the nuts on one of the terminals slightly loose, and with some shuffling back and forth of the terminal I could wedge it hard enough onto the tapered battery terminal to provide a good enough connection, or I could disconnect the terminal as required without needing any tools. You run the risk of forgetting to disconnect the starter battery and allowing it to go flat. If you use this setup, ensure you disconnect the negative terminal only to prevent accidental short circuiting of the auxiliary battery on the body of the vehicle, and obviously do not allow the loose negative terminal to touch the positive side of the battery, unless you are trying to start a fire.

Manual Isolator Switch

Rather than playing around with battery terminals every time you disconnect and reconnect, a switch can be added in circuit. The switch can be a basic isolator switch or a relay that is toggled in and out via another switch, usually positioned inside the vehicle cabin. It could also be a connector or anderson plug which you manually disconnect. This system still relies on manual intervention to ensure the starter battery is not depleted.

The switch can be placed in one of two locations. The first location keeps the starting battery powering the vehicle accessories such as interior light, stereo, etc. This has the advantage of not interfering with the vehicle’s standard electrical distribution so that it is easy to revert back to factory arrangement, and allows the starter battery to contribute to some of your power requirements so that your auxiliary battery is not cycled as deeply. This results in a net reduction in battery wear across the two batteries and some extra usable battery capacity. It also keeps the starting battery in circuit ready to start your vehicle regardless of the position and condition of the switch (for example if the switch fails). It also places less stress on the switch, as starter motor current does not flow through it. The problem with this arrangement is that it allows your starter battery to become depleted from the vehicle’s accessories.

manual isolator switch

The second location for the switch shown below completely isolates the starting battery so that there is completely no load on it. The battery will stay charged indefinitely (apart from self discharge), but the battery will not contribute to your power use, the switch will be under greater stress during starting and the failure of the switch may make your vehicle unable to start.manual isolator switch position 2Instead of a manual switch or relay, the relay can be driven automatically from your accessories being turned on. So when you turn the keys to accessories the relay is energized and closes the circuit. This removes any manual intervention, but adds the complexity of wiring from an accessories power feed to the relay. It can be placed in either location in the two schematics above with the same pros and cons as using a manual switch, relay or connector.

With either the manual switch or accessory driven relay, your auxiliary battery will contribute to starting your engine, which is useful if your starter battery is tired.

Voltage Sensing Relay

A voltage sensing relay is the most common dual battery arrangement. Most dual battery isolators are of this type. The voltage sensing relay isolates the starter battery when the starter battery’s voltage goes below a setpoint level. This allows the starter battery to contribute to your power requirements to a small extent, up to the point where it is isolated. It is fully automatic, does not require any manual intervention and does not require any control wiring from accessory feeds or other switches so is easy to install. When your motor is started, the alternator will charge the starting battery and raise its voltage. The voltage sensing relay will close once the voltage exceeds a setpoint level, joining the two batteries together and allowing your auxiliary battery to be charged from the alternator. The “S” indicates the sensing terminal on the voltage sensing relay.

voltage sensing relay

The voltage sensing relay must be in the location identified in the schematic, otherwise once it opens you would never be able to start your engine. Your starting battery is still at risk of being depleted by the vehicle’s accessories. It also does not permit the auxiliary battery to contribute to engine starting once the starter battery’s voltage is below the switching setpoint voltage. This will prevent you from otherwise being able to start the engine if the starter battery is flat but the auxiliary battery is fully charged. Some voltage sensing relays can be forced to close, for example through a switch or connecting a link or jumper. This allows your auxiliary battery to contribute to starting the motor.

Dual Voltage Sensing Relays

If you have some sort of charging system for your auxiliary battery (eg solar panels), the single voltage sensing relay has a drawback. Once it opens, it prevents your charging system from charging your starter battery, and your starter battery will be slowly depleted by your vehicle’s accessories. Adding a second voltage sensing relay in the reverse direction resolves this issue. The batteries will be joined if the voltage on either side is high enough. So once your charging system has charged your auxiliary battery, the second voltage sensing relay will close and your starter battery will be charged. Apart from avoiding depletion of your starter battery, this arrangement allows your starter battery to contribute to your power use every charge / discharge cycle, so you have some extra capacity to run your loads longer, and it allows the auxiliary battery to contribute to starter motor cranking. It also reduces the depth of discharge on your auxiliary battery, resulting in a net reduction in battery wear across the two batteries. It, if the charging system on the auxiliary side permits, also gives your starting battery an occasional higher charge voltage above the standard voltage of the alternator, which reduces sulfation and extends battery life. There are dual voltage sensing relays available that perform this function as a single unit – closing when either terminal is a high enough voltage. So the system can be arranged with either two standard voltage sensing relays or one dual voltage sensing relay.

The charging system (solar panels) is not included in the schematic. They are treated separately in the solar panels section.

dual voltage sensing relay

DC-DC Converter

Charging your auxiliary battery with your vehicle’s alternator is good but not great. It does not provide a boosted voltage output after a battery cycle down. This means, in cycling applications, the battery will suffer from sulfation which will accumulate with each cycle and eventually cause premature battery failure. Further, the alternator’s limited output voltage means charge rate becomes very slow as the battery approaches 100% state of charge. Also it does not provide an adjustable output to suit a battery with unusual float and cycle voltage ratings. A solution is to use a DC-DC converter. This is an electronic multi-stage charger that is powered by your alternator. During certain times of the charge cycle it ramps the voltage up to around 14.5V. This charges your battery faster in the final stages which may offer a higher state of charge. The higher voltage also reduces sulfation which reduces the damage caused by each cycle down and extends battery life. A further advantage is that it separates the charging between your starter and auxiliary batteries, so you can use an auxiliary battery with unusual float and cycle voltages, provided the DC-DC converter can be configured to charge at those voltages. Another advantage is that the cranked up voltage can help offset some volt drop if you have a long cable run (for example to a trailer) and are not able to keep volt drop within acceptable limits.

DC-DC converter

DC-DC converters are good but they are not perfect. They have the following disadvantages over traditional voltage sensing relays:

  • Reduced charge rate during bulk charge: DC-DC converters are usually rated to around 20A or so. If your battery is able to accept more current, the DC-DC converter will not be able to provide it. Alternators can typically charge up to around 100A, which is especially useful if you have an AGM battery which is capable of being charged at very high charge rates (30 to 40A for typical AGM batteries or more for very large batteries). When there’s other loads connected to your auxiliary battery, such as fridge etc, this further reduces the charge rate from a DC-DC converter as some of the current is diverted to the loads whereas the alternator will be able to supply those loads whilst still supplying as much current as the battery will take.
  • Overcharge: DC-DC converters always charge at elevated voltage on startup even when the battery is already fully charged. It takes a short period of time for the charger to recognise that the battery is fully charged and revert to float voltage. This may cause a small amount of unnecessary overcharge and the associated grid corrosion. When not cycling the battery, the voltage should remain at the float voltage at all times (usually around 13.8V). The alternator voltage is chosen for a reason – it is an appropriate float charge voltage for most batteries. Startup overcharge is minor. More serious is overcharge caused by loads triggering the DC-DC converter to go to cycle voltage when the battery is already fully charged. See section below “Overcharge from DC-DC Converters and Solar Regulators.
  • Inability to supply large loads: if you have a big compressor that uses 50A or a power hungry appliance hanging off an inverter then your auxiliary battery will become depleted even with the engine running because the DC-DC converter can’t keep up with the load. This could be a problem if the high load is sustained, for example if you have the only good compressor and need to pump up all your mates’ tyres, or you have a high maintenance wife who needs to use the hair dryer on camping missions.
  • Does not allow a charging system on your auxiliary battery to charge your starter battery: You can’t backfeed from the auxiliary side to the starter side. Some DC-DC chargers, with dedicated solar inputs, will allow solar panels to charge both the starter battery and auxiliary battery.
  • Does not allow the auxiliary battery to contribute to cranking: You can’t use the auxiliary to crank your engine if the starter battery is flat or if it’s failed completely. This is a problem if you’re out bush. I did have my starter battery fail whilst travelling around Australia and was able to use the auxiliary to start my hilux since I use VSRs.
  • Too slow when charging battery banks: if you have a bunch of batteries in parallel sharing the charge from a DC-DC converter then you’re going to be waiting a long time to charge the batteries. Also, since the batteries may not charge identically, there’s increased chance of overcharging the batteries since those that are charged quickly will be held at the cycle voltage for too long. Usually batteries of similar age and type do charge at about the same rate since the higher charged battery will naturally sink less current and the flatter batteries will catch up.

DC-DC Converter Myths

I’ve noticed a lot of misinformation about DC-DC converters so I’ve added this section to help clear the water. The most commonly reported myth is that your alternator cannot fully charge a battery. Often a figure of 80% is quoted as the maximum charge that an alternator can provide, but I’ve seen figures as wild as 60% being reported. I have found exactly zero substantiated references to support these claims. I have seen exactly zero adequate explanations to support these claims. It’s not true. The figure of 80% sounds like an arbitrary number that some marketing guy made up. Nothing special happens to a battery’s chemistry when it reaches 80% state of charge. In reality any voltage above the voltage dictated by the battery’s chemistry will charge the battery to 100%. You can verify this by looking at charge characteristic curves on battery datasheets – the capacity approaches 100% for any charge voltage. It is true that the final portion of charging will be slow from an alternator. Perhaps extremely slow depending on the output of your alternator. An elevated voltage will charge the final few % faster and help reduce sulfation.

On average, under cycling conditions, an alternator will provide a greater state of charge. After an overnight discharge, an alternator will charge faster than a DC-DC converter during the bulk charge stage. This accounts for most of the battery’s capacity. For a typical AGM battery size of 100Ah , the alternator may charge at say 40A whereas the DC-DC converter will be limited to its rating, typically 20A, or even less at elevated temperatures. With loads like fridges connected, the DC-DC converter will charge the battery even slower as many amps are diverted to the loads, whereas the alternator has excess capacity to supply the loads as well as continue to charge the battery at 40A. So in a typical scenario, driving from camp site A to camp site B, or driving from camp site to fishing spot, or running your motor specifically to top up the batteries, direct charging off the alternator will be charging over twice as fast as a DC-DC converter. That’s over 20A more current. Unless driving for a very long time, the battery will never be fully charged in this scenario. It is only once the battery is almost fully charged that a DC-DC converter will charge faster than the alternator. On average though, for a typical cycle of the battery, you’ll have more capacity charging from the alternator.

I’ve seen other myths reported about DC-DC converters. For example that they are “better for your loads”, or provide “better isolation”, or that they help your starter battery charge to a higher capacity or improve longevity in your starter battery. These are all untrue. Disadvantages of DC-DC converter are numerous as illustrated above.

Some claim that if cost is not an issue, a DC-DC converter is universally the best solution for a dual battery design. Even with infinite budget, I do not believe this to be the case, although DC-DC converters do have advantages that should be considered. Any design is a compromise.

If your auxiliary battery is able to accept a high charge rate from the alternator and will see regular charging from adequate solar and / or an intelligent mains charger then I prefer the advantages of a dual VSR setup.

If you have a fixed voltage alternator and its output is lower than the specified float voltage of your battery then you probably need a DC-DC converter. If you have a smart variable voltage alternator that spends a lot of the time at reduced voltage and won’t respond to loads by cranking up the voltage then you probably need a DC-DC converter. Or if you never have a solar or mains system giving regular high voltage top ups to give the battery 100% charge and reduce sulfation then again you should consider a DC-DC converter.

Remember, a DC-DC converter is the most expensive solution for a dual battery system. Therefore it is the preferred solution for anyone selling this stuff. Keep this in mind when assessing marketing information.

DC-DC Converter with Shorting / Bypass Relay

Some of the disadvantages of the DC-DC converter is that it does not allow your auxiliary battery to contribute to starting your motor, will not allow a charging system on your auxiliary battery to charge your starting battery and charges too slowly during the bulk charge phase of a depleted battery. A solution is to add an extra relay that directly connects your batteries, bypassing the DC-DC converter. The relay would be manually activated whenever you need the batteries directly connected. The relay needs two contacts of opposite sense – one to connect the batteries together and one to open the supply to the DC-DC converter. This shuts off the DC-DC converter, preventing damage which could result from shorting the input and output of the DC-DC converter. The DC-DC converter may protect itself from destruction if its input and output are shorted but it’s a good practice to disconnect the DC-DC converter when shorting.

So if you have a depleted battery and you’re driving a short trip from camp site to fishing spot, you could activate the switch and top up your battery more quickly. Or if your starter battery is struggling to crank your engine you could activate the switch so the auxiliary battery helps.

DC-DC converter with relay

More Fancy DC-DC Chargers

Some battery system vendors have their own solutions for overcoming some the of limitations of  DC-DC chargers. Some may offer their own solution that achieves a similar result to having a bypass relay. Some combine solar charging input with alternator DC input. Some add on top of that a mains AC input. Examples include smart bypass and battery manager. Building a complete system using these most fancy combinations of a dual battery system vendor’s products can run into many thousands of dollars. A similar result can be achieved with a dual voltage sensing relay, solar regulator and separate mains charger.

Modern Variable Voltage Alternators (Smart Alternators)

Some modern vehicles have engine management controlled alternators with various modes of operation. The voltage is cranked up to recover energy from a decelerating vehicle. The voltage is also cranked up if it is detected that the battery needs a charge. The voltage is ramped down at other times to save fuel. An intermediate voltage is used when the battery is fully charged but there are constant loads energized such as headlights. The system models the starter battery’s state of charge and uses inputs from various loads and / or load current sensing to determine the different modes of operation. This works great with the factory arrangement but if you want a dual battery system the following problems may arise depending on the implementation:

  1. The system only elevates the voltage during energy recovery mode which is not enough time to charge the aux battery properly.
  2. The system’s model calculates that the starter battery is fully charged and sets itself into fuel saving mode which reduces the voltage output. The aux battery does not get recharged.
  3. The current flow to the aux battery and / or auxiliary loads triggers the system to permanently elevate its voltage output. If the starter battery or aux battery are fully charged then they suffer from overcharge and diminished life.

Solutions to these problems are:

  1. Force the system into fixed voltage output mode so it behaves like an old style vehicle. There may be a jumper or relay contact that needs to be shorted to activate this mode. The manufacturer / dealer should be able to do this. This is my preferred solution. Fixed voltage output means predictable results and tolerance to any combination of loads and wiring arrangements.
  2. If the factory system has load current sensing, then ensure the dual battery system is arranged so that any current flow to the aux battery / aux loads are also measured by the load sensing.
  3. Use a DC-DC converter. This will crank up the voltage to ensure the aux battery is charged. The starter battery is still at risk of overcharge, depending on how the extra load is interpreted by the system.
  4. Wire it up like an old school system and see if it works. It may work well depending on how the charging system has been implemented on the particular vehicle.

Charging 12V Systems from AC Mains

There are many 12V battery chargers available. Less sophisticated chargers are simply unregulated power supplies. These are ok, but do not boost the voltage to charge faster and minimize sulfation, and do not terminate the charge once the battery is fully charged, risking overcharge. Ensure you manually disconnect these chargers when the battery is full charged, or after say a maximum of 48 hours when charging a flat battery. Usually these types of chargers have a fairly low voltage output and so will take a while to cause significant overcharge.

Electronically controlled and regulated chargers are the best solution for charging 12V lead acid systems. They offer higher charge rates, optimized charge profiles to extend battery life, and will terminate the charge when the battery is fully charged (backing off to float charge voltage at this point). They offer reconditioning cycles which attempt to dislodge sulfur from electrodes and restore some battery capacity. Using these chargers may extend the life of your batteries.

Some people permanently mount an AC charger in the vehicle or trailer. It is permanently wired to the 12V system so that it’s just a matter of plugging it in to begin charging. This is useful if you are visiting powered camping sites regularly, such as those found in caravan parks. Make sure you have a switch to isolate the AC charger on the DC side when not in use, otherwise it may sink some current whilst turned off and flatten your batteries. My CTek intelligent charger draws a few hundred mA when switched off and connected to my battery. Most other chargers will probably be similar.

To get around the fact that, at the start of your trip, a setup with a VSR relay may yield a state of charge less than 100%, you can hook up the mains charger half a day or a day before you leave. This will top up your batteries to maximum capacity whilst still affording all the benefits and simplicity of a VSR system. You can also run your fridge without depleting the batteries so that it’s ice cold when you leave.

Charging 12V Systems with Generators

Many generators have a 12V supply which can be used for charging 12V batteries. Usually the current capacity is quite low – around 8 amps. So charging from a generator using its 12V output will be slow. It’s also wasteful, under loading the generator and thus wasting fuel. The generator is almost idling and you need to let it run for hours. Another problem with the generator 12V output is it is not regulated nor does it terminate the charge when the battery is full. If the output voltage of the generator under no load is around 13.8V or below then it’s fairly safe to leave it connected to the battery for long periods. A higher voltage will eventually overcharge the battery and reduce battery life.

A better solution is to purchase a regulated mains AC charger. Don’t use the generator’s 12V supply. Get a regulated charger and plug it into the generator’s AC source. The benefits include faster charging and less fuel wasted. Also, with the right type of electronic charger, the charge will automatically terminate when the battery is full, preventing over charge.

There are dedicated 12V petrol chargers available. The design typically has an engine turning a vehicle alternator. Capable of delivering up to 100A, they are the fastest and most fuel efficient way to charge a 12V battery by generator. However they will not terminate the charge when the battery is full and lack the flexibility of a generator that can power your 240V appliances as well. It might not be practical to invest in such a specialised generator.

Charging 12V Systems with Alternator Booster Diodes

A booster diode is connected to the alternator’s regulator to trick the alternator into increasing its output voltage. As a battery becomes full this will help charge the battery faster. A voltage increase of 0.5V is typical.

A vehicle’s alternator has no regard for the state of charge of the batteries. It will keep trying to charge indefinitely. For this type of charging arrangement, the voltage should not exceed about 13.8V or as defined under “float charge” on the battery’s datasheet. This voltage is the best compromise between charge rate and minimizing overcharge, grid corrosion and elevated temperatures. This is why car alternators regulate to around 13.8V and why most battery datasheets indicate 13.8V as the float charge level. Alternator booster diodes will permanently elevate the voltage, leading to increased grid corrosion, higher battery temperatures, increased risk of venting and reduced battery life. If your alternator voltage is below 13.8V and a booster diode brings it to 13.8V then that is ok. Boosting to higher than 13.8V and holding it permanently at the elevated  voltage will reduce battery life.

For more information on battery charging, visit

Solar Panels

Solar Panels can be used for charging your batteries. They provide a good solution for those that want to be self sufficient and go on long camping missions through remote areas. They are available in various voltage and power ratings. More than one solar panel can be used in parallel to combine their power output. Solar panels joined in parallel work most efficiently if they are the same. If they are the same, you can design it so that they both generate power at their optimal operating points. Mixing different panels together gives a compromised operating point. It will work but the panels will not operate as efficiently.

Solar Panel Poly or Mono

Silicon solar panels have two basic construction methods – polycrystalline or monocrystalline. There are slight differences between poly and mono cells. Mono are slightly more expensive, require more energy to make, and are slightly more efficient. Poly are slightly cheaper, use less energy to make so are better for the environment, are slightly less efficient but have a slightly better temperature coefficient. That means at elevated temperatures the poly cells become more efficient.

The differences are only slight. It’s largely irrelevant. Find a solar panel with good efficiency and good temperature coefficient and at the right price. Whether it’s poly or mono does not matter.

Solar Panel Sizing

How much solar panel do you need? You can get a rough idea from the loads you want to run. It’s best to work in power when calculating solar panel load. It avoids confusion when running appliances at different voltages (for example AC appliances through an inverter). The current provided by a solar panel is also difficult to calculate due to the complexity of their optimized voltage vs current relationship. So work in watts (abbreviated to W) and watt-hours (Wh).

Calculate your load

Determine the power use of each appliance and how long they will be operated for. Check appliance datasheets or nameplates. If their consumption is in amps, multiply by their voltage to yield their power. If they are AC loads driven by an inverter, multiply the power by 1.1 to take into account the losses in the inverter.

You want to calculate the total Wh consumed in a 24hr period. Multiply the power consumption of the appliance by the number of hours it runs per day and sum together with all the other appliances.

As an example – a large fridge that is rated at an average of 2.1A at 12V, a small fridge used as a freezer rated at 1A at 12V (double the rated current use when used as a freezer – 2A instead of 1A), an LED light rated at 0.5A at 12V that operates for a couple of hours per day, and an 240V AC television rated at 180W used for a couple of hours per day.

Large fridge Wh: 2.1 x 12 x 24 = 605Wh

Freezer Wh: 2 x 12 x 24 = 576Wh

LED Light Wh: 0.5 x 12 x 2 = 12Wh

TV Wh: 180 x 1.1 x 2 = 396Wh

Total energy consumption over 24hr period: 605 + 576 + 12 + 396 = 1589Wh

Calculate the Size of Solar Panels to Supply the Load

The size of solar panels you need is the energy consumption in a 24hr period divided by the number of hours in a day that the solar panels produce energy for. For number of hours, we use peak sun hours (see explanation below). If we assume we are travelling in sunny areas with 7 peak sun hours per day we get:

Solar Panel Rating = 1589 / 7 = 227W

So in this case you need around 227W of solar panels. Or do you? What if it’s cloudy? What if your panels are dirty? What if your panels are not faced directly at the sun and at the correct angle? What if you leave the lights on longer than calculated? What about the loss in panel efficiency with elevated temperatures? What about loss in panel efficiency as they age? What about the fact that your solar regulator is not 100% efficient? What about the charging efficiency of the battery? What about the variation in load from the fridge depending on ambient temperature? What about variations in irradiation due to season? What about variations in irradiation from different locations as you travel around? What if, at the same time, you want to chill a carton of warm beer, charge your laptop, charge your torch and crank some tunes on your stereo?

So the equation would become incredibly complicated if you were to try to work it all out. You need to multiply by a fudge factor. The fudge factor is a number anywhere between 1 and infinity. It’s simply down to probability of running out of power. The fudge factor should be at minimum 1.2 to overcome just the efficiency losses in charging. Assuming you run your system 24 hours a day 365 days a year, if your fudge factor is 1, then you’ll run out of power most of the time, since the system is not 100% efficient. If it’s 1.2 you might run out of power 100 days a year. If it’s 1.5 you might run out of power 20 days a year. There is no way to guarantee power, you are simply shifting the probability. If you are powering life critical systems your fudge factor might be 10 or more. A typical fudge factor might be say 1.3. So in our example it changes:

Solar Panel Rating = 1589 / 7 x 1.3 = 295W

For touring with vehicles, a good sized solar panel is around 100W to 130W. It’s a compromise between economies of scale and how awkward and difficult to access / install / maintain / vulnerability to vibrations and movement. Larger panels get pretty heavy and 130W is towards the upper end of what sized panels I would take on a touring vehicle, but you can go as big as you want depending on personal preference and practicalities of your install. Assuming 100W panels, we need:

Number of solar panels = 295 / 100 = 2.95

So you’d need 3 x 100W panels.

Peak Sun Hours

Peak sun hours is a way to standardize how much sunlight a particular area receives at a particular time of year. It’s the equivalent number of hours per day when solar irradiation averages 1000 watts per square meter. The units are kWh per square meter per day. I used 7 hours in the calculation above. Peak sun hours for many places in Australia during summer is around 7 hours. Some places it’s more, some places it’s less. Seven is a reasonable estimate for Australia in summer. What about winter? In southern capital cities on the mainland you might expect 2 to 3 peak sun hours during winter. In Hobart in winter you could be around 1.5 peak sun hours per day. So you can see there is a lot of variation depending on location and season. This has to be taken into account when sizing your panels. If the above system needed to work in Hobart in the middle of winter then, using the same 1.3 fudge factor, the calculation would be:

1589 / 1.5 * 1.3 = 1377 W

So you would need around 14 x 100W panels.

Locations in northern Australia experience little variation in peak sun hours over the changing seasons. It’s always around 6. In summer (wet season) the temperatures in northern Australia are high and stay high overnight. The daytime maximum might be for example 40 degrees and the night time minimum 29 degrees. It might stay above 32 degrees for 18 hours a day. This is a huge heat load on your fridge. Add this to the fact that the extreme heat is causing you to drink a carton of XXXX Gold every day and with only 6 peak sun hours to charge your battery, you have a situation where many systems will not cope. To handle this situation you’d use peak sun hours of 6 and a fudge factor of maybe 2.

Solar Panel Positioning

You want your solar panels pointed directly to the sun, perpendicular to the sun’s rays. If you have multiple panels then some suggest they should all be positioned the same. Actually each panel individually should be positioned as close as perfectly perpendicular to the sun’s rays as possible regardless of the position of the other panels. The voltage vs current relationship and the resistance in the electrical distribution will allow the better positioned solar panels to provide more energy even if it’s not as much as when all panels are ideally positioned. Of course the maximum energy is obtained when all panels are ideally positioned in which case they will all be positioned the same.

If your panels are fixed to your vehicle / trailer etc and you’re parking up for a day or more then you want your panels to be facing north if you’re in the southern hemisphere. Unless it’s summer and you’re in the tropics above the tropic of capricorn in which case you may actually need to face south! If you’re just parking for a short period of time then face wherever the sun currently is or maybe slightly to the west to allow for a couple of hours of movement.

Connecting Your Solar Panels – Direct Connection

You can connect your solar panels directly to your auxiliary battery. This is the simplest system but it’s quite a poor arrangement. There is potential to overcharge and damage your battery. A solar panel will continue to charge to its open circuit voltage, which is usually around 17V for panels used in 12V systems. This is much too high for lead acid batteries and will cause excessive grid corrosion and pressure venting and rapid failure of the battery. Small panels will still lead to the overcharge condition, it will just take longer. Another negative aspect to this arrangement is that the solar panel is not allowed to operate at its optimal voltage. The voltage is clamped to whatever the battery voltage is. So your solar panel will not be able to achieve its rated power output and it will take longer to charge your battery.

solar panel direct connection

Solar Regulators

To overcome the limitations of a direct solar panel connection to your auxiliary battery, a solar regulator is used. Some solar panels come with an integrated solar regulator. Some dual battery isolators and DC-DC converter vendors also provide solutions for solar charging and distribution. For the purpose of this article I assume a separate third party solar regulator. Solar regulators charge your battery according to an optimized charging profile, reducing sulfation and grid corrosion and will terminate the charge at the correct voltage. The best solar regulators are of the type Maximum Power Point Tracking (MPPT). These regulators load the solar panels according to their optimized current and voltage characteristics for the given level of irradiation. This ensures your solar panels are operating as efficiently as possible, providing you with the most energy and highest charge rate possible. MPPT solar regulators are the best solution for connecting solar panels. To size your solar regulator, sum the total power output of all your panels together and divide by 12 to give you a current rating. The solar regulator needs to exceed this value. So if you had 2 x 80W panels, the current would be 2 x 80 / 12 = 13.3A. The rating of the solar regulator needs to exceed this.

solar panel solar regulators

Some solar regulators have a “load” connection. I have not shown this connection in the diagram above. This is explained in the next section.

If your solar panels will be permanently in the sun then you may want to consider a way to disconnect the solar panels when the engine is running. Otherwise it will be feeding in parallel with your alternator which might not be a problem but could cause damage (unlikely – the solar regulator should drop back to float voltage when the alternator is feeding power) or it could cause an engine check light. You could achieve this disconnection with a normally closed relay opened when the vehicle’s keys are in the run position. If your panels are covered when not in use or folded up and put away then you do not need to worry about this potential issue.

Overcharge from DC-DC Converters and Solar Regulators

DC-DC converters and solar regulators crank up the voltage to around 14.5V to charge your battery faster. This elevated voltage is acceptable during recharge and is indicated on battery datasheets as cycle voltage. Chargers sense the current flow and once the current flow reduces below a certain setpoint this is interpreted as the battery being fully charged and the voltage is reduced back to float level (about 13.8V). If you have a load that regularly draws current (like a fridge) then the DC-DC converter or solar regulator will interpret this as the battery requiring a charge and will crank up the voltage accordingly. The charge voltage will be cranked up whenever the load is running even if the battery is already fully charged. This is a problem. The fully charged battery will be experiencing overcharge. Grid corrosion will increase, temperature will increase and pressure will increase. Venting could occur and battery life will decrease.

This is a serious problem for designs utilising DC-DC converters. Since the DC-DC converter is running all the time whenever the engine is running there’s a high chance that the battery will experience overcharge conditions regularly. The problem is less for solar regulators since this will impact the battery only when the solar panels are deployed and typically after an overnight cycle down where the battery is in a discharged condition.

A solution for those running DC-DC converters is to separate the loads via some additional relays and circuitry. Some DC-DC converter vendors may offer their own solution. This problem can be avoided completely by using a voltage sensing relay instead of a DC-DC converter. For those using solar, find a solar regulator with dedicated load terminals and run your loads from these terminals. The solar regulator is configured to differentiate the current between the load and the charge terminals so that it will elevate the voltage only when the battery is sinking significant current and not when the loads are running. If you don’t do this the regulator cannot distinguish between the load and charging the battery and will elevate the voltage whenever the loads are drawing current.

Large short term loads should not be connected to the load terminals of the solar regulator. Instead they should be connected direct to the battery, as the large load will pull the voltage down anyway and you do not want to overload the load terminals on the solar regulator. So for example your inverter or air compressor should be connected directly to the battery.

Minimising Depth of Discharge

If you don’t have any way of charging your batteries whilst out on camp, or you only have a small solar panel, or if you know you are camping in dense forest, or if you know it will be very overcast, or if you don’t have a DC-DC charger and you aren’t driving long enough for the alternator to fully charge your battery, then there’s a few things you can do to help keep your beer cold and minimise the depth of discharge of your aux battery.

  1. Connect your vehicle to a mains battery charger the day before you leave and keep it charging until you depart. Preferably use an intelligent charger. This ensures your batteries will be fully charged before leaving for camp.
  2. Turn your fridge on the day before you leave. This ensures your fridge and its contents are cold before leaving for camp.
  3. Do not put warm stuff in the fridge when you leave for camp. Put it in the fridge the day before or put it in your house fridge and transfer to the car fridge when you leave.
  4. Set the fridge to a very cold temperature whilst charging on mains and whilst driving. This accumulates the coldness which saves energy when running off batteries. A setpoint of -2 degrees is good. Put beer closest to the cooling elements, since the beer won’t freeze at -2 degrees.
  5. Fill up your fridge as much as you can. This maximises the coldness that can be accumulated which saves energy when running off batteries. If you have spare space fill it with bottles of water, preferably frozen.
  6. Freeze stuff in your house fridge and put it in the car fridge just before departing. This might be meat or bottles of water or bait. The frozen stuff keeps the fridge cold and reduces the load on the fridge.
  7. When you arrive at camp, set your fridge to a higher temperature. Something like 7 degrees is ok. Beer still tastes pretty good at 7 degrees. Note anything above 4 degrees could lead to accelerated food spoilage and associated food wastage or poisoning.
  8. If you run your engine at any time during camp, set the fridge to a very cold temperature again, say -2 degrees. This ensures the fridge runs flat out whilst your engine is running. As soon as you shut down the engine set the fridge back to 7 degrees.
  9. Avoid opening the fridge. Wait until everyone is ready for the next round of beers, then swiftly retrieve the beers, keeping the fridge open for as little time as possible.
  10. Use a fridge cover to help insulate the fridge.
  11. Wrap your fridge in blankets or sleeping bags or any other insulating material but take care to not obstruct cooling fans or ventilation.


Safety note: Inverters produce high voltages which can cause harm or death. Install according to instructions provided by the inverter manufacturer. Ensure the inverter cannot be exposed to moisture, dirt or dust. Connect loads directly to the inverter outlet to avoid the requirement of any permanent 240V distribution within the vehicle. If permanent 240V distribution is required this must be installed and checked by a certified electrician and must conform with relevant standards.

Inverters convert 12V DC to AC and allow you to run AC appliances from your 12V DC system. In Australia the AC voltage is 240V 50Hz. Electronics and step up transformers are used to change from direct current to alternating current and to increase the voltage.

Sizing your inverter

Your inverter needs to be sized to be able to supply steady state and peak loads. The electronics in your inverter have limited overload capability so it is important to check peak loads to ensure your inverter will be able to supply it. Most inverters are able to supply double their rating for short periods. To size your inverter for steady state conditions, add the wattage of the loads you will run simultaneously. To size it for peak loads, you need to find out what the peak loads are for the appliances you are powering. For example a rule of thumb for electrical motors is 5 x rated power during startup. TVs, fluorescent globes, etc all have startup inrush that needs to be taken into account.

As an example, let’s assume we are running a 200W drill and a 60W incandescent light globe. The steady state calculation is 200 + 60 = 260W. For peak load, take the 200W rating of the drill and multiply by the inrush factor typical for electric motors (5): 200 x 5 = 1000W. So you need an inverter that can supply 260W continuous and 1000W peak. This would usually mean an inverter with a 500W continuous rating. It has to be oversized to be able to start the drill.

Modified Sine Wave vs True Sine Wave

AC appliances are designed to work with AC voltage supplies that are smooth and follow a sine wave profile. This sine wave profile is inherent in traditional power generation techniques (rotating generators) but not when generating power with electronics as is the case for inverters. Inverters can be either modified sine wave or true sine wave. Modified sine wave has a square wave voltage profile. True sine wave inverters use pulse width modulation to offer a smoother output closer in profile to a sine wave. The square wave of modified sine wave inverters does not matter much for inductive loads like motors or resistive loads like heating elements. However capacitive loads, such as the front end of most power supplies (laptop chargers, phone chargers, battery chargers, etc) can be worn out by square wave voltage inputs. The square wave causes current spikes at each edge of the wave. This can lead to premature failure of capacitive appliances. The increased failure rate is hard to quantify. Smaller inverters are inherently less able to provide much of a current spike due to their higher internal resistance, so small inverters offer less risk. For example I use a 120W modified sine wave inverter to charge our laptops without any issues after many years of use. The larger the inverter, the greater its capacity to source high current spikes, and the greater the case it is to go for a true sine wave inverter. I chose a true sine wave inverter for my 2500W inverter.

Supplying High Power AC Loads with Inverters

The current on a 12V system is a limitation for using inverters to supply high power loads. Volt drop and cable sizing becomes an issue. It becomes difficult to manage volt drop and cable sizes become impractical as power becomes large.

A typical 12V system might be good for say 100A on the 12V side. This means a maximum AC power of just 100 x 12 / 1.1 = 1090W (divide by 1.1 to account for inverter efficiency). So regardless of the size of your inverter, a typical 12V system might max out at around 1100W. Slightly higher peak power could be attainable. I use my system regularly to supply an appliance that uses around 1000W. I reckon it would be good for a bit more.

A well designed 12V system that could be reasonably fit in a car or camper trailer, with large batteries, large terminals, large diameter cables, soldered connections and short cable runs, may be good for around 200A. Rounding up, let’s say it will be maxing out at around 2400W, which is the maximum rating of household appliances running off standard power outlets in Australia.

So you could potentially run a 2400W kettle or toaster off a well designed 12V system. They are resistive loads so do not have any inrush current. You’d be depleting your batteries pretty quick though. What about if you wanted to run a 2400W motor? Say a compressor? It’s becoming impractical. The inrush is around 2400 x 5 = 12000W. This translates to around 1100A on the DC side after taking into account inverter efficiency. Cable sizes would be too large to terminate onto typical terminals found on batteries and inverters. At such a high current you’d need a bank of batteries to combat volt drop purely from the internal resistance of the batteries, regardless of the size of the cables you use. Even if you had two batteries that together could supply 1100A according to their datasheet, it would come at a significantly reduced voltage – maybe down to around 8V, which is below the threshold that most inverters could tolerate, so you’d need more batteries to combat startup volt drop. Mind you I have never tried to build a system that could start at 2400W motor, so I am speculating. If you’ve done it successfully let me know!

It depends on the nature of the load. If the load requires low torque at startup then the inverter will probably restrict its output and slowly accelerate the motor. But if you need high torque on startup then I reckon you’ll struggle starting a 2400W motor with a 12V system.

Lithium Batteries

I have not used lithium batteries in dual battery setups myself. I have helped troubleshoot a couple of lithium systems on my travels. Some things to note about lithium compared to lead acid:

  • Lithium can be discharged deeper and cycled more times. This means you can get away with a smaller battery and it should last longer.
  • Lithium is much lighter than lead acid batteries.
  • Lithium loses much less capacity at high discharge currents compared to lead acid.
  • Lithium usually requires higher charge voltage and accurate detection of full charge. This means you rely on a specialized and expensive lithium DC-DC charger and if it fails whilst out bush and you try to directly charge from the alternator then the lithium battery will not charge properly.
  • Lithium batteries will not absorb overcharge, not even a trickle charge. Overcharge will cause rapid wear and possible catastrophic failure. This could be especially risky if charging direct from the alternator, for example if the electronic charger fails whilst on the road.
  • Lithium DC-DC chargers may trigger elevated charging voltages when loads like fridges start up (see section above “Overcharge from DC-DC Converters and Solar Regulators“). This means, even with sophisticated DC-DC chargers, a lithium battery may suffer from overcharge and a significant reduction in service life.
  • Lithium batteries are very expensive.
  • Lithium may not be as tolerant to mechanical disturbances and may be more likely to have internal failures, especially if going off road.
  • When they fail, lithium batteries often catch fire, as reported on commercial aircraft and Tesla cars and more recently Hyundai electric cars. Lithium batteries used in dual battery systems (usually LiFePO4) are less susceptible to fires than some other lithium chemistries, but still have not proven themselves long term.
  • Lithium batteries are not very tolerant to high temperatures, and wear out quickly when they get hot. This is why electric cars have sophisticated battery cooling systems with radiators / heat exchangers. A second battery in your car does not get such luxuries and may fail prematurely due to heat.
  • Capacities of lithium batteries are often over-stated by their dodgy chinese vendors, eroding some of the benefits of lithium. Advertised capacities vary wildy for similar cells. Lead acid technology is much more established, and a lead battery of a certain form factor always has a capacity close to other batteries of similar size and weight.
  • There can be an emotional bias towards lithium since it is the newer and more fancy technology and therefore attracts more status in the vehicle modding community. This may lead to advantages of lithium being over-stated.
  • For an average Joe who just wants cold beer, going lithium could accidentally trigger a sequence of events that lead to a very fashionable and expensive state-of-the-art dual battery system costing many thousands of dollars. I feel bad when people get lead to believe they need to spend many thousands of dollars to keep their beer cold. Some people are spending out of control amounts of cash on their dual battery systems. They’ll be stuck at work paying for that system rather than be out exploring the outback and drinking cold beer.

This is not a comprehensive analysis of lithium vs lead acid but for me I prefer lead acid. Lead acid is too cheap, reliable, easy to recharge, resistant to vibration, tolerant to overcharge, resistant to high temperatures, easy to fix and easy to find replacements. If lithium batteries are catching fire on planes I’m a bit concerned about what they’d do over a few billion corrugations.

If you combine the effects of high temperatures, lack of cooling systems, possible overcharge and vibration, the actual life of a lithium battery might be far less than the theoretical life, which erodes a lot of the long term value that lithium supporters claim.

If a battery failure were to occur I’d feel pretty bad about blowing away over 3 times the money of an equivalent lead acid battery. There is extra risk in putting so much money into a single supposedly longer lasting device, compared to having money in the kitty for when failures occur. However lithium does have advantages and is a worthy solution that should be considered in a design, particularly for its weight and depth of discharge advantages.

How does long term ownership costs compare? If you calculate the ownership costs based purely on battery price and maximum theoretical lithium life span then lithium may or may not end up slightly cheaper than lead acid depending on how expensive the lithium battery is and the assumed cycle life and capacity. There are some cheaper lithium battery options now available (at slightly more than double the price of lead acid) but I doubt they provide the same capacity and number of cycles as the brand name lithium batteries that cost 5 times more. The winner depends so much on the assumptions and prices chosen when calculating the total cost. But if you go spend 8 grand on the most fancy lithium setup in the world with super extreme plus battery management system and touch screen interface with automatic wifi firmware updates and remote bluetooth control with full snapchat integration and a huge LCD monitor dedicated to displaying how much status you acquire in the 4WD modding community then there’s no doubt, in terms of long term ownership costs, you’d be much better off with a simple lead acid system.

Cable Sizing: Current Ratings and Volt Drop

Cables are sized according to volt drop and current capacity. Cables have a current carrying limit which cannot be exceeded. Current travelling through a conductor will cause volt drop across the length of the conductor, which needs to be kept within limits.

Cable current ratings are defined in manufacturer’s datasheets. Values are dependent on heat dissipation and insulation tolerance to elevated temperatures. Calculate the current that will go through a cable based on what it is feeding. For example if it’s feeding a 1000W inverter, then the current through the cable would be 1000 / 12 x 1.1 = 92A where multiplying by 1.1 is to take into account inverter efficiency. If the cable is to join your starter battery with your auxiliary battery, then you want to assume a large current so that you auxiliary battery will charge quickly and be able to contribute to starting your vehicle if the starter battery is depleted. At least 100A is a good figure for sizing this cable.

Volt drop is calculated by current x resistivity x length. Resistivity is usually given in Ohms / km so make sure your distance is also in km. Resistivity is resistance per unit length, and is governed by the cross sectional area of conductor and type of conductor. The actual values are provided in manufacturer’s datasheets. Ensure you include the negative return cable in your calculation. It means you need to double the distance of the cable run.

As an example, let’s say you have a 3m cable run from battery to the 1000W inverter mentioned above. The cable needs to be rated to 92A. Looking at datasheets for a particular manufacturer, this yields a minimum cable size of 25mm2 which has a resistivity of 0.734 Ohms / km. The total distance, including return cable run, is 0.006km. So the volt drop would be:

Volt Drop = 92 x 0.734 x 0.006 = 0.4V

Volt drop needs to be kept within the limits of the equipment being fed. For example many inverters can tolerate down to around 10.5V. The smaller the volt drop, the more efficient your system is. A typical figure in electrical design is 5%, which in a 12V system is 0.6V. So the arrangement above passes the test. A more stringent criteria for a 12V system would be say 0.2V. In this case the cable fails and it needs to be oversized to pass the volt drop criteria. The smallest cable size to provide less than 0.2V volt drop at 92A is 70mm2, which has a resistivity of around 0.27 ohms / km. The volt drop would be:

Volt Drop = 92 x 0.27 x 0.006 = 0.15V

For your heavy power cable runs, for example connecting the main battery to the auxiliary battery, and to a large inverter, it’s important to size the cable correctly to avoid overheating it and avoid volt drop issues. Too much volt drop will mean your auxiliary battery takes too long to charge or your inverter cuts out. An easy option is to use the largest cable that is still practical to work with and terminate. This is around 70 to 95mm2 for typical battery and inverter terminals. Larger cables become difficult to work with but it depends on the installation.

Below is a table listing various cable sizes, typical current ratings (derated for being in contact with other surfaces according to Australian Standards), typical resistivity, and a volt drop calculation to provide a guide for cable sizing. Everything in the table is straight from the manufacturer, apart from the maximum current at 3m distance which is calculated by current = volt drop / (resistivity x length). The table refers to PVC/PVC copper cables. When purchasing cables, make sure you specify mm2 of copper so that it is not confused with overall cable diameter.

Cable Size AWG Resistivity Current Rating Overall Diameter Max current for 3m cable length and 0.2V volt drop
mm2 Equivalent Ohms/km Amps mm Amps
1 17 18.2 13 2.8 1.8
1.5 16 12.2 16 3.1 2.7
2.5 14 7.56 23 3.6 4.4
4 12 4.7 31 4.5 7.1
6 10 3.11 40 5.1 10.7
10 8 1.84 54 6.0 18.1
16 6 1.16 72 6.9 28.7
25 4 0.734 97 8.4 45.4
35 3 0.529 119 9.4 63.0
50 2 0.391 146 10.9 85.3
70 2/0 0.27 185 12.4 123.5
95 3/0 0.195 230 15.2 170.9
120 4/0 0.154 267 17.3 216.5
150 300MCM 0.126 308 18.8 264.6
185 350MCM 0.100 358 21.1 333.3
240 500MCM 0.0762 428 24.1 437.4
300 600MCM 0.0607 495 26.9 549.1
400 750MCM 0.0475 577 30.6 701.8
500 1000MCM 0.0369 668 34.1 903.3

Terminations, Connections and Cable Routes

Terminals and connectors should be the right size for the cable. If the wrong size is used, the terminal can become a hot spot when under load and a potential point of failure. There is debate as to whether crimped connections should be soldered. Usually soldering provides a stronger connection less prone to becoming a hot spot, but some suggest soldered connections are brittle and will crack from vibrations. Bolted terminations must be tight and double checked before energizing the system.

It’s handy to connect your solar panels, lights etc via plugs. It’s extra work and doubles the amount of terminations, but it means you can easily remove them when you’ve finished your camping trip or if you have a problem with the equipment or its mounting system. Plugs should only be able to be made one way and should be held captive through some sort of latch. Anderson plugs are a popular solution. If you do a lot of offroading then expect some mounting and mechanical issues with your devices in which case being able to remove and refit them easily is important. Make a judgement as to whether it’s worth the effort of using plugs on a case by case basis.

Cable lengths should be minimized to reduce volt drop and reduce the amount of cable potentially exposed to damage. Cable should be run within convoluted split conduit to protect them from mechanical damage. Avoid sharp bends, especially on larger cables. Avoid passing across sharp edges, such as those created when a hole is drilled. If passing cable through a drilled hole, first fit some split rubber seal around the hole or some other measure to cover the sharp edge. Rough roads and corrugations will otherwise cause the cable insulation to fail at the sharp edge.

The Complete Installation

A complete system comprises of a dual battery system, solar panels, a solar regulator, an inverter and all the cables, connections, terminations and fuses / circuit breakers between. Add to that your auxiliary loads which are not covered here – fridges, lighting, extra cigarette lighter outlets, USB chargers, etc. In my opinion the best dual battery system is the dual voltage sensing relay with solar charging. It provides a good solution to charging the auxiliary battery from the alternator, sharing loads between the starting battery and the auxiliary battery, enabling the auxiliary battery to contribute to motor starting and enabling both batteries to be charged from a solar panel charging system. An advantage over the DC-DC converter is that it enables a higher charge rate, which means your auxiliary battery will be topped up quicker when your motor is running. It has a further advantage over the DC-DC converter that it does not keep the auxiliary battery at an elevated voltage all the time, which can cause unnecessary grid corrosion for no benefit. When you are cycling the battery, the solar panels are deployed and the solar regulator takes over, charging the battery to an elevated voltage with an optimized charging profile that reduces sulfation and charging the battery as fast as possible with an elevated voltage. It will also top up the starter battery with an elevated voltage, reducing sulfation on the starter battery. The batteries are cycled down every night so are not held at the elevated state of charge, which minimizes grid corrosion. This is exactly how the lead acid batteries are specified to operate according to their datasheets. A reduced float charge of 13.8V when not cycling, and an elevated charge of around 14.5V when cycling.

However each design has its compromises. I’d consider a DC-DC converter with a bypass switch, especially if I didn’t have solar. The end user needs to decide what suits his or her application and budget.

The schematic for a complete system would look something like this, excluding fuses and circuit breakers:

complete 12V system - dual batteries, inverter, solar charging system

A battery condition monitor is also useful. These can be as simple as a voltage display. Voltage is a good indicator of a battery’s charge state if the battery is rested and not under load. More sophisticated condition monitors are available that calculate available battery capacity and other parameters.

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See also:

Manly Thermomix Review

Design Compromise

Emission Systems – Worth Tinkering?

Corrugations – Fast or Slow?

Why No Diesel Performance Chip?

After Market Add-ons and Other Touring / Camping Stuff

Our Electronic Gadgets for Touring / Camping

Camping / Touring / Travelling Phone and Internet Setup

Disconnect Negative Terminal when Welding

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  1. Great article, thanks!
    One question about the DC/DC converters tho, most of the ones I can find are 24 to 12 volt…but Wouldn’t I need a 12v to 12v if my battery is feeding 12v?

    • Hey Dave I think the 24V refers to the input voltage. You might be looking at a charger designed for 24V charging systems (like on trucks and stuff). Or maybe some chargers have a wide input range (10 to 24V or something). I can’t tell exactly from the info you’ve given. Check the datasheet and make sure the charger can take 12V input and is good for charging 12V on the output. There’s heaps of chargers made for 12V systems, it’s the most common arrangement.

    • Phenomenal Article ! BEST Part, you’re Clearly not Selling anything. It is EXTREMELY Frustrating trying to (research);when all of the responses are from those with some sort of “Agenda” … either as a means to ‘justify’ their own install or to Sell you something that you Likely do not need.

      Really Awesome, Ta ! Based on your great explanations, it seems that the (Dual) VSR Set Up is Best for my rig. I got a little confused as to whether I should be sourcing one Dual Sensing VSR, or Two (plain) VSRs. The Dual Sensing are nearly ‘unobtanium’ in the US. I have Yet to find one. Aus. , NZ & UK sell them, but at Extreme Shipping expense. I refuse to buy Anything directly from China.

      Am I off-the-mark on the single Dual Sensing vs. two (reg.) VSRs ?

      Thanks Again !

      • Again Joe, Ta ! I’m pretty much settling on the WirthCo Battery Doctor 20092 for my Application. Tentatively. This morning however, I was reading an article that stated Lithium batteries (can) use 100% of (rated) aH capacity, while traditional Deep Cycle AGMs are only 50% (?!?). News to me. If you had already blogged about it, I missed it because Lithium was not previously a consideration.
        Lithium’s WERE not considered due to Cost, and fear of vulnerability to Fire. But this 100% vs. 50% ‘factoid’ has me a bit chafed …..
        Thoughts ?
        I’m under the impression that since AGMs have an entirely different composition, therefore Charging Profile, Tom Lithium’s, that I might have to re-engineer my Entire Plan, should I go that route …..

      • What you suggest is essentially true. Lead acid batteries can be discharged as deep as you want but the damage to the battery accelerates disproportionately with depth of discharge. Cycling down to 0% severely reduces the number of cycles on lead acid batteries. 50% is considered a good compromise between available capacity and cycle life. This is mentioned in the battery sizing section of the article. Lithium can tolerate deeper cycles better. There is a small lithium section in the article too. I personally prefer lead acid but lithium has advantages that should be considered.

    • Hi Joe, thanks for so much unbiased info! its been doing my head trying to sort out what would suite my 89 75 series troopy when it comes to dcdc vs VSR! So many opinions and many seem biased thru one way or another. Just wondering have you used lead crystal batteries at all? Also wondering if you have seen Andrew St Pierre Whites rant on DCDC’s (video called ‘battery charging in 4wd’s’) verse VSR’s and what your reply would be to his claims that an alternator doesn’t give the correct type of charge to the second deep cycle battery, cheers!

      • Hey Rob I think lead crystal batteries are just over priced AGM batteries but i dunno much about them. If they were really that much better the technology would propagate through the industry.

        Yeah I’ve seen Andrews vid he is wrong. Refer to article. Andrew is a great story teller and entertainer but needs to brush up on his electrical fundamentals. There’s a butt load of cringe-worthy “theory” in that video.

  2. hi joe,Great site, we have a camper van with dual batteries connected via single VSR. House battery is loosing all its acid when running 2000w inverter. would your double VSR system help by taking load off deep cycle battery by diverting current direct to invertor

    • Hey Derek what load do you have connected to the inverter? Dual vsr won’t help at very high current because the load will pull the voltage down and the relays will drop out. You can try running the vehicle – the alternator may be able to hold the voltage up so your vsr stays energized and the load is shared. But even then the voltage may drop enough to open the relay. A solution is to force the vsr to stay closed regardless of voltage. Some have a switch or something to do this otherwise a jumper can be installed on some to force it to close. I need to do this when cooking with the thermomix otherwise the relays drop out. Another solution is bigger battery that can cope with the high current or adding more batteries in parallel.

  3. Hi Joe

    Do you have any Dual VSR devices that you could recommend? I am in the process of designing a dual battery and solar system to have in my campervan (Mitsubishi Delica L4000) I am building for a 8-12 month trip around Australia (well, actually just the southwest, south and east coasts) starting in March.

    All I have to do is ensure that the VSR is capable of sensing the voltage of both starter and aux battery right? I am thinking of something like the Enerdrive VSR which is around $80 delivered online.

    I’d like to finalise my design this week so I can start ordering all the equipment now, so when I finish work in a month I will have it all ready to be put together and installed.


    • Hey Karl I had a quick look at the enerdrive VSR and yes looks like it senses voltage on both sides so will do the job. I have not heard of it before though. Redarc make dual VSRs that are well proven and reliable but also expensive. If you can research some reviews for the enerdrive and if the company looks ok and there’s evidence of good support then I’d probably give the enerdrive a go. As long as you are ok with the risk that it may not be as reliable, but also might be perfectly reliable. Good luck with your design and install and your trip around oz. You’ll love it.

      • Hi again

        With a VSR, does the proximity to the alternator or cranking battery matter? I was thinking to have the VSR mounted above the aux AGM battery inside the cabin (behind the driver’s seat) on a panel along with the other surface mounted equipment (fuse box, output panel etc). This will have around ~3.8m of 13.5mm2 cable from the primary battery to to the VSR and then about 0.80-1m of cable from the VSR to the aux battery.

        I posted up my design here, not sure if you can view it if you’re not a member..

        I also posted a quote from this article with a link, hope you don’t mind.


      • Yo karl linking back is good mate cheers. VSR position shouldn’t matter, once it’s closed it’s just straight through like it’s not there. Only time it would matter is if your aux battery is sinking lots of current the voltage will be pulled down, maybe enough to cause the relay to open. So closer to starter battery is better in this case. If you’re going to run something that consumes 100A like an appliance off an inverter then this problem will manifest. If you have a decent sized aux battery it could charge pretty fast also. Most people wouldn’t check this and get away with it nearly all the time but you might want to calculate voltage drop to the vsr based on max expected current, to be sure.

  4. Hi Joe

    I’ll be using a 100AH AGM battery. No inverter or power hungry devices. Just a single 47L ARB fridge (peak around ~4.5A and cycle around ~0.87A/h average), some USB powered gadgets (GPS/smartphones/tablet), a rechargeable LED lantern and a 80w notebook PC for a couple of hours a day. I’ll have the aux battery rigged up to a 120w (2x60w panel) folding solar panel kit during the days when we are camped up and not in transit. When we are in transit the alternator can do the work instead to keep the two batteries charged.

    My proposed system was going to be a bit more simple than your recommended one, with the power from the solar going through a basic MPPT regulator and 50A circuit breaker directly to the aux battery to charge that. As I mentioned before I am looking at the Enerdrive VSR but I am still unclear if that will actually allow two way power flow (from alternator and the solar) to BOTH batteries to keep them both fully charged where possible. I have noticed some VSR have a feature which will allow you to, at the push of a button, to perform an emergency link between both batteries in the event that the starter battery is too drained and needs a jump from the aux battery.

    A system where power from either solar or alternator will constantly charge both batteries would be ideal as obviously I want to avoid the chance of getting a flat in the middle of nowhere. I am going to use the primary battery to charge USB devices (GPS and smartphones mainly) from the front desk cig jack while we are driving, but I also have a 12v LED lamp and LED interior lights that we will be using for a few hours each night. I know LED drain a lot less power than the old style lights, but always having the starter battery at full charge will be reassuring.

    I am planning to leave around mid-March, so it’s all coming to the pointy end of design and planning, and I now have to commit to a power system and start ordering all the components so I can assemble it once I am finished with work in a few weeks. Good times!

    • All sounds good mate sounds like you have a good arrangement and yes the vsr you speak of will work both directions. With a 100Ah agm allow about 50A max charge rate and check the volt drop to make sure your vsr won’t drop out. Have fun and may your beers be icy cold!

      • Hi Joe

        I’m nearly there! Just in the process of choosing between the ABR Sidewinder DBi140 and the Enerdrive VSR.

        The Enerdrive is dual voltage sensing as you mentioned, but it doesn’t have the emergency override feature.

        The DBi140 is also dual sensing, and it has the option to add an emergency override button somehow (not sure how complicated or expensive this is, as they say it’s requires an optional push button). The DBi140 is the same price as the 140A Enerdrive VSR unit.

        One more question, the isolator needs to be earthed. In one of the schematics on the ABR website they show a separate earth cable going all the way back to the starter battery negative terminal. Considering I would like to have the VSR inside the cabin behind the driver’s seat this means I will have to have two separate 4-5m of cables running back into the engine bay. This seems a bit overkill. Do you think it is safe and effective to simply connect the earth wire from the VSR to the earth terminal of the aux battery, as the earth cable from the aux battery running back to the starter battery will be a thick 13.5mm2 cable. Or should I just run two separate earth cables?

        I get paid tomorrow and will be ordering as many components and cables as I can, so I have it by the end of the month. 🙂

        Thanks again for your previous replies


      • Hey karl I think under very high current the negative at your aux battery might float up causing your vsr to drop out of earthed there even though the starter battery still had more than the cutoff voltage across it. Splitting hairs though. Unlikely to have a problem earthing it anywhere convenient, especially if you have a fat cable as the negative return from your aux.

      • Yeah I have the non dual sensing abr. Works a treat but I ended up chucking in another vsr in parallel in reverse to get dual vsr function. Emergency override is good. I used it when my starter battery failed and I use it whenever cooking with the thermomix as the voltage drop at such high current causes my relays to drop out.

      • Hi again

        To correct an error in my previous post I have been informed by ABR that the DBi140 is NOT dual sensing.

        The ABR Bi150 does have the dual sensing feature, but at a higher cost ($99 vs $79 for the DBi140).

  5. I’d suggest do some more reading..

    Firstly you can NOT plug 12v solar panels directly to a 12v battery EVER(with the exception of a extreme emergency to start your car). It is extremely dangerous and will kill your battery in a matter of days or even hours if the solar power is high. That is if you don’t blow it up and put your self in hospital.. ANYONE reading this MUST use a solar controller to charge 12v battery.

    Secondly, a dc\dc charger is the only solution of you want to get the best our of your agm if you have typical cranking battery for your starting battery. You should avoid parallel setups of different battery types.

    • Hey Luke heaps of people connect their solar direct to battery. I’ve seen it plenty of times in my travels. So I guess, by definition, you CAN do it, because people ARE doing it. It’s a pretty crap arrangement and will damage the battery but it’s worthy of discussion so I included it in the article. Helps in understanding. When the battery is accepting a reasonable current it’s ok. The voltage gets pulled down and the only disadvantage is operating the solar panel at an inefficient voltage. As the battery becomes fully charged the voltage rises and you have a problem. If you remove the panel at around 14.5V it’s ok. Let it float higher and you boil off the electrolyte and corrode the grid. You’ve pretty much just repeated what I said in the article but used “blow it up and put yourself in hospital” instead of “overcharge and damage battery.” So your point is moot.

      DC/DC charger is the only solution? Did you even read the article? Care to address some of the points raised where a DC/DC converter does not get the best out of the system rather than just making an unsubstantiated claim?

      • I think you will find they are running a very small cheap solar controller, about the size of a relay. In fact allot of panel kits come built in.

        The other exception would be a small solar panel under 50watt. They generally only put our about 15volt.

        There is not a 12v battery in existence that would Handle 19volt for much over 30hours at best a few days. So sorry don’t believe what you have claimed to have saw.

        I did read your article and you really talked down a dc to dc charger… I do agree with most of your points, and if your running identical batteries then I would agree that perhaps a dc charger isn’t the way.. but if you are running a cranking battery with a agm for example then a dc charger is the only way to get the benefits out of your agm. Running a cranking battery with a agm in parallel will reduce your agms life. The positives outweigh the negatives in this setup..

        I will add you mentioned a dc dc override to help start your car is a good idea, but the positive of not having run large cables is lost when you do that..

        So your points regarding dc dc chargers where not bad, I just don’t think you quite understand drawback of running agms in parallel with non would just be better of buying a cheap cranking battery.

      • So you say direct connection is universally not possible then say possible for certain panels? I think you contradict yourself. The same rules apply regardless of panel size. The difference is how quickly an unsafe condition will occur. I think you’ll find that there are people who just buy a cheap panel and hook it up. I’m pretty familiar with this stuff so unlikely to get it wrong on multiple occasions. I’ve tinkered around with heaps of installations on my travels as people seek advice and troubleshooting help. Can you explain how agm life will be reduced without a dcdc converter? My agm has a float voltage specified at 13.8V, exactly my alternator voltage. Which is also the float voltage for typical cranking batteries. Actually dcdc converter can decrease battery life if it spends too much time at elevated voltage. Voltage should surpass the specified float voltage only after a battery has been cycled down. Not every time the charger is switched on. Elevating to cycle voltage without a cycle will damage the battery. A battery that hasn’t been cycled must be kept at it’s rated float voltage. Some installations cause the dcdc converter to never enter float mode, if there are loads downstream of the charger. The loads cause the charger to permanently remain at an elevated voltage since it interprets the current as the battery requiring a charge. This causes premature battery failure. If float voltage is around 13.8V for an agm (it usually is) then the alternator is the perfect supply for maintaining the battery whilst not cycling. It’s the definition of float voltage and is specified by the battery manufacturer. How will this reduce battery life?

  6. Its not the charging part that is the issue.

    You have explained well that an alternator charges agm well, and that 13.8 is the recommended float voltage for most agms winch is true for most.. there is however the maintenance issue that will not happen ever by an alternator.. Il get into that in more detail later.

    My point is not so much the issue with your alternator charging a agm… your points are valid to the point I wont argue with them.. its the issue of running a agm battery in parallel with a cranking battery… and the issue is not the charging its the discharging of the two units in parallel..

    Il get my text book out and get some references for this.

    • Yeah get back to me if you find something. I don’t think it matters. The batteries will be around the same voltage and hence won’t even really know the other battery is there. Then once the starter battery voltage drops slightly it’s isolated by the vsr. If you left them permanently connected then the slight voltage difference could cause problems but no system leaves them permanently connected.

      Only thing with the alternator is you miss the elevated voltage for de-sulfation. But this is offset by the disadvantages of the dcdc converter so you need to way it up. Point is dcdc converter isn’t universally the best. If you have solar panels then you’ll get the de-sulfation anyways.

  7. Nice work Bloke.

    What are you thoughts on where and how a 240/12V battery charger should be wired into the mix (or specifically for a caravan or camp trailer). Would connecting this to the battery (and in turn to the output of your solar charge controller) upset the charge controller at all or would it be better to fit a relay to disconnect the charge controller when the 240V charger is available?


    • Hey Craig if you’re panels are covered when you connect the 240V charger you will be ok since the solar regulator is designed not to take reverse current otherwise it would discharge your batteries. If your panels are permanently in the sun there could be a problem. More than likely everything would protect itself but it’s hard to know for sure. For example if the solar regulator’s output is slightly more than the 240V charger or if they interpret “fully charged” at different times (this would cause a large difference in output voltage) there could be a problem with them working in parallel. I’d prefer to have the solar regulator isolated to be safe. Make sure you isolate your 240V charger from the battery when not using it since they usually sink a few 100mA when connected and switched off. I probably need to expand the 240V charger section and include it on the schematic to make it more complete. Will do that one day. Thanks for your feedback.

  8. Yep. I think a 240V coil relay connected to the mains input to change over the battery between the mains and solar chargers would probably be fool proof enough, except if the solar charge controller also has a load cut off, then this complicates things a bit and would mean the house load would also have to be switched (DPDT relay), so under solar charging, the load can still be cut off to protect the battery from over discharge, but when there’s 240VAC available the load’s connected directly to the battery so there’s still 12VDC available.

    Sound right?


    • Yeah sounds good, a little annoying with the loads connected to the solar regulator. My panels are folded up and covered when I use the 240v charger. I just pop the hood and hook it up with alligator clips rather than having it hard wired. I’m using it less and less these days with caravan parks charging a fortune for power to cater for fifth wheelers with air con and plasma tvs when all I’ve got is a fridge. Extra $10 a night means I can buy a new battery every 30 nights so I prefer to just cycle the battery and use solar.

  9. can i run a 240 v smart charger from a 12v to 240v pure sine invertor off a dual battery to charge 4x 120 amp hr batteries in a camper trailer

    • Hey Gary yeah you can do that. It’s wasteful in terms of energy efficiency and wasteful in terms of equipment, stepping up to 240VAC then back down to DC. But if you already have the gear then it’s an option. I’d feel a bit uncomfortable running an inverter all the time, especially if it’s a cheapo no brander. They have fans (moving parts) and get pretty hot. I don’t know how they’d handle a duty cycle like that. The usual application is for short time periods. Plus there’s the safety issue of having live 240VAC in your car all the time, driving in wet conditions, puddles, dust that can attract and hold moisture, vibration. Take care where you put the gear. You’ll draw a few more amps compared to a DC system but if it’s off your alternator then shouldn’t matter much.

      • thxs joe. i have 2 66 amp hr optima yellow tops working as dual batteries. can i charge the 4/ 120 fullrivers in trailer in parallel with the optimas, or should i run seperate wires from the dual battery electrics to charge them from alternator while driving.i have a honda 2 kva and a 20 amp 6 stage charger, was thinking of getting a 50 amp projecta charger as an alternative to the alternator . bit long winded but thxs in advance cheers gary tut

      • Hey Gary, unless you have separate DC chargers, the batteries are in parallel no matter how you run the wires. Just need to make sure, if you daisy chain them, that the cable can take the current and wont yield too much volt drop. It’s usually ok to charge different batteries in parallel since most lead acid batteries have very similar cycle and float voltage levels. Check your datasheets to verify – if the voltages are all within a few tenths of a volt then should be fine. Yeah a big 50A charger off a generator is a good way to charge when you need serious charge current rather than idling your engine all day.

  10. If I use your suggested dual VSR setup with 2 Exide Extreme batteries in my 4WD (with Engel in back connected to one), how is this complicated by the addition of an Anderson plug connection through to my camper trailer which will have 2 gell batteries running a larger fridge? Would a DC-DC charger (e.g. CTEK) now be ideal in the trailer because different sort of battery from the car? Or does the logic of using another dual VSR in the trailer continue on?

  11. Joe, to give you more info following on from the above Q before you attempt to answer it, I do not have the 2 Exides in the LC 200 yet. And I don’t own the trailer yet ,but it will be the Vista Crossover. It comes with 1-2 Haze N70 GEL 100 ah batteries (these are not AGM). I am impressed with the concept of the dual VSR in the car. But not sure if there are special requirements for charging the GEL’s in the trailer. Another dual VSR between the aux batteries in the car and those in the trailer- esp because so different in type? DC-DC charger may provide too high a voltage for the GEL’s?

    • Hey Harry you can have one isolation system (for example a single dual sensing VSR) and run all these batteries. Just chuck them all in parallel after the VSR. An anderson plug in between makes no difference. This will make the batteries in the car and trailer behave like one system. They will share loads together, share charging together, share a solar system together. If you want the trailer to be isolated (for example if you’d rather the car batteries go flat and still have capacity left in the trailer) then you’d need two isolation systems. Either two VSRs or a VSR and a DC-DC converter or whatever. The pros and cons don’t change between VSR vs DC-DC converter. Check the datasheets for the trailer batteries vs the car batteries. If they’re all your typical 13.8V float and 14.5V cycle (or around about there) then no problems having them all in parallel all the time. If the voltages are a bit different you might prefer separate isolating system so they aren’t discharging in parallel. If the voltages are unusual then you might prefer a DC-DC converter that can be configured for the voltage requirements of the batteries.

      If the batteries are all typical voltage I’d just have them all in parallel hanging off one dual sensing VSR. Simple, good load sharing, ability to charge everything from one point and plenty of current direct off the alternator for charging (a DC-DC charger will be pretty slow at charging all that stuff in parallel). Check your volt drop on the cable run to the trailer – you need to make sure cables are adequately sized. Some people think DC-DC converter is best for long runs since it boosts the voltage. It’s a valid point but you can make volt drop as low as you want through cable sizing.

  12. Are you using a fuse between the battery and inverter and if so what size fuse are you using? This would have to be based on the maximum surge? 5000w

    • Yo Steve nah I have no fuse, the inverter has its own protection and my battery is right near my inverter in the tub with very short cable links so cable risk is low. You can put a fuse in especially to protect cables. The rating is hard to say. Fuses don’t blow instantaneously. They can tolerate inrush. I’d size the fuse according to the continuous rating of the cable. If you size the fuse for inrush you can have a situation where the fault doesn’t cause the fuse to blow and you start a fire.

  13. Hi Joe,
    Thanks for your article it really helped me to understand a little about what I needed and what I didn’t.
    After reading through it about 20 times I decided on 2 vsr’s a solar panel and mppt regulator along with a decent agm house battery. I just have a couple of things that I don’t understand. You say that the vsrs will allow the house battery to assist in cranking. How does this work if the vsr is only working when the voltage is over 12.7v? Isn’t that only when either the solar or alternator are active? And if the house battery is helping then shouldn’t the cables be at least the thickness of the cable running to the starter? I bought 2 Narva vsr’s rated at 140A continuous but would think cranking a battery would exceed that. I carry a jumpstart battery pack so I don’t really care if the house battery helps crank or not but I would like to understand it.
    Yours, Adam

    • Yo Adam glad the article helped. Yes you are right when cranking the voltage will drop and the VSRs will open. You need to be able to force one of the VSRs to remain closed. I have a little jumper on mine. Some might have a switch or something. I’ve had my starter battery completely fail whilst in outback Queensland and ran off my aux for a few days until we got to Brisbane. I also force one of my VSRs to close when running heavy loads on my inverter (100A or more), otherwise the volt drop causes the relays to open and prevents the alternator from keeping the aux battery charged.

      Your VSR rating of 140A should be ok for cranking. Current when cranking might be around about that, could be bit more or bit less, but it’s very short term so shouldn’t melt anything unless you are having problems starting and are continually cranking for a while. If you want to allow for cranking solely from your aux then yes you need some fat ass cables capable to carry the current and avoid excessive volt drop. If you would just like to share loads a bit then it doesn’t really matter, smaller cables to your aux will naturally limit the current since the volt drop will be more than your starter battery which will prevent much current from flowing. But you still want cables big enough to avoid too much volt drop so you can quick charge from the alternator.

      Good luck with your setup dude, have fun.

  14. Thanks for the Blog Joe. Very informative. I currently have a dual battery system to keep my large fridge running and a few other electronics. Im using two identical dual purpose AGMs isolated with the National Luna intelligent split charging system. Now I’m looking to add solar. My plan is to go a bit overkill with 130W solar, a 20A MPPT charge controller, and dedicated deep cycle AGM battery (100+ Ah) as the auxiliary. My question is: since I have the two identical dual purpose AGMs, is there any problem with wiring them in parallel to use as the main? I understand that set up can be a problem if there is a weaker battery, but are there other issues with that arrangement?

    • Hey Josh I’m not 100% clear on what you propose. Do you want to use your 2 x old batteries combined as your starter? You say “wiring them in parallel to use as the main.” Main is starter? Or are you putting a third battery in parallel to make bigger auxiliary capacity? Putting different batteries in parallel is ok if the voltage specifications are similar. But putting an old battery in parallel with a new battery can be bad depending on how worn the old battery is. The old battery may not retain charge and as it’s voltage drops it starts discharging the new battery as well.

      • Yes. Im hoping to use the 2 batteries I currently have as my starter, wired in parallel. I then would add a dedicated deep cycle as my auxiliary. The 2 AGMs I have are identical, and were purchased together but one has been cycled and charged by my alternator. They both still hold similar volts when fully charged. I went ahead and wired my 130W panel w/ 20A controller and it’s working great. I work out of my truck in remote parts of the Southwest US and am hoping to have more power than I can use whether or not I’ve been driving. The thought about combining the 2 to use as my starter would achieve enough power to not worry if I play the stereo in camp for too long. Also an auxiliary with more amp hours that is a true deep cycle seemed like a better long term fit now that hopefully the solar will keep it charged up. Loads of sunshine in the Mojave desert! My question is: should I be concerned about the battery than is my current auxiliary not performing as well as the one that is my current starter? If I wire them in parallel are there any downsides to that setup if they are indeed healthy and very similar?
        -My electrical experience is fairly limited to this project and I’m still very much learning. Thanks Joe!

      • Yo Josh putting different batteries in parallel is usually ok for charging and supplying loads since the current will inherently share between them. The problem is leaving them permanently connected where slight differences in charge retention could cause one battery to discharge the other. This isn’t a problem when the batteries are isolated by a relay or something once the voltage drops (as is the case when using a VSR between batteries). In your case you will have them permanently connected in parallel. That is fine and is done all the time with battery banks in industry but usually all the batteries in the bank are same type and age and will suffer the same charge and discharge cycles throughout their life. If both batteries are healthy and similar voltage specification then you should be fine but if one starts failing further down the track it could cause problems. I think we’re splitting hairs though. I’d do it.

        South West USA ay. Me and my wife travelled through there in a van. We camped at Mojave nature preserve and checked out the Joshua Trees and did some hiking. Also visited a few of the national parks around there. Had a ball. I love desert areas. The isolation, untouched natural beauty and sunshine are great. It’s amazingly relaxing and I feel really good when I in these areas. Even when I drink a million beers and breathe in smoke all night I wake up in the morning feeling a million dollars. I love banging around the aussie outback which is similar to south west USA in a lot of ways. What work do you do that lets you camp out in the desert? Got any jobs going?

  15. Hello Joe,

    Excellent write up and very comprehensive. I know you have written alot but the value in this reply section has been as informative as the blog itself so I hope you are open to answering my questions.

    Two things that interested me in this blog were the relay bypass of the dc dc charger. This would be a great option for my system but would it mean another full length run of cable? (Battery and charger are in the rear of the vehicle) My other question is how do you “solder all connections and not just crimp”. Does this mean solder after crimping or another system altogether…I am prepared to do this but don’t know what it entails. I have heeded all your points on the dc dc charging system but I still feel this is the best compromise for me and my needs.

    Any help would be greatly appreciated and Thankyou again for an informative blog.

    • Hi JHood for the bypass relay you’d tap off the DC/DC charger if the terminals are big enough. You’d just have short links from the input and output of the DC/DC charger to the relay so you wouldn’t need to redo the entire cable run. If the terminals on the DC/DC charger aren’t big enough you’d chuck a couple of terminal blocks either side and use those.

      I’m not an auto-electrician so may not be the best man for advice on soldering. Yes it means solder after crimping. The way I do it is crimp the terminal onto the cable then heat the terminal up and feed some solder inside the terminal. You need a good source of heat to get the inside of the terminal hot. It’s not like using a soldering iron to solder something to the outside surface. You want the solder to melt and get drawn inside the terminal. A gas blow torch does the trick.

      Good luck, thanks for looking. May your beers stay cold.

  16. Thanks again Joe. I’ll keep an eye on the batteries to make sure they continue to perform the same. I spend 5-6 months a year doing wildlife biology field work. I feel very lucky to have so many similar mornings waking up feeling awesome out there (even when I’ve enjoyed too many fine brews). The state of California has passed legislation to meet renewable energy standards (thanks Arnold Schwarzeneger) in coming years. This has resulted in large scale solar and wind projects ramping up and conflicting with federally protected flora and fauna. That’s where folks like me come into play. We inventory project areas for sensitive species before and during construction efforts. Between the renewable energy boom, all the transmission corridors associated with those projects, and various military reservations looking to expand their boundaries, the mojave desert continues to feel the pressure. It can be very rewarding and also bittersweet to have such a deep connection to a shrinking eco-system. I appreciate your expert advise.

  17. Two things: A proper DC to DC charger-regulator does not just switch over to alternator voltage on your Aux battery. It monitors battery levels and charges accordingly. Also, when a load comes online (say the fridge), although this drops the line voltage, the charger only lifts the voltage to the specified level, nothing more nothing less. You implied that the charger wouldn’t know and would overcharge. This is not the case.
    C-Tek have invested huge amounts to produce excellent products, that don’t have any moving parts. They are very reliable and I notice you have a 240V version for caravan parks. As you say they are multistage “clever” electronics to get the best out of our batteries. Maybe check out the D250S DUAL by C-Tek. Relatively inexpensive and well engineered, this product has inputs for solar, crank battery, Aux battery, all separate. It will never overcharge either battery, always stop when charge is full, and will most likely extend the life of both batteries, due to it’s ability to multistage the crank battery with sunshine when the engine is off !!! Something that your alternator can’t do. Beat that ! Pete.

    • Hi Pete. You haven’t explained anything here, just generic marketing stuff. DC-DC converter is perfect in all scenarios. Can you explain actual voltage levels and how the regulator monitors battery levels and how an overcharge condition would be avoided when other loads are connected?

      Yes DC-DC charger monitors battery level via charge current and charges accordingly. When a battery is fully charged it drops to float voltage which is the same as your alternator voltage. This prevents overcharge. If you have some other load then the DC-DC converter will not drop to float. It will maintain the battery at the elevated voltage and cause overcharge.

      If you have a solar charging system then you get the advantages you describe through the solar regulator. If you do that in parallel with a simple voltage sensing relay then you get the best of both worlds.

  18. Very interesting post. Can I cut to the chase here and ask a simple question. What is the sensible max inverter wattage one could use for say 15 min. continuous. Given a sensible capacity alternator and one or two DS batteries even the AGM ones. No solar. I had an idea to use one of those smaller electric grill oven things if pos. Do have gas but gas versions of these ovens don’t seen too efficient. Just seeking some thing between Baby Q BBQ and stove.
    With engine idling ? yes/no?

    I once had an air pump with an air con clutch. Could one not have a second alternator driven by such a clutch?

    • Note to moderator, ( couldn’t see how to edit) Please add,
      Is a portable generator of suitable wattage a better option. In the interest of simplicity?

    • Yo Otzi how are ya dude? A typical system has the capacity so run a pretty big inverter. Volt drop and cable sizes become an issue however the current supply capability of a combined alternator – lead acid battery system is pretty high. For example alternators are usually rated at around 100A. Let’s assume we leave the engine running and get 80A. Then if you want to utilise half of 100Ah battery’s capacity in 15 minutes that means a current draw of 200A from the battery. So total current from battery + alternator is 280A. This equates to 280 x 12 x 0.9 = 3024W. Derate it a bit for volt drop and heat accumulation and safety margin and inaccuracy of the calcs and your typical 2400W max rating of Australian appliances is feasible provided the appliance does not have high startup current. Heating elements for cooking usually do not have a high startup current. But now you can’t keep your fridge running without excessive deep discharging (it’s better to no deplete a lead acid battery past 50%) so you need to leave the engine running to recharge the battery, probably for a few hours. So it’s annoying. That’s why cooking with electricty whilst out in the bush is usually not done.

      Two alternators ay. I’ve never seen it done. I’m not sure how the alternator regulators would respond to being in parallel. If the regulated voltages were slightly different or the output ripple was out of phase there could be problems. They’d have to be closely matched.

  19. Thanks for the reply Outback Joe. In my 4WD days the usual camp kit was carried, simple as. But now, my more geriatric years driving a commercial van (NOT camper van!) the travel approach is of more gentile comfort, and of some what longer duration. Weber Q baby on the back door and those Chinese market cooker things with a disposable canisters, different/odd I will grant you but they work. The little oven idea is/was a bit left field but if it worked, figured it would be handy. Any way we’ll get there.

  20. Hi Joe , Great info on all articles in your site..well done. Quick Q: Wife and I like to travel usually in the troop carrier only without the camper, We are replacing our start battery and our deep cycle with two AGM dual purpose , I run a Redarc isolator between the two. O

    Our 110 Waeco drawers a bit and often we listen to music at night, if we park up for two or three days my second gets depleted quickly. I don’t like the idea of carrying solar BUT could I carry a third battery in the rear which would be in parallel, hopefully this would suffice over several days.

    Do I need a DC/DC ?
    Is this setup OK?


    • Yo Paul yeah just chuck another battery in parallel both fed from the same isolator. Direct through isolator to alternator is better – two batteries in parallel may sink 60A when charging which the alternator can supply easily plus extra to keep your fridge running. DC/DC converter will be too slow, total output only 25A or whatever the rating is minus whatever your fridge is using. You could put in a DC/DC to speed up the final top up but I’d only do it with a way to bypass and go direct to alternator.

      • Thx Joe

        Can you explain direct through isolator to alternator is better? I believe my vsr is between the two batteries at the moment

        Thx Paul

      • Hey Paul yeah your vsr is in the right spot. What I mean is when the vsr is closed it’s just like having the aux batteries connected directly to the alternator which will yield maximum charge rate.

  21. Dual Voltage Sensing Relays
    I’d like to buy 2 Dual Voltage Sensing Relays with setpoint levels both high & low.
    Could you tell me which relay to buy and where to buy them?

    • Hey Marc do you mean adjustable setpoint voltages? Quick search of “adjustable voltage sensing relay” yields a few hits but I have no experience with them. I think redarc will customize the switching voltage for you when you buy it but can’t be easily adjusted afterwards.

      • Hi Joe

        Thank for your efforts and information

        I have made my battery box with 80mp battery, I am charging it with solar charger controllers ..sometime I need to charge the battery while driving at night…will it be ok to plug the input of solar charger controller ( DC 12V) to the cigarette lighter or directly to the starter battery.

        If this is not OK … is there any devise to charge the battery through cigarettes lighter 12V

        Thank you

      • Yo Suliman interesting question. I’m not sure how the solar regulator will behave since the voltage of the alternator is quite a bit below the voltage of a solar panel. Plus it may make something go screwy with the solar regulator’s algorithm for tracking peak power. It might work. You could just wire the cigarette lighter to the output of the solar regulator so that it is in parallel with the solar regulator directly to the aux battery. Schematically it’s the same as what I illustrate in the drawings.

  22. Thanks for the article Joe. Some questions though. For an auxiliary battery installed in the back of a wagon, the battery should have a fuse or circuit breaker right, but what size is required for cranking the engine through it, in my case a 2.8L turbo diesel. Also, I am thinking of using a 50 Amp Anderson plug as the connection to the battery, so that I can easily remove it. Given that this is rated less than the VSR, is that a potential problem?

    • Heya Daniel I have a 100A in line fuse in mine and it’s worked cranking off the auxiliary when my starter battery failed. I’m not sure exactly what current the starter motor draws. It may be a bit more than 100A but as long as you’re not cranking for a long time 100A fuse is ok. Fuses do not trip instantaneously but rather according to a trip time curve that varies according by the amount by which the fuse’s rating is exceeded.

      Your fuse size should be less than whatever the lowest rating item in the circuit is. So if you’re using a 50A plug and your cables, VSR and terminals are rated at more then 50A then the fuse should be less than or equal to 50A. In my system I have VSRs rated for big current and cables rated for big current and no plugs or connectors. Also if you plan on running anything rated at more than 50A such as power hungry appliances on an inverter then your plug won’t suffice.

      Good luck with your setup. Happy camping.

  23. Hi Joe, let me start by saying I know in my head what I want to do but have no idea how to do it so hoping you can help,
    I want to set up my camper trailer so I can power everything via generator or solar during day whilst also charging batteries so at night we can run off the batteries.
    I also have an Anderson plug on my car so could charge batteries whilst driving?
    I have no idea how to go about this, would really appreciate any advice

    • Yo Brian your question is too general for me to answer adequately, apart from generally describing dual battery systems, which has been done already in the article. There are too many variables. Have another read of the article and figure out what you can then let me know if you have some specific questions.

  24. Joe, excellent article – very helpful, clear and concise. One comment is that I’ve read an article by some crazy guy on the North coast of NSW who argues that running a solar panel into your starting battery all the time gives significant fuel savings as it reduces load on the alternator, and thus the engine. have you come across this argument?

    • Yo Roger nah don’t waste your time dude. In round numbers, it takes roughly 20kW to move a decent car at highway speed. If you have light electrical loads then you might be using say 5 amps or about 60W running the engine electrics and some low power loads. So your fuel saving will be 60 / 20000 = 0.3%. If you have the fridge running and the air con fan on and stereo on you might be using 15A or 180W. If you have enough solar panel to offset that then you will save 180 / 20000 = 0.9%. Lighting loads are disregarded since the solar panel only charges during the day. If you then average this across less than ideal sunlight conditions and panel angle and driving at night you’ll be saving bugger all. A lot less than the wind resistance from the solar panel.

      If you were a massive tight ass you could run no electrical loads at all and remove the alternator completely and have a small solar panel on the dashboard to keep the battery charged. Removing the alternator completely is good because it removes all the mechanical friction of the belt and alternator, plus the weight. Plus the electrical loads. This might combine to save you a couple of % fuel but you wouldn’t be able to use anything electrical or drive at night.

  25. Hi Joe! I have come across your blog while Googling for info about dual battery systems. Extremely interesting and hopefully useful very soon. There is a wealth of information here that I am still to digest all. My wife is getting long service leave next year and we are planning to go touring around oz for six months from May. Unfortunately, we are complete newbies when it comes to touring 4WD, having never owned one. Most of our time spent in the bush is usually on foot carrying everything we need on our backs. I noticed that you are in Perth, so are we. I would love to have a chat and pick your brain ( which seems to work very much like mine: once an engineer, always an engineer 😉

      • Enjoyed the rant about the Thermomix direct marketing, it pushes my buttons too. I still have to read the bit about the device itself. Yes Joe, a quick email to touch base would be great. Just started dabbing into the alphabet soup! It kinda goes like this: ARB, TJM, OL, MW, SBR, MJ and that’s just the first pass!

  26. Hey outbackjoe just checking your blog and your last block diagram shows the inverter connected to ac and your battery charger connected to an outlet, could be bad for your health.

  27. Hey joe, I wrongly assumed the ac load was actually a supply input eg caravan park, which would in turn recharge your batterys via a battery charger. What really confused me was the power point schematic, I thought this was an output when in fact it was an input. And they say a picture tells a thousand words. Not in my case. Cheers

  28. Great article, clear and factual. I love it.
    As every system seems to be a little bit different, I have some questions about what I would like to do with mine (camper trailer with aux battery in the camper dedicated to a fridge).
    I would like to install the VSR in the camper to reduce the change in the car to running cables (later completed with a MPPT solar controller). is this not recommended?
    I usually see 100A fuses inline for 6 b&s cables, but as I read previously, this means that I have to use anderson plugs that are > 100A and not the standard SB50? Or should I fuse the cable with 50A fuses, assuming that the aux battery will never draw more than 50A?
    And last ;), is it possible to use a circuit breaker to bypass the VSR (manual override if I want to use the aux battery to kick start the car)? If not, what other type of heavy duty switch should I be using?

    Thanks to any generous person that may be around to answer!

    • Hey Arthur you can chuck the VSR anywhere. The risk with longer cable runs to the sensing side is that it may cause the VSR to chatter at high currents if the voltage drop is too much. High current on longer cables can cause voltage to drop below the sensing voltage, VSR opens, current flow stops, voltage rises, VSR closes, process repeats. I have a problem with mine at around 100A where I need to force it to stay closed using a jumper.

      Yes you should size the fuse to the weakest link. 50A fuse is good as long as you don’t have a large bank of batteries or large inverter loads or need to crank your car through it. If you want to crank your car you need something bigger.

      Yep circuit breaker should be fine to bypass it. Just rate it sufficiently high. Mine has a jumper that I can plug in to force it to stay closed which achieves a similar result. Some VSRs have a button or something to force them to stay closed.

      Happy camping and may your beers stay cold.

      • Thanks Joe for these precious advices that I will definately use.
        Very good point for the 50A not to be enough to crank the engine, I forgot that bit… I will use bigger anderson plugs and 100A fuses!
        I’m already dreaming of the cold beer in the outback. It will be a first, then sounds unreal right now 😀

  29. Hi Joe.
    Was wondering if there was an easy (or any) way to switch the source of the ACC power? Ie I would like to have my stock acc circuits running without the ign on AND somehow have it on the AUX battery side without re running all their wiring.
    Cheers, Dave

    • Hey Dave, have a read of the “manual switch” section of the article and check out the second diagram in that section. You need to put a switch or relay in that location. You could have a relay energised only when the key is in the “on” position. This will isolate the starter battery unless you are running or starting. You need to find a feed from the “on” position of the ignition and use that to energise the relay. Note the switch has to endure cranking current and is a potential point of failure that will prevent you from being able to start the engine.

      If you don’t want to leave your keys in to run accessories you need to re-wire the power supply of those accessories to be constant.

      • I must admit it was my annoying ign key “chime” that prompted me on this subject, I know there’s ways to kill it, but now it’s got me thinking of ways to run acc on both sides.
        Is there a main acc “out” I could tap a switching feed from the aux battery so I could switch all acc feeds to hot without the key? Or is it more complicated than that?
        Failing that, sounds like the switch position you mention is the only way to isolate the cranking battery and still have the acc. circuits live ?
        Thanks again for a great article…and making me think!

      • Yeah might be easier to kill the chime, was pretty easy to do on the hilux. Yes you could bypass the ignition and feed the entire acc circuit with a separate supply. I’ve never done it before so not sure if there’d be any problems with doing so or if it would be complicated because of the immobiliser in modern vehicles. You could just run a separate supply from your second battery to the accessory you want to run. For me my acc runs off the starter battery as standard and I need to leave the keys in. This could be a problem for example if we decide we’ve had enough fishing and we coop up in the car and watch movies all day. But my arrangement means the starter battery is always charged by the solar panels and I over did it on the solar to make sure there was always excess for the starter battery even in poor weather.

  30. One of the most plain speaking (that is good thing) articles I have read. Thanks outbackjoe I would imagine you put alot of time and effort into creating this article.

  31. Hi Joe, great article but it gets a bit confusing when you read everything. What I have is a trailer that has 2 x deep cell batteries mounted on it to drive a motor that draws about 8-9 amps. The motor will run for about 60 min to complete its work. Can I connect the 12VDC line from my trailer plug ( Axillary pin ) directly to the batteries in parallel to charge them when the vehicle is running. The trailer motor will only be operating when the vehicle is in motion so it will be continuously charging. Will this work.


    • Hey Doug yeah that will work but I don’t know what the rated current capacity of the plug and cable is in the trailer supply. The problems you could experience are overloading plug / cable and accidentally discharging your starter battery when the engine isn’t running if you leave the trailer connected with a load.

  32. Hi Joe,

    I’ve got a duel battery system with a redarc controller in my ute to run a 50L Waeco fridge/freezer and some lights. It works well and dosen’t run out of charge as when I’m camping I drive most days (beach driving/4wd ect).

    For Christmas I was given a 20 watt smart solar system. Would this be useful for me to use or could it cause dramas to my system?


  33. Joe,

    I have a newer Tacoma with a 400 watt (100 watts driving 400 watts at idle) 120v outlet in the bed. I want to run 2 AGM deep cycle batteries in parallel (12v output) in the bed using the inverter outlet for a smart charger (recommendations on charger) to charge to bed batteries. They will be completely isolated from the vehicle electrical system. Is this feasible?

    Thank you for your time!!!

    • Hey Ty yeah you can do that. Some of the AC chargers, since they’re not usually permantenly connected, sink a bit of current when switched off. Which means it will slowly deplete your batteries. I noticed this with my C-Tek AC charger. Apart from that C-Tek is pretty good. I dunno about other brands. Also, with 2 AGM batteries, depending on their size, might take a pretty long time to charge with an electronic charger.

  34. Hey Folks,
    Apologies for a out there question: I want to install some electrics into my camper trailer (as usual on a budget), and want to be able to charge the battery both from the car’s own electrical system, and also from the mains power at home. My understanding is that to do both of these I would need a DC-DC charger, and also a mains battery charger.

    My questions is this: Can I instead just buy the mains battery charger (multi stage charger) and also buy a decent 12v-240v inverter and use those instead. When I am at home or in a caravan park I use mains, while I am on the road I run the mains charger from the inverter which is being powered by the car to charge the second battery in the trailer.

    Hoping you can point me in the right direction!

    • Yo Tom O yeah you can do that. Check out a few comments up, Gary Tuttle asked the same question. The inverter / electronic charger combo is more complicated, less safe and less efficient then either a DC-DC charger or simple voltage sensing relay. But if it means less equipment or something it may be an option.

  35. Hi outbackjoe

    Great web page mate – This has probably been done to death but to help me out – I have a 80ah start battery in my Landcruiser 76 – I have a Anderson plug at the rear of my vehicle to future proof my self if I was to buy a camper – I wish to splice into this circuit and put a Anderson plug in the inside rear of my car to charge a AGM 120 ahm battery to run a 12V fridge and a few Leds when I go to the cape – the battery box will be in and out of the car and will be charged via solar on extended stop overs in one place but I wish to give it a fast charge using the alternator while moving between camp sites. I have a Anderson plug built into my battery box to effect this.

    Is this ok – do I need to install anything between the 2 batteries

    Thanks for your time


    • Yo Wax. Your question “do I need to install anything between the 2 batteries” is tantamount to asking “how do you do dual battery systems”. I wrote a whole article on that! Check out the sections “The Simplest System – Direct Connection of a Second Battery” onwards and consider the pros and cons of each arrangement. If you are manually disconnecting every time you stop then provided your loads are downstream of that connection point you don’t need anything except some fuses, at the risk of depleting your starter battery if you forget to disconnect. It’s all described in the article.

  36. this is Robert from USA . i am using 120 v ac invert-er . will that directly replace your 240v ac w/no other changes

  37. Hi Joe, great article, very informative.
    A question for you…. have heard tell from supposed experts that we shouldn’t run 12v fridge freezer from load terminals on a 160w solar panel with 20A mppt regulator and 150ah (two 75ah connected in series) battery. And That it should be run straight off battery. You thoughts? and why would a mppt regulator have these terminals if they weren’t to be used.??? Is it because of voltage drop in two sets of cables….. one for the panel to battery and another for fridge to load terminals. What if cable run is over 5metres…to get to the “sun” because of camping under such lovely large shady trees, and it is the tropics. Your thoughts would be appreciated.
    Kind regards

    • Hi Gumtrees

      Your regulator should be located near the batteries. This maximises the length on the high voltage side (the side to the solar panels) which overall minimises voltage drop. Then the cable length argument doesn’t come into play since it will be a similar distance to the fridge from either the battery or the regulator. Anyway I think it would be easy to have a big enough cable to handle the few amps a fridge draws with minimal voltage drop even at several meters if you’re regulator is at the panel but it is a bit annoying to have to run two cables all that way.

      If you don’t use the load terminals the regulator will keep the battery at elevated voltage whenever the fridge is running which will overcharge your battery and wear it out. Seek an explanation from your experts on why they would not use the load terminals, let me know if you find anything interesting.


  38. To use a 20m cable (more choice of position) from solar collector to controller then battery, would a relay (say at the 10m halfway point) assist in minimising drop in voltage ?

    • Hey David nah a relay wont help. Just maximise the cable run at the higher voltage (direct from solar panel) and minimise the cable run at the lower voltage (after regulator).

  39. Thank you very much, it’s so bloody confusing for me! Standard cables are “6mm”, I am getting “10mm” (presume external); what would be the % relative drops in voltage from 12V solar panel to regulator at: 10m, 15m, 20m & 25m? My camping observations; often difficult to find maximum sun close to semi shaded van (regulator site).

    • at 25m it’s 50m return, resistivity of 10mm cable is 1.84 ohms / km which yields a resistance of 0.092 ohms. Assuming 17V at the solar panel, at 300W this translates to a volt drop of 1.62V. Power loss is 28W. Scale this up or down according to distance (for example half the cable distance means half the volt drop and half the power loss). If you use 35mm cable it would be 0.46V volt drop and about 8W power loss. Note cable sizes are cross sectional area of conductor rather than some external diameter or something.

  40. Hi Joe

    I’m planning a system like the one described in your final diagram, but that can also run from a 240v hookup. The regular loads would be lighting, fridge and maybe something else. In your system, these loads would run through the solar charge controller and to the battery, which would be being charged by the battery charger. As I understand it, voltage drop caused by the load could risk overcharging of the battery by confusing the charger. Plus it seems a bit convoluted. Is there a more direct way of using the 240 to power the 12v loads that bypasses the battery and maybe even the solar charge controller? Can chargers act as a rectifier (or whatever is the opposite of inverter)?

    Secondly a basic wiring question, what’s the principles for where to locate fuses or circuit breakers? On the positive side of cable runs? Every cable run, or is that excessive?


    • Hi Ralph

      I don’t think there is any easy way to protect automatically the batteries from overcharge from a smart charger. Really you need a charger designed with dedicated load terminals. Everything on the market that I have seen is basically a dedicated battery charger and not really made for long term connection to batteries and loads. I’d be interested to see if anyone knows of a suitable device. One way you could do it is get a charger that can output a fixed float level voltage independent of load (fixed to say 13.8V, or even 13.5V to be safer if you are connecting for very long periods of time). This would keep your batteries topped up and supply loads without overcharging batteries. Another way is when you think your battery is fully charged (been on charge for a day or so) you could isolate your battery with a manually activated relay or something.

      My C-Tek AC charger has a “supply” mode which is a fixed voltage output but the voltage is too high, over 14V I think. Otherwise the supply mode would be ideal.

      Not sure what you mean by charger act as a rectifier, can you explain? There will be a rectifier within any AC to DC conversion.

      Every bit of positive cable, if you trace it back to the battery, should have a fuse somewhere. You can either protect each cable run individually with a fuse or common up the cables and have a common fuse upstream. Put fuses as close as you can to the battery to minimise unprotected portion of the cable. It’s all explained in the article under the “Electrical Protection” section of the article.


    • Yeah some chargers have a “supply” mode where they behave as a fixed voltage supply. I don’t know about the one in your link, something like this would be better:

      According to the info on the web, in supply mode the output is fixed to 13.6V which is a pretty good level you can leave at to run your loads and keep your batteries safely topped up. It does not bypass the battery though. Apart from some solar regulators I have not found any chargers with dedicated load terminals to differentiate between load and battery. I have the older version of that charger and the supply voltage is too high.

      No worries dude glad it’s helpful.

  41. Hi Joe

    Thanks for all your hard work, very informative and much appreciated. You obviously know your stuff, no marketing bias, great stuff!

    My set up is a Troupy with 327 watt Sunpower e20 series solar panel 65v open voltage to be fitted to pop up roof. Then will fit a Victronics blue 100/30 MPPT solar controller to either a 260 amp AGM Lifeline or Fullriver auxillary (or possiblly a 200 amp, maybe 300 amp Lithium). The latter batteries are the way to go but rediculously expensive. And those sold in Aus don’t have a track record as yet. The Fullriver is the least expensive option and should still work well, for how long who knows.

    I will have two VSRs or dual and a 3 step 240v charger not huge in amps. Will run two 40 litre compressor fridge/freezers, Engel as fridge and Eutectics as freezer hopefully from the solar controller and not the auxillary if that is possible from the Victronics solar controller. The high voltage of the Sunpower solar panel makes it difficult to find a suitable solar controller other than the Victronics 100/30. I will run Laptop, maybe very small TV, mobile phone, sat phone, GPS. Envisage lots of remote camping very occasionally caravan park overnight stop only.

    Should I upgrade the standard Toyota alternator for higher amps and at the same time get a big 4 wiring upgrade from alternator to starter battery,chassis, engine,VSRs or is this a waste of money? What about size of cables from starter battery /dual VSR to Auxillary battery – install much larger cales? Should two VSRs or dual be solid state?

    I haven’t yet grasped how the alternator’s charge current (via the dual VSR) and the solar panel/solar controllers charge current work as a unit when they are both connected to the auxillary battery at the same time. When driving in full sunshine, which one charges the auxillary first? They can’t both charge at the same time ? How does the connection of these two charge devices to the auxillary battery work to properly charge it? The alternator is not a 3 step device, the solar charger is? How does one device read the other and know when to cut in and out? A dc dc charger is 3 step device and its just install and connect, no big 4 wiring upgrade or alternator amp upgrade needed.There are 30 amp dc dcs out there which is better than the amps from a solar panel. How many more amps are you likely to get from a standard alternator? Same question arises as to how a dc dc charge device and a solar panel/controller charge device work together to fully charge the auxillary battery?

    I’m not plugging dc dc charge devices, everyone in a RV already has a charge device, the alternator and its amps can be increased, then all it needs is a 3 step voltage regulator to ensure the starter battery and an auxillary battery is FULLY CHARGED CORRECTLY. When reading the marketing about products they give the impression is all you need to do is buy their product and all will be good. But there are so many stories of batteries not being charged correctly.

    Your thoughts will be greatly appreciated



    • Hi Rod

      Most Toyota alternators are rated at around 100 amps. A 260 amp hour AGM is pretty big and might be able to sink slightly more than 100A but more typically around 80A or less. The existing alternator will do the job, you would squeeze a faster charge rate out of a bigger alternator when you also had your high beams on and aircon on and the starter battery needed a bit of a top up. Bigger alternator is an option but not necessary. You should use some fat ass cables between batteries and VSRs. This will minimise voltage drop and maximise charge rate and also affords engine cranking from the aux battery. For such a big battery I’d use the biggest cables you can terminate onto the terminals. Probably around 70mm2.

      When you have two charging systems in parallel they will both charge the batteries simultaneously. The sharing of the work happens automatically. It’s like having batteries in parallel. The system with the higher voltage will source more current. As long as the voltages are similar there should not be a problem. You may encounter a problem if the output voltage from the solar panel is too high for the alternator. On my hilux I get an engine check light when this happens but no damage is done. There is potential for damage and safest option is to cover your panels when driving, but what should happen is as the batteries become fully charged and the voltage rises and current reduces the regulator should automatically back off the voltage to around the same level as the alternator when it enters float mode.

      Not sure what you mean by DC-DC charger is just install and connect. So is a VSR. Upgrading wiring impacts both DC-DC chargers and VSRs. If you have a 30A DC-DC converter it may need 40A on the input. If your cables are too small there will be too much voltage drop and the DC-DC converter will not be able to provide 30A. The DC-DC converter still gets its energy from the alternator. The amount of energy that can be provided can be limited by cable sizes. If cable size is limiting charge rate with a VSR it will also limit charge rate with DC-DC converter. You need fatter cables with a VSR only if you want to utilise all the potential capacity greater and above what a DC-DC converter can supply.

      With an alternator you can charge up to the rated current of the alternator. It’s hard to say what the actual currents will be but we can take a guess. Lets assume you have some fat cables and say your starter battery is charged and you aren’t running much stuff in the cabine and the auxiliary battery can charge at 80A (reasonable guess for a 260Ah AGM) and your fridge and freezer are running and using 5A each (total 10A). The alternator will provide 90A: 80 to the battery and 10 to the fridge and freezer. The 30A rated DC-DC converter will provide 30A: 20 to the battery and 10 to the fridge and freezer. With VSR the battery gets 80A charge. With a DC-DC converter the battery gets 20A charge. So the alternator is charging 4 times faster.


  42. Hi Joe

    Thanks for the info.

    If I find the alternator and solar controller do not work well together/as a team, rather than covering the solar panel while driving could I just fit an off on switch after the solar regulator and tun it to off when the engine/alternator is running, turn it to on when stopped and install a reminder flashing led in the dash in front of the steering wheel, flashing when the solar switch is on?

    Do you think my standard vehicle alternator will eventually fully charge the 260 amp AGM battery (or 200 amp or 300 amp Lithium) if I went for the latter type?. Some people say a vehicle alternator will never fully charge a starter battery let alone a large amp capacity auxillary battery because a vehicles alternator regulator is not a multy step device. Do you know why they are not multi step?

    I had hoped the alternator would provide a fast bulk charge and then the solar voltage controller would take over and properly charge ie top up the auxillary. I’ve read the Victronics 100/30 3 step solar panel controller can have its voltages re programmed to suit the chosen batteries needs, I hope this means with some reprogramming of the solar controller I can get the alternator and solar controller to work as a team ie the alternator to provide a fast bulk charge and then have the solar regulator take over and finish the job . Your thoughts?

    I have estimated my draw on the auxillary battery as about 1,000 watts per day worst case. Probably more like 800 watts but would rather over estimate. The 327 watts of solar panel is the most that will fit on the roof of my Troupy so only way get more solar watts is carry a fold up portable which I don’t want to do. You said a 260 amp AGM is a big battery, Have I have over sized the proposed battery? I had originally thought a 210 amp Lifeline battery should do the job but am worried about the contents of my two fridge freezers if camped out somewhere for a week and the sun is not particularly friendly. Maybe a 200 amp Lithium will be a better choice if I can find the dollars, because they can be drawn down 80% versus only 50% for an AGM. Your thoughts?

    My one solar panel is large in watts (327) and in its dimensions. It will be mounted so there is an air gap under it. In mounting it I plan to fit a wide cross shaped tmber support underneath in the centre of the panel between the back of the cells and the roof of the vehicle. I may put a strip of rubber on the framework. I hope this support will help the panels better deal with road corrugations. Time will tell. I wonder what other RV people have done?

    If I use 70mm2 will I be able to fit them to two VSRs or a dual? Any idea of a suitable VSR of dual for this size cable? I want to be able to start the vehicle from the auxillary battery in an emergency so would like to fit big ass cables to auxillary. I assume in line fuses can be bought for that size cable. Do I need to fit a fuse at each end of the positive cable, one close to the starter battery and another close to auxillary battery?



    • Hey Rod

      Whilst you’re driving you are not cycling and letting it run off alternator only is fine. Then when you park up you use the panels and it charges as much as possible. This is what I do. My panels are deployed only when parked up. I have had them run in parallel and apart from an occassional engine check light from overvoltage it seems to work fine. The solar regulator backs off it’s voltage to match the alternator once the current is low enough for the voltage to start rising significantly. What problems have you had running in parallel? To help with running in parallel you could put a diode in so that once the output from the solar regulator exceeds the alternator it cannot backfeed to the alternator and the rest of the car’s engine management system. But this would prevent your starter battery from getting any charge from the solar panels.

      Yes a standard alternator will charge anything to 100% but it will take a long time for the final few %.

      Big battery buys you time when there aint much sun. It’s a trade off. There is no too big or too small. I prefer to oversize the solar panels so that they can cope even in overcast conditions. Worst comes to worst just run the engine for a bit. Saves you carting around 100kg of lead for tens of thousands of kilometers. I dunno much about lithium batteries apart from them being expensive. I would be worried about their resistance to constant vibration. Also they do not tolerate overcharge very well.

      I think my cable is 70mm2, I have redarc isolator and you can see a picture of it with the cables terminated here It is capable of cranking my starter. It is protected by inline fuse.

      I have never seen such a large panel used in mobile applications so I can’t comment much on how it will go. I don’t get involved much with big motor homes and stuff.

      Yes fit fuse at each end of cable as close to battery as possible.

      Good luck with your build up.


  43. Thanks Joe.

    I have been fact finding before building my solar charge, alternator charge or dcdc charge system for a large capacity auxillary battery to operate two fridge/freezers, invertor, laptop, small TV, sat phone mobile phone, GPS, coffee grinder etc. About 800 to 1,000 watts of draw each day. Lots and lots of remote camping stopped for days at a time.

    Have virtually decided to build my own 4 x 3.2 volt lithium cell battery and battery management system, 180/200 amp or maybe 300/360amp.

    Have been informed Lithiums should not be charged at more than 14.2 volts as one cell can in time end up being over charged and damaged. So have to be careful about the amount of volts from the alternator when charging while engine is running and maybe fat cables to the lithium auxillary will not be necessary. I will need to find out what volts I get from the alternator at the lithium which will be housed in the back of my Troupy.

    There are lithium battery management devices available on the net and some are inexpensive such as mini-bms a cell balancing device. Lithiums also need under an over voltage device which apparently can be simply controlled with an over voltage fuse or circuit breaker. They also need an under voltage device which can just be an an alarm in a dash mounted battery management viewer.

    I am going to source quality brand lithium cells such as CALB, Winston or Sinopoly and have a go at building my own battery and battery management system. Sourcing quality cells at the right price is the biggest issue, not the battery management devices

    If there are other readers interested in Lithiums , those who have already built or are planning to build their own who are interested in sharing knowledge and experience, my email is



  44. Hi Joe, great write-up mate. I am about to start installing a dual battery system for my new cruiser. I have already purchased a redarc bcdc1225-lv (in a bit of a rush so not sure if it is suitable for my desired deign). I intend to have a system similar to the diagram you provided above under “the complete installation” heading. in addition to that I would like the option to charge the starting battery from a solar panel and 240v charger. I would also like the option to bridge the two batteries for high load occasions (e.g. winching or inflating all my mates tyres…) and to take advantage of the high amp charging ability of the alternator. I have a general understanding of 12v electronics but still find it quite confusing. I was hoping you could shed some light on what my options might be and if the redarc bcdc1225-lv is needed or not.

    Thanks in advance,


    • Hey Ross you can use the DC-DC converter and chuck in a bypass relay as I describe in the article. Then you can manually join the batteries up when you are winching or when you want a quick top up from the alternator. If you use the DC-DC charger as your solar regulator it may not be possible to charge your starter battery from the solar. Maybe the DC-DC charger can do it, check with redarc. Otherwise buy a separate solar regulator and use the bypass relay when you want the solar to top up your starter battery.

  45. Hey joe, I’m in the process of finishing up my setup on my 2012 Nissan D40 Navara, my question is what battery’s am I best of running? I will be using a voltage sensing isolator, and I will be running a waeco cf-80 fridge a radio and some led lights at night, now I was told that deep cycle battery’s are no good in a setup like this because the alternator rapid charges and the deep cycle battery’s need a trickle charge like a dc-dc setup, which I really don’t want to buy as I already have the other system. Are the AGM deep cycle battery’s better for this isolator setup or am I better off going with a couple of big starter type battery’s? Any input you could give me would be fantastic.
    James Burns

    • Yo James AGM is best coz they will accept a higher charge rate. But any battery will do, a standard deep cycle will not be forced to accept an unsafe charge rate. It’s higher internal resistance will naturally limit the current.

  46. Thanks Joe,

    You have explained things so very well and have talked me out of a DC-DC charger. Happy travels!

  47. Hi Joe, I have just read your whole blog, the most relevant down to earth info I have been able to find. Thanks for sharing your knowledge and time. Ralph & Karen.

  48. Hi Joe,
    Thanks for your excellent blog. I have an Anderson plug wired to the back of my vehicle with a VSR to charge the batteries in my camper. I want to add an auxilary battery in the back of the vehicle on the tray. Do I need to run it through a sepparate VSR?

    • Hey Chris just get a battery that matches what you got in camper. Put them all in parallel with the single vsr. It will behave like one big battery. No need for anything else.

  49. Hi Joe,
    Thanks for that. It makes it big, 340 Ah consisting of 7 batteries! This means the voltages should equalise after I wire them together? With this much current running around the vehicle I guess I’d better fit some fuses.

    • You don’t need to duplicate what you have in the camper. You can have just one battery in the car. Just use the same make / type / capacity. That way they all charge and discharge together better. You should protect all positive cables with a fuse regardless of how many batteries you have.

  50. No worries Joe. The camper came with 6 batteries and I am adding one auxiliary in the vehicle. They’re all AGM, but the new auxiliary isn’t the same make or capacity as the ones in the camper which are only 35 Ah each. I often parallel charge and use different capacity LiPo batteries of the same voltage for RC applications with no problems. You wouldn’t recommend this for my vehicle set up?

    • Yeah should be ok if all agm. Maybe check datasheets to ensure voltages are the same. If they are same type of batteries with same voltage specification then no problems chucking them all in parallel together. Even if not the same, all that happens is some batteries may not receive a full charge and some batteries will discharge more deeply. So you can always put them all in parallel but best to match them as much as possible.

    • I need advice: I got a 400w 240v wind turbine and a 300w 12v solar collector to charge a 150ah battery. The dual controller provided (from China) to charge the battery has a problem. The wind turbine capacity is okay however the hand book specifications for the dual controller state that it is only rated at 12.5amps from the solar collector, not the 25amps required. My attempts to get the China company to supply an appropriate dual controller seems to have fallen on deaf ears. What can I do to utilize the controller I have for wind but make appropriate purchase to include my solar? Does anyone know of a commercial product/business that I can contact. I live in Darwin NT Cheers, David

      • Hey David. Check the voltage of the panel and the voltage range that the regulator can handle. The panel will operate at a voltage greater than 12V. You might find the panel achieves peak power at 30V which is only 10A at 300W. Otherwise search ebay for “wind solar regulator” and find something suitable for the voltage and power ratings of your turbine and panel.

  51. Hi Joe. Extremely interesting stuff, particularly regarding the DC DC charger and the myths about them! As a result I am looking at installing a PROJECTA DBC150TK 12 VOLT 150AMP DUAL BATTERY SYSTEM KIT SMART ELECTRONIC ISOLATOR to my 2010 Mitsubishi Challenger to charge the 100aH AGM battery that I have in my camper trailer. Two questions though if I may. Firstly, do you think that this is suitable, and secondly, if so, which location would be preferable to situate the isolator, in the Challenger or in the battery box on the Camper?

    • Hey Mick yeah I think that isolator will do the job. You can put the isolator wherever you want. At high current levels it’s better located close to the alternator / start battery so that it doesn’t drop out due to volt drop. This probably wont matter if just charging your battery but if there’s anything that needs a heap of power in your camper then best to have the isolator close to the alternator with a short cable run.

  52. I have a dual controller for 400w, 240v wind turbine and 300w, 12v solar collector to charge a 150ah deep cycle gel battery. Unfortunately the input for the solar cells can only handle 12.5amps, not the 25amps required.
    Can I just use my controller for the wind turbine, but instead of the outlet going to the battery, can it go to a dual DC controller (so I can the solar input) then have the combined output going to my battery?
    It seems that it’s cheaper to buy a dual DC controller than to upgrade my dual AC and DC controller.
    Desperately seeking informed advice
    Cheers, David

  53. Wow, really liked that information. You seems pretty well knowledgeable on DC systems and charging. I have a question here for you, I’m looking for a similar application, but a bit different. I’d like to hear your recommendations on this.

    We are camping in the woods, with a Tent Trailer (12V battery bank), and I have an Electric Golf Cart (48V Battery Bank) over there, for traveling. I plan to setup 2 x 250W solar panel on top of my Tent Trailer, which would be the maximum space available. I’m only there on weekends, so I’d like to charge back both battery bank during the week, while I’m not there.

    So, first, it should always charge the 12V battery bank in priority, and then jump on the 48V bank. During the week-end, the 12V battery bank will always be connected to the Solar Panel systems, but not the Golf Cart. So the Golf Cart could be connected or disconnected multiple times during the week-end. When the charging of 12V battery bank is almost done, and there is available power output from the solar panels, I’d like to have that unused power to charge the 48V battery bank. Is this possible, and how?


    • Hi Jonathan. You can do it using a voltage sensing relay, two solar regulators and two standard relays. One 12V solar regulator and one 48V solar regulator. The voltage sensing relay has its sensing terminal directly connected to the 12V battery. Use the voltage sensing relay to control two relays that feed the solar regulators. When the vsr is energized (12V battery is charged) then close the relay that feeds the 48V solar regulator and open the relay that feeds the 12V regulator. The problem with this is it will not allow the 12V battery to be fully charged. If you can specify a VSR activation voltage to be higher than normal (say 14.4V) with a programmable vsr this will give the 12V battery a better charge but still not fully. Best solution would to have charge current monitored and when this reduces below a certain level use it to energize the 48V system. Latch this in and use the VSR to energize the 12V system if the voltage drops. Or find a solar regulator that has an output that indicates it is in float mode, latch this in and unlatch when the vsr opens. You need to make sure the voltage from the solar panels is adequate for the input of the 48V solar regulator.

      Another solution would be to use the VSR to energize the 48V system whilst keeping the 12V system energized at all times. This would allow the 12V battery to be fully charged. Problem is I don’t know how two solar regulators in parallel will behave since they will be each trying to modulate the solar input to maximize their individual charge rate. It could go unstable.

      Another solution would be to supply the 48V system from the output of the 12V solar regulator via a DC step up device and again control feed to the 48V system by a VSR so that the 12V system gets priority.

      This system requires more thought and design effort beyond the scope of a quick reply in the comments of a blog. There might be some off the shelf products that I’m not aware of that could help.

      • First proposed solution makes sense, which is what I’m thinking to do. Even better, I’ve found some MPPT Charger, that supports auto detection of battery voltage 12V/24V/36V/48V. By using a controller like Arduino, voltage and current monitoring sensor, I could probably program a sequence that jumps from 1 bank to the other, by using 3 relays (1 for each battery bank, and 1 feeding the charger), so I can make it reboot, to redetect the right battery voltage. Only thing I’m concerned of, is disconnecting the PV panels from the Charger when running at full current, it will create a surge, and don’t know if the charger can support those surges. Problem with this, when the 12V battery is at the end of charging, it is not using the maximum power of the PV Panels, so the PV panels are not running efficient for a while, and I’d like to be able to use that energy to charge the 48V batteries at the same time.

        Second proposed solution, that’s what I found also, it could be unstable, and won’t work. Midnite The Kid is supposed to be able to connect in parallel, since they communicate together, and only 1 runs the MPPT, but talking to their tech, it’s just marketing for now, it seems it’s not working yet.

        Third solution, is probably the best, in terms of efficiency. I can have both charging at the same time, and if I need priority on the 12V, I can just shut the other one down. I can probably either use a step-up or step-down, as long I can control each battery bank with a relay. After doing some website search, I just can’t put my hand on a good step-up or step-down converter. The best would be a 48V-12V step-down converter, with a fixed ratio, that way, the charger would be always seeing like a 48V battery bank, and do it’s charging sequence properly.

        Thanks for your comments, really appreciate it.

  54. Hi Joe, trying to come up with a simple dual battery system for a 2015 Prado. Every 4×4 shop I’ve walked into just tells me that I need the $1500 Redarc / Optima system, but surely there’s got to be a cheaper option that will do what I want?!? I just can’t seem to find a shop in Brisbane that will give me options – the info in your article has helped explain things but I’m still at a loss.

    I’ve been looking at the Piranha DBE140SF Dual Battery Management System paired with an Exide Stowaway 95AH battery to go under the bonnet. Would this system work in my vehicle (has a variable voltage alternator)? Only running an Engel 45L Fridge and a couple of LED lights at night. Any cons I need to be aware of? Thanks in advance!

    • Hey Adrian that Piranha product is perfect for a simple voltage sensing relay system. Will it work with fancy new alternator? I dunno, I have no direct experience. I speculate that you should be able to arrange it so that the variable voltage system detects the extra load and cranks up the voltage to accommodate, and thus sufficiently charging your second battery. The risk is the variable voltage system does not crank up the voltage and your second battery does not charge properly. Otherwise it would be fine for short trips powering a small fridge, provided the size of the battery is big enough. I’d love some feedback on anyone who has successfully done a simple VSR system on a trendy new alternator. The other option is to set the alternator to operate in fixed voltage mode. This should be possible via a jumper or link or something on the regulator system, you might have to check with Toyota.

      In terms of batteries, I’d get one of the more reputable AGM ebay specials. There’s some brands on ebay that are pretty cheap and have proper websites and datasheets and some ok experiences using them (including myself). I prefer AGM coz they can take high charge current straight off the alternator.

  55. Hi Joe,
    Just after some advice. Planning on a big trip around Australia in my 2005 hzj79r, have currently got a red arc SBI12 solenoid installed (as its the older model it’s not an ECU controlled alternator) I have a yellow top d27f as the cranking battery. Haven’t got a deep cycle battery yet but can only fit a 100-105ah battery under the bonnet, can you recommend a battery? I will be buying an Engel mt80fcp which claims its draw is between 0.5 and 4.2Amps. And also running some led strip lights in the tray. What would be the best setup, solar? Generator? Combination? What size solar panels would I need? Are the cheap ones on eBay that claim to have a mppt regulator any good or should I pay more for a local brand?

    • Hey Jonny I have bought ebay AGM batteries from brands with some internet presence, website, detailed datasheets etc, and they seem ok. You probably need around 100W worth of solar but depends how much sun you’re getting and how reliable you need it to be (see solar panel sizing section in the article). I don’t like generators, with their noise, ongoing expense, size and weight. Nor do I like the culture of high power consumption devices becoming part of the camping setup when a generator is available. So in my opinion I’d go solar all the way unless you’re predominantly in dense forest or cloudy climate. The ebay stuff is usually ok particularly if you have the know-how to fix any assembly issues, but sometimes you get unlucky, go for an item with detailed specs and good seller feedback, most of the time it does the job.

  56. Hi Joe

    Have read everything twice you know a bit ah ,was wondering if you think this is ok or what you would change.I have a caravan already set up with 240 v and am setting up a 12 v system this is what I have 4 x monolite 12v 100 amp batteries a redarc dcdc 1240 ( do I need this or is their a better way) a 24 volt fridge, 4 x led light strips a 5 switch panel with lighter socket ,2 x 12 v fans, a fuse box 50 amp inline fuses 2 x250 watt solar panels , a amp meter voltmeter, 12/24 v step up charger and a water pump , will be putting in a inverter at a later stage. The cable runs are all very small the questions I have are how to put it all together , are you able to mud map it for us , and am I missing anything bud
    Thanks for your time Joe Graeme…… Trying to keep it cold 24/7

    • Hey Graeme sounds like you got the bits you need. Check the current rating of your charger, with that many batteries you may be better off with the bulk charging current from a vsr. There’s many ways to do it. Would take an essay to respond. Best to try to understand everything in the article then let me know if you have specific questions.

  57. If I am using the dual batteries system, how important it is that the main battery MUST be the same as the auxiliary battery? For example, can one be a standard acid battery for the main start up battery and an AGM battery for the auxiliary battery? It is better to have one 200Ah auxiliary battery or two 100Ah auxiliary batteries in parallel? Or it makes no difference.

    • Hey Patrick as long as the batteries have similar float and cycle voltage specifications then it doesn’t matter much. My starter is standard lead acid and my aux is AGM, connected by VSR. Works great. One 200Ah would be cheaper and simpler install than two 100Ah. But with two you have some redundancy if one fails, they are probably slightly less likely to suffer mechanical failure from bumps and vibration, you can install in different places to balance out weight and you can handle the batteries with one person without breaking your back. Depends a lot on the space you have to install it. Whatever is more practical.

      • Is a 200Ah (one or two units) auxiliary battery overkill for my camping purpose? Most of the forums I read, users tend to have one 100Ah battery if running the 12v appliances that I have in mind. They are: 24″ TB, 55 litre fridge, microwave (for heating food only), 3 led lights installed in the van, charging laptops, ipads, and camera batteries. You suggested phone batteries can be from the car’s main battery. If 100Ah battery is a bit under, should I be getting a 150Ah battery?

  58. I was thinking of having a 300W inverter installed on my campervan to charge laptops, smartphone, camera batteries, etc but in light of what I read about the potential risk and danger of having 240v cables in a vehicle, may be that is not a good idea after all since we can get many 12v appliances and gadgets to use the 12v system. Would you concur with this view?

    • I charge my laptops and cameras with inverters plus run the thermomix. My phone is off 12V USB chargers. Don’t run your inverter continuously, install it in a clean dry place, and don’t run the high voltage cables through areas where there is risk of physical damage and exposure to water. If you do that the risks are low. But an all 12V system is also a good solution, bit safer maybe.

      • I think on your website, you mentioned you are using a 2,000W inverter in your vehicle. Is 300W too small a power inverter or should I be using a 2,000W or 3,000W inverter if I mainly use it to charge laptops, camera batteries, occasionally ironing a shirt for formal occassion (very rare) or other uses.

      • I have two inverters. To charge my laptops I have a 125W inverter. This means I can use a small efficient inverter with low standby current when supplying small loads like laptops and cameras. Then I use big mumma for big stuff like the thermomix. Check your iron power rating and make sure you get something big enough.

        There is no ideal solution. Everything is a compromise. Always using one big inverter can waste energy for small loads but it means you don’t need two inverters. Having one small inverter is small and cheap but then you can’t supply large loads.

  59. One more question. What type of batteries (i.e. AGM or Gel batteries) are more suitable for camping purposes using Solar Panels and alternator for charging?

    • Straight off alternator – AGM is best. Off solar panels or DC-DC charger – standard deep cycle gel battery might be better. See in my article under “Types of Batteries”.

      • Sounds like I should use the standard deep cycle gle battery since my plan is to have two 120W solar panels, one permanently mounted on the roof and the second one is a portable one that can be manually hooked up when I am in a shaded site and can put that portable panel to a sunny spot. It is also pointless installing both panels permanently to the vehicle’s roof because when the vehicle is moving, the battery will also be charged from the alternator. If I have two 120W panels mounted on the roof and with the alternator running, there is a high risk that the batteries may be overcharged and be damaged if the regulator is not working properly and the charge is still coming from the alternator when the vehicle is moving. Do you think having one 120W portable panel is a good idea?

        In order to increase the life of the battery, I was wondering it is best to disconnect the auxiliary battery by a switch that links the alternator and the solar panel to the auxiliary battery if I am not using the vehicle for camping for about the 10 months in a year when I am not away from camping and only using the vehicle as a second car to ferry kids around the block. If I do that, then the auxiliary battery won’t be charged and will go flat over time and this might create more problem. Once the auxiliary battery goes flat, I will recharge it before I go away for camping. Is this a bad idea and no cost saving to increase the life of the battery?

      • Whatever you do, keep the battery fully charged. Leaving lead acid batteries in a depleted state causes permanent loss in capacity. If I had a DC-DC charger I’d disconnect it and just charge the aux battery on a 240V charger once a month or something. Straight off the alternator I’d just leave it. The alternator voltage is a nice float voltage that is safe for long periods when you’re not discharging the aux battery.

      • Also, if I am using the second 120W portable panel method, what is the maximum distance length wire I can have from the solar panel to the aux battery before it starts to become too inefficient? What size wire should I use for this purpose given that it will be exposed to the weather and external elements such as people walking on top of the cable?

      • You can have the cable as long as you want, longer distance needs bigger cable. Use a pretty big cable and you should be right for fairly long run, maybe 35mm cable. Put your solar regulator near the battery rather than with the solar panel, this maximises efficiency, since the longest cable run is at higher voltage / lower current.

  60. Great blog Joe. I have a 2006 Dodge Ram 2500 diesel truck that I will be storing for about 8 months. I purchased a 10 W solar panel kit which included a 7 A regulator/controller. Do you think the dual batteries can be maintained with the solar panel on the dashboard (with 5-6 hours solar exposure) and the regulator’s battery output plugged into the 20 A cigarette lighter type power plug located on the dashboard? Thank you for your help.

    • Hey Joel yeah man I think that will do the job, as long as you don’t have more than a couple of hundred amp hours of battery capacity. Lead acid batteries discharge by a few % a week. As long as you are putting in one or two % per day (allow a bit of margin for less than ideal conditions) then you should be fine (10/12*5 = 4.16Ah / day).
      Most cigarette lighter plugs are only active when the accessories are turned on. You need to make sure the solar panel is connected to a constant feed to the batteries.

      • Thank you very much for your reply Joe. As a follow up, I tested solar panel with the regulator’s battery output plugged into the cigarette-lighter-type accessory power plug and it only charged the slightly smaller battery, not the main cranking battery. So I’m thinking the smaller battery is probably blocked so it cannot discharge the cranking battery. Now I’m going to run the regulator’s battery output wire through the truck door and hood and connect directly to the larger cranking battery with clamps. Hope it works and charges both batteries. Cheers!

      • Hi Joel do you know what isolation system you have between the two batteries? Coz if it’s a DC-DC charger it could cause problems and should be disconnected if you are going to join the batteries in parallel via the solar regulator. If it’s just a voltage sensing relay it doesn’t matter.

  61. Joe I don’t know what the isolation system is. But I just finished my testing with the solar regulator battery output connected directly to the cranking battery, and yay!, both batteries were charged with both reading at between 12.7 to 12.9 V throughout the day and the charge was maintained overnight! So thank you again for your help and great advice! Cheers!!

    • OK cool. I hope you don’t have a DC-DC charger, coz when the voltage gets high enough it will turn on and cause problems since its output is shorted to its input via your solar regulator.

  62. Hi Joe, great job. I have a Redarc 20A solar regulator – dual battery outputs, no load output. I have only one auxiliary battery, could I use the second output to run my fridge ? Thanks Nico.

    • Hey nicolas yeah I think you could run your fridge off it, but I think you will need a battery there, otherwise the regulator may not turn on that output.

  63. Hi Joe,
    I’d like some advice regarding a dual battery system. I have an MN Triton and want to set up a system that will handle a fridge and a couple of led camp lights, a uhf and charge my phone, gps and a camera, for overnight trips and the occasional long weekend. It’s hard to get a straight answer from sales people. I’ve found a Projector VSR 100K kit on sale for $90. Will this do what I want?

    • Yeah man that will do the job. For long weekends, without solar or other charging, you’ll probably be running out of power and discharging the battery well below 50%, depending on the size of your aux battery.

      • Thanks for the quick reply Joe.
        Any suggestions on what battery I should get. I want to keep it all under the bonnet.
        If on a long weekend I’m starting to drain the battery too much how long would I need to run the motor to top it back up. I obviously don’t want to kill my battery, should I also invest in a battery monitor to see how low it’s getting?

      • I’d get an AGM coz they can take a higher charge rate which means faster top up. If you get an AGM of around 100Ah and run some decent cables to it then it will take a decent current, say 40A. Then to get your battery from say 40% to 80% will take about an hour (probably more since charge rate will reduce as battery gets more charge).

        A battery monitor is handy but not necessary. It’s easy to do some calcs to figure out roughly how much power you use and how long you need to run the engine, refer to article on how to do it.

  64. Hi Joe, I have just bought a 2015 Colorado, which has one of those variable voltage alternators doh!. You have converted me to the VSR over the dc charger as it would be more suitable for my needs. But what I would like to ask is, as I have a Piranha DBE 180S Isolator, which is ignition controlled to isolate the main and Aux battery when the ignition is turned off, (by a wire usually connected to the windscreen wiper power wire). Do you think I could just connect the ignition sensing wire to the main Battery via a low voltage cutout to then act the same as a VSR? This way I could hopefully get it act as a normal VSR without having to purchase one.
    Any thoughts?

    • Hi Paul, do u have any pictures of where the ignition wire is where u tapped into? Im trying to find it on my colorado to install dual battery kit. Thanks!

      • Nah, sorry I haven’t even got that far yet Kristy. Just thinking of a few ideas, do I use the old Piranha or use something like the Intervolt ebi Programmable VSR. But I have now been told the Colorado does not have an ECU controlled Alternator.

  65. wow what a good read . I am setting up a race car trailer with a live in area so a lot of what you guys are doing applies too me as well . I need too run led lights at night and my 45 lt engel fridge a small tv and microwave and kettle . ok I have a 200 watt solar panel on the roof of the trailer feeding into a ctek dc to dc charger ive just bought a 280 amp agm battery heavy sucker that was and a 3000 watt pure sinewave invertor . I plan to run the fridge and led lights straight from the trailer battery and the microwave and tv from the invertor . I also have a 3,5 kva genset when required and the trailer battery can be charged by the towcar on the move as well . a ford ranger with smartcharge turned off.
    we only spend 2 nights at most at each track so im hoping I have enough battery and charging power to cook our meals and watch tv after racing . I have not finished setting it all up as yet and im welcome to any suggestions to improve the set up I have a feeling I may have over engineered it all but too much is better than reading in the dark lol.
    cheers dean

    • Hey Dean yeah I think what you propose will do the job. That’s a big battery, the solar might be a bit light on but for only 2 nights you should be good with 280Ah. Is the Ctek DC-DC charger MPPT? Coz if it’s not it may not operate your solar panel very efficiently.

  66. Hi Joe,
    i have just purchased a 100w solar panel and 20A regulator. Im just after some advice on connecting to my single battery? i dont draw much load with my setup (only fridge for trips and lights), but decided to get the solar if i was stuck in the middle of nowhere with a dead battery.
    just a query as to the setup i should run, was planning on panel to regulator and then regulator direct to battery.
    should i include a switch in between to turn off system whilst driving during the day (due to alternator working) or will the regulator do that job for me? and should i chuck a fuse in at each end of cable run?

    cheers, Frank

    • Hey Frank if your solar panel is going to be in the sun whilst driving then you might want a switch to turn it off since it may cause troubles with your alternator. But it may not pose a problem, depends on the car’s charging system and the regulator. Put a fuse at the battery end for sure. Typically you don’t fuse at the solar panel end.

      • Thanks for getting back to me mate.
        It’s an older Hilux so now fancy alternator and the regulator is an adjustable one. Presume I could set it up so it just couldn’t charge at a higher rate than the alternator and that shouldn’t pose a problem?

      • Yeah based on experience it should be ok either way but in theory there could still be a problem which is why I am hesitant to say she’ll be right. The solar regulator pumps out a higher voltage than the alternator. Two voltage supplies in parallel at different voltages poses a problem. On my car I have once experienced an engine check light (P0622 generator field / F terminal circuit) due to solar panels deployed. No permanent damage was done. Usually the regulator settles to float mode which matches the alternator close enough and the system is ok. But there’s a risk that the higher voltage from the solar regulator could backfeed into the alternator and cause a problem with the alternator’s regulator. I have never actually heard of damage being caused in this way.

  67. Hey Joe, Great stuff mate, very informative and truly brings out the spirit of adventure in us all i rekon!

    After months of talking about it I have recently begun collecting parts for my dual battery system for my old T4 transporter.

    So far I have acquired a CTEK 250s Dual DC, which I intend to connect to a small solar system a little later on (possibly a different van).

    Anyway, I just had a few questions regarding protection, batteries, inverting and metering as I’ve tinkered with auto electrics in the past and another fried loom isn’t good for the soul i feel 😉

    In terms of load, I intend to run a 35-40l fridge, small led strip light, 12v fan, phone and laptop charging facilities.

    As i mentioned she’s an old girl so won’t be heading out too far, just day camping, weekends etc and thus the power demands won’t be too drastic before a drive to fill her up again.

    So firstly, do you think and 80ah agm will be ok for running this for a few hours a day? i know there are useful examples of calculations above however I’m fairly limited on space and would prefer to spend more on a smaller (physically) & better quality battery than go overboard with a monster.

    I also intend to be able to monitor the charge remaining in the aux battery if possible, is this possible by simply running lcd voltmeter across the battery? if so will a 10-30v 100A DC one do it?

    can you perhaps suggest an invertor that would be suitable to flip it to AC for the above loads? i’m really not to sure as to how this part of the system works tbh.

    Finally, in terms of protection, I intend to stick a couple of 100a fuses in line at the battery ends of both positive terminals. Is 100A ok if I’m using 13.5mm cable and runs of no more than 3 metres?

    Thanks in advance mate, again, really great source of info and a pleasure to read into your experiences.



    • Hey Damo

      80Ah should be ok for what you need. If you’re camped up for two nights without running the engine then you might have a problem. Yes voltmeter is ok for monitoring battery but charging current / load current complicates the measurement. Measure a rested battery under no load. You may get a feel for how much the voltage is pulled down whilst running the fridge and other loads, so you can still use voltage to approximately indicate state of charge even under load.

      For charging laptops, I use the smallest, cheapest inverter possible. This maximises efficiency and minimises cost. If you have a big laptop you might need something a bit bigger. You can see what I use on this page:

      You can use a smaller fuse than 100A since the CTEK is limited on it’s current capacity. Check it’s datasheet for max input / output current and pick a fuse a bit bigger. This will provide better protection.

      Good luck.



  69. i was wondering if you can help or confirm what I want to do in a Solar setup.

    Currently, I am only using a portable 180W Portable Solar Panel Kit (beside generator) which when hooked up to the van’s 50Amp Anderson Plug charges 2 x 100Ah Deep Cycle Batteries in the front boot which are connected in ‘parallel’.

    Having just completed a lap of Australia, I find that when towing, and the car connected to the van’s 50Amp Anderson Plug, I am not putting enough charge into the batteries. I believe that most of the cars charge is running the Gas/240v/12v Dometic Fridge. So, to rectify the problem, I want to permanently fit a 200W Solar Panel to the van’s roof. The van was pre wired for Solar during construction which lead from a junction box on the roof down to the front boot.

    Checking where to position the new panel/s on the roof, I find that I have an area approx. 750 x 2000mm.

    Checking Panel sizes on the Internet, I am finding that to get away by using only one panel, the average size is 1830 x 808, these being to wide. So I ave resorted to using 2 x 100W (can only find them as a folding panel kit) each panel approx. 930 x 670 and fix them side by side on mounting blocks.

    Now, firstly to my original Portable Solar Kit, I believe I have to remove its Regulator (read somewhere that you can’t use two Regulators – conflict with each other). Still wanting to use This Portable Solar Kit in my new setup, I will connect them together in ‘parallel’ using MC4 T connectors.

    Then, the new panels to go on the roof, also join these in ‘parallel’ with MC4 T connectors. After mounting, then connect them to the preinstalled wiring in the junction box on the roof with the wiring leading down to the front boot. In the boot, I will then use another 2 x MC4 T connectors and run another supply line to a 50Amp Anderson Plug to be fitted to the outside of the van. This will allow me to pick up the supply from my original Portable Solar Kit.

    From the other end of these 2 x MC4 T connectors, run the wiring to a 30Amp MPPT Regulator to be fitted in the boot.

    Now, I not sure, from the Regulator, do I the ‘positive’ to the ‘positive’ on one battery and the Regulator ‘negative’ to the ‘negative’ on the second battery OR, from the Regulator, do I run a ‘positive’ the both batteries and the ‘negative’ to both batteries.

    Also unsure as to what size cable – the preinstalled is a twin core B&S. Also, do you know anything about installing a ‘Mobile Safety Switch’

    Any help or suggestions greatly appreciated.

    • Hi Mark

      It doesn’t matter how you connect to the batteries, schematically it’s all identical. Positive to one battery and negative to the other battery is to help distribute voltage drop evenly but I don’t think it matters much for two batteries installed next to each other.

      Cable size depends on current and cable length. For your solar panels something like 16mm should do the job.

      Mobile safety switch is to protect from electrocution from inverters. You can use one if you want. It trips the power if there is earth leakage or if the earth voltage rises. They are more common for motor homes or caravans where you have a permanent distribution network for the AC system.


      • Hi Joe
        Just another question!!!

        Can you advise if I can connect different wattage panels (in parallel) to the same regulator/controller?

        I want to permanently fix 2 x 100w OR 2 x 120w panels in parallel to the roof of the van? Before the wires get to the regulator/controller, I want to put in another branch leading to the outside of the van with a 50Amp Anderson plug on the end. This will then pick up 2 x 90W panels on a portable kit which are also in parallel.

        Then, at times with both sets of panels connected, I will have a total of 380W OR 420W going to the controller. At the moment I am planning on using a Tracer 30Amp MPPT Controller. Will this be big enough, or do I go to a 40Amp.

        Any advice greatly appreciated.

        Safe travels.


  70. Hey Mark you can mix different panels in parallel but it will mean they will operate at a less than ideal operating point. You’ll lose some efficiency since the regulator will not be able to simultaneously optimise current vs voltage vs irradiation level for every panel. If you choose panels with similar voltage, current and power specifications then probably the reduction in efficiency won’t be much.

    380 / 12 = 31.6A so your 30A regulator may not be enough in bright conditions.

  71. Great article and great amount of comments and replies.

    Quick question though: I’m looking at putting a dual battery setup in my Hyundai i30cw. I cannot add another thick wire to the positive battery terminal of the start battery (no room left). As the car is still under warranty I don’t want to change anything, so I’m wondering whether connecting the cable and vsr to the positive cable plugged directly into the alternator will be ok? Surely this is very similar to connecting directly to the battery. Seems less messing around and I can mount the vsr to the firewall right near the alternator too making less cable run.

    Planning in having a 50-80ah AGM battery in the boot area with Anderson plugs between so I can remove battery when not travelling. A couple of years ago I ran a heavier cable to the rear aux socket as the original cable couldn’t handle my fridge. Also installed a low voltage cutout too so I could still start car in the morning, but on some trips either the fridge didn’t last all night (warm night) or lasted but couldn’t start car in the morning (not enough power to heat glow plugs). we don’t usually stay in the one place for very long and when we drive it’s usually for at least 3 hours anyway, so should charge ok. I have a 120w solar panel too that I plan on using of an afternoon when arrived at camp.


    • Hi Neil

      It doesn’t matter where you connect, electrically it’s identical. Unless your car has some fancy load sensing device and a variable voltage alternator. Then you need to ensure you connect in a location where the load sensing will detect the current flow. Don’t forget to fuse the cable as close to the source as possible.

      Have fun, may your beer be cold.


  72. Hi, back from a another bush camping trip and not sure I have my 12v electrics set up correctly. Aux batteries seemed to have drained far too quickly (fridge had a flashing fault light which is generally caused by low voltage).
    Camper trailer has 2 (new) batteries which are charged by:
    – a permanently fixed 240v smart charger when at home or at a caravan park.
    – alternator and dual battery setup when driving.
    – solar panel with controller when bush camping.
    Batteries were fully charged when we left and I used the solar panel momentarily when it wasn’t raining. We don’t use a lot of power when camping. The fridge is a small evakool and we have a few led lights.
    Why are our batteries draining so quickly?
    Is it the permanently wired 240v charger? Or running the fridge while solar charging? And if so why do these things create problems?

  73. Hi Stuart

    There’s a good chance the 240V charger could be causing the problem. See in the article under “Charging 12V Systems from Mains” I suggest having a switch to isolate it on the 12V side. I think my charger sunk about an amp or so from the battery when not powered up, which is quite a bit when accumulated 24/7.

    If not the solar regular could be taking some current, or a problem solenoid in a relay. Or the fridge light stuck on. Or a minor short in your lighting or something. You need to go around measuring current at each device. You can do this with a tong meter or you can use a normal multi-meter by disconnecting the wire that feeds a device and putting the meter in series.


  74. Thanks. I will find some time to set it up in the yard and give your suggestions a try. All the electrics are neatly tucked away in the camper and only accessible when set up (a real design limitation). I think I should be able to figure out an isolating switch on a panel that I can get to when it is packed up. Since I need to be able to charge the batteries at home then isolate the 240v charger until we are back on 240v.
    Thanks for your reply and help. I am sure the same problem has been covered in earlier posts (but the terminology and specific details were beyond me) so I appreciate your time in answering my particular problem.

  75. Hi Joe

    Excuse my ignorance but I have what I believe to be a very simple question and that is: VSR’s come in different amp ratings, how do I know what amp rated VSR to install in my system? I have a Mazda BT50 start battery with a slide-on camper. The slide-on camper has 2 x 68 watt and 2 x 72 watt solar panelS, 2 x 120Ah AGM storage batteries, a 15 Amp CTEK Charger and a 140 Amp VSR.

    Your advice would be appreciated.



    • Hi Colin size your VSR to suit the biggest current it’s likely to have to handle. This is based on how much current your batteries can charge at, what other loads you have, the rating of your alternator and whether you want to be able to crank the starter from the aux batteries. Your 140A VSR should be big enough for your needs.

  76. Hi Joe

    Thanks for the reply, I have been searching the web for the answer without success. Your Design Guide provides a wealth of information on the system which I will be reviewing.

    Thanks once again


    • Hi Joe

      Following on from my previous question, in your The Complete Installation diagram you show 2 x VSR’s installed. Would the BEP DVSR 710-140A perform the same function given I have a similar setup to your Complete Installation diagram (less the Inverter?

      Once again your advice would be much appreciated

      • Hey Colin nah that VSR you have is only single sense. That means it won’t close to allow charging from your aux side. See in the article under “Dual Voltage Sensing Relays”.

  77. This is an example of poorly thought out design. You do not parallel two batteries without some sort of load sharing, because if the batteries have different internal resistances (which they ALL do), you’ll end up with one battery charging the other one until the voltages are equal. Once that happens, the battery with the lowest internal resistance with self-discharge a little faster than the one with higher internal resistance. That starts the charge-discharge cycle up again. At the very least, you end up with two totally dead (and possibly irretrievably damaged) batteries. At the worst, you overheat one battery and may even start a fire. If you can maintain a charge (even a trickle charge) on the pair, or maintain a slight discharge, you will not have this problem as long as the discharge rate is high enough to avoid charge-discharge between the parallel batteries.

    Personally, I would NOT parallel two (or more) batteries in any system without an automatic (or manual) disconnect such that when there is NO charge or discharge cycle, the batteries are all isolated.

    • Hi Jim

      Why not word your comment nicely? Your comment is an example of poorly thought out concepts and lack of understanding of theory.

      Load sharing is inherent in paralleled batteries of the same chemistry and voltage. The battery with less resistance sinks more current during charge. The battery with less resistance sources more current during discharge. Both batteries are always at the same voltage under all conditions since they are directly connected. When under no load and no charge the voltage is still the same and therefore no current flows between the batteries. If one battery self discharges faster, the other battery discharges as well to balance it out. This is exactly the same as the load sharing under load. There is no difference. Self discharge is no different to having a small load. When two batteries are in parallel the two self discharge loads are also paralleled and the batteries share that load just like they would any other load.

      This is backed by the last 100 years of putting batteries in parallel. Everything has parallel batteries, from laptops to UPSs to off the grid systems to vehicles.

      Which parallel batteries are you referring to anyway? This article is a design guide that covers many different arrangements. Did you read the article?


  78. Hi Joe, I have another question. I was planning on installing an Optima D34 AGM battery under the bonnet behind the start battery as it is an easy install using the Piranha dual battery tray on my 2015 RG Colorado. But looking at the charge specs for the Optima it states the float voltage to be between 13.2 and 13.8V and I have not seen the dash voltmeter go below 14V during day to day driving. I am afraid this will over charge the battery as I was going to use a VSR. Will this mean I have to go down the DC DC charger route?

    • Great article! I have been researching dual-battery setups for a few months and found mostly conflicting information both online and from those supposedly experienced in the field, and salespeople/retailers (presumably pushing product they stock) including those I’ve known for quite a few years. It’s difficult to know who to trust, and I like to see evidence/proof rather than just take people’s word for their claims. I was leaning towards a Ctek D250S as an alternative to an isolator and separate MPPT solar regulator until I read the article…

      In regards to Paul’s comment, I recently returned from an interstate trip where the cranking battery failed in a Hyundai Imax (dropped a cell, and within weeks after 2yr warranty ran out!). I suspect it was due to the extended driving with headlights, driving lights, and air-con running, which I presume kept alternator regulator voltage higher than “float” as the continuous load would’ve been >20A (for hours at a time). After reading Joe’s article, I suspect the Ctek D250S would result in the same affect on a secondary battery as it would be “overcharged” (excessive volts) when driving for extended periods. Is this correct? It seems strange there is no such warning (that I could find) on Ctek’s or various retailer’s websites of this “issue”, and your article is the only instance I have found such a claim. (Note: I know you have nothing against Ctek or DC-DC chargers/converters, as you have clearly endorsed such devices in several or your responses to comments to your article).

      I also tend to concur with the few comments in reply to the article regarding adverse affects of simply connecting a secondary battery in parallel when (~15 years ago) I used to have two Odyssey 1000 batteries in parallel (no battery isolator or regular, other than the alternator) where two batteries failed (again, dropped a cell) within the space of 3 years. If I remember correctly, in both instances it was the secondary battery (in the boot) which failed, and not the one in the engine bay (and both were replaced under warranty). I was told at the time to always parallel same battery brand/model/size and never different sizes as one would never charge fully; the larger battery would only charge to the same capacity as the smaller battery. I never knew if this was factual as I never tested the “theory” as I always used identical batteries.

      Perhaps I’m just unlucky when it comes to batteries! 🙂

      • Hey Rich cheers for your feedback, glad the article was useful. Yes you are correct it is possible for DC-DC charger to overcharge the battery if there are other loads that trick it into thinking the battery needs a charge.

  79. Hi Paul hmm not sure about that. What happens to your voltage if you go on a really long drive, does it eventually drop a bit? Keeping the optima at 14V all the time isn’t ideal but then it’s only 0.2V over, might be worth contacting optima to see what they reckon. Otherwise use a DC-DC charger that has a float voltage within the specification. Problem with DC-DC charger is, if it’s also feeding a fridge and other loads, you’re overcharge could be even worse (as described in the article). So you could just put up with the 14V from the alternator.

    Also, can you find anywhere in your vehicle’s documentation a description of how the voltage is supposed to regulate? Sounds strange to keep it at 14V all the time, would be good to verify if that’s correct.

    • Unfortunately I can find nothing in the vehicles documentation, but I have read on the web about a small number of the starter batteries boiling. They had put this down to maybe it dropped a cell, but it’s possible it may be due to overcharge too. The longest drive I have been on would be only 90 mins, but I would’ve expected the volts to drop in that time. I will keep an eye on it and maybe look into this a little more.
      Thanks Joe

      • I’ve been testing my cars alternator (Hyundai i30cw Diesel) with my ScanGuageII and it wouldn’t drop below 14.3v and mainly sat on 14.8v. I thought good it’s not controlled by the ECU or anything.

        I recently bought an 8 stage battery charger (240v) and hooked it up to the battery. It started off at 5 bars out of 10, which means the battery wasn’t being charged very much from the alternator. I should probably mention that I have been running a fridge from the start battery and on many occasions have taken the battery down to the fridge’s cutout of 11-11.5v.

        After charging over night and driving to work the next morning, I noticed the voltage now drops down to 12.6v during normal daytime driving. Having headlights on and/or the windscreen wipers the voltage jumps back up to 14.3v again, but hasn’t been as high as 14.8v yet.

        So, Paul, you might want to check your battery and go from there.

  80. Hi Neil thanks, After some thought I put the Ctek 7Amp charger on the battery to check if it was fully charged, because after every start the volts would begin at 12.3 then rise to 13.5 then jump to 14.8 and slowly go down to 14.0-14.1V. It took about 2 hours to go into the last stage(Float), so maybe it was down a bit. I will leave it on overnight and see how it goes.

      • Started up this morning and the volts jumped up to 14.8V and then after a short time slowly came down and would hover between 13.8 and 13.9V. So it seems the battery was never fully charged.

  81. I drive approximately 20 mins into work daily, so nothing too short. Today the voltage wouldn’t go below 14.1V, so I dont know whether the drives are too short, or the battery takes a bit to charge. There are no added accessory loads on the battery when it is stationary except whatever the vehicle has as standard like alarm etc. So I am still unsure if I add an auxiliary battery it will be over charged.

    • Yeah seems strange. I would have thought 40 minutes charge every day would be plenty, especially since the voltage is elevated. I’d expect the voltage to ramp down after only a few minutes from startup if the vehicle is driven regularly with no extra electrical loads. If you find out any more let us know.

      • Well I gave it about 8 hours on the Ctek using the “Supply” mode which is for maintaining batteries it runs a straight 13-8V up to 7A. After starting the vehicle the Voltage dropped very quickly down to 13.8V and after driving approx 20 mins even went down to 13.6V.

        Thanks Joe

      • Yeah that’s how I’d expect it to behave every startup if the vehicle is driven 40 minutes per day with no extra electrical loads. See how it progresses.

  82. Good Evening,
    I have a 2013 Diesel Hilux that has been set up with a deep cycle battery as an auxiliary, I am thinking about fitting a second aux. deep cycle battery in the back of the tray. What I need to know is what equipment I would require to set this system up. I am also toying with the idea of fitting a fixed solar panel on the cab to assist in charging the two aux. batteries. Would this be possible.

    • Hi Stephen yes you can add another battery and charge with solar, there’s many ways to go about it, I don’t want to write a marathon post in this comment. The answers are already in the article, try to understand as much as you can and let me know if you have some specific questions.

  83. One brand of isolator that people may not of heard of is traxide ( They have some really good info and diagrams that explain the difference between DC to DC battery chargers and standard battery isolators. I have a traxide isolator in my Disco (recommended by my Landrover dealer) and have no charging issues with my dual battery setup. Their diagrams really helped me get my head around the whole dual battery charging thing, as there is so many different opinions

  84. The issue I see is prolonging battery life. The Alternator as i understand doesn’t have the battery charge profiles. And wouldn’t frequent charging shorten the batts life quickly? Most peoples wet batteries are in poor cond with lost capacity, is this largely from the alternator lacking the specific profile? I have the SBI12 with my aux running fridge on day trips & don’t know if I should be just charging up my batts with external multistage ac charger along with small on board solarpanel w/regulator OR just get the DC-DC charger? My fear is the alternator while driving is shortening the life of my batts some what.

    • Hey Jman charging by the alternator, if it’s at the float voltage, does not damage the battery. A battery is damaged by discharging (sulfation). The deeper the discharge, the greater the damage. Charging at a higher voltage can reverse some of that damage. For a starter battery, since it never gets a deep discharge, sulfation is not usually a problem anyway. Starter batteries charged solely off the alternator often last 5 years, sometimes longer, if the vehicle is driven every day. For batteries that are deeply discharged, occasional higher voltage minimizes sulfation. You don’t want it too often though, since higher voltages cause increased grid corrosion and other problems related to over-charging. You definitely don’t want it too long if the battery is already fully charged. If the battery is not fully charged then extra voltage can help charge it faster as well as get rid of some sulfation. After a very deep discharge you’d want to fully charge the battery with an elevated voltage as soon as you can.

  85. Do i have to earth 12v batteries, Going camping and i have 2 engels which will will be connected to the 12v battery , do i have to ground the battery with a earth stake, will be using solar panel to keep battery charged

  86. Hi joe I have hilux extra cab with a dual battery system running a red arc vsr running from the main battery too the back of the ute connecting a 120 amp/ hr battery. Which runs 60 litre engel and a few accessories plugs for lighting . My question is I have another red arc vsr both are the 100 amp one’s , I have been given a battery holder from arb that fits in the engine bay what I would like to do is fit the second vsr to the main battery and add another battery for more accessories inside the cab. Will haveing two vsr running of the same main battery running two separate battery’s work. Any help would be appreciated

    • Hey Tony yes two VSRs will work. You’ll have two independant systems. Another way would be to use only the existing VSR you have already installed and have the extra battery in parallel. This would mean both your batteries combine their capacities to feed both your fridge and the extra accessories you’re adding.

  87. Hi Joe,
    Great website.
    I am researching a dual battery system for my jeep wrangler. I am considering a vsr like the enerdrive or redarc.
    I have read countless posts online.
    The vsr raises a few issues:
    1. The JK alternator is to a degree variable voltage – will the vsr effectively charge the auxiliary batter or should I look at a DCDC converter (like a Ctek). The issue with a dcdc converter is that it is a bit of work to modify it so that the auxiliary can be used as a jump battery if the main goes flat.

    2. Would a low auxiliary battery demand more load and therefore the variable
    alternator increase its output (ie the charge is pushed across the starter
    battery and it who sense more charge is needed for the auxiliary)?

    3. Is it simple to add a solar or 240v input into a vrs system?

    4. Would there be any issues having a substantially larger (in terms of AH)
    AGM battery as the auxiliary in terms of charging etc (the Jeep has an
    optima AGM as its main).

    I would be most grateful for your responses to the above.

    I look forward to hearing from you.

    • Hi Lochie some answers for you:

      1. If your alternator outputs at or above the float voltage of the battery then it should charge fine. If voltage is lower then you probably need DC-DC.
      2. Depends on how the system is implemented. If it has load sensing, and you connect it correctly, then it will see the extra load and crank up the voltage. But this is highly dependent on the specific implementation in your vehicle.
      3. Yes. The article covers it.
      4. No issue apart from that it will sink a large amount of current to charge. You could have a situation where the VSR continuously cycles in and out, if the alternator is not able to maintain the voltage.

  88. Hi Joe,
    A very helpful website. I much appreciate the clarity of your explanation!
    I have a Hiace Camper with 2 x deep cycle house batteries, 300W solar panel, Votronic 350 MPPT, 240V battery charger, and a single-sensing VSR. I’ve worked out how it works and realise that (after leaving the front interior light light on for a few days) that the panel and/or charger do not keep the engine battery charged. Will replacing the single-sensing VSR with a dual-sensing VSR solve this problem, enabling the panel to charge the engine battery too?

  89. Hi Joe, great site, good info, thanks.

    Could you answer this question please.

    My system is on a yacht, its more like a floating caravan. I have 2 x 40 watt solar panels. Each panel feeds to a 100 amp AGM Deep cycle battery via a Victron PWM regulator. The boat has a 2 cylinder diesel with a 50 amp auto type alternator. The diesel can be hand started, no worries. The main use of this system is to run a fridge. The fridge gets through a bit of juice due to lousy insulation and been next to the engine. I start the engine on either battery and sometimes after been moored up for a few days need to parallel up the batteries to get the diesel going.

    Recently one battery has died, 4 1/2 years old, the other is 4 years old I,m thinking of replacing both batteries. The design of the boat is such that to replace one you need remove both batteries. No FUN!! I have been advised to use 2 x 6 volt golf cart batteries in series, giving 12 volts 220 A/H, reason is, better made commercial battery and better price. I’ve also been advised to just replace the 2 batteries and leave the system as is.

    What do you think?? Would you have you ideas to improve this system?? Would a dual VSR relay work with the solar to each battery??

    Regards Mike

    • Hey Mike. I dunno much about 6V vs 12V. To be honest I don’t think it makes much difference, any good battery will do. You only want a VSR if you want to have two systems together that can automatically isolate from each other, for example keep one battery for fridge and one battery for starter. Sounds like you have two separate systems, either of which you can use to start the engine.

      • Thanks Joe,
        I’ve been doing a bit more thinking about my system and am now thinking that going for one large AGM battery , about 270AH. Paralleling up the solar panels, 2x 40watt, and using a MPPT charge controller to look after the battery. The diesel can be hand started and has a Paris Rhone 50 amp car type alternator. The engine would normally run for 1/2 hour, its only used to get us out to the bay.
        The reason for going with one larger battery is I have been advised they are more efficient that 2 batteries of 1/2 the AH. How true this is I dont know.
        What do you reckon?????
        Regards Mike

      • Hey Mike one big battery is fine. One guy reckons 6V batteries are better coz they are better commercial grade battery, one guy reckons big battery is better coz it’s more efficient. I don’t reckon it makes much difference. Go with whatever is cheaper or more practical to fit. Handling one big battery can be a bummer. Consider if you’ll ever be by yourself and need to move the battery to access something or fixing some electrics or whatever.

  90. Hi Joe, I have read through your info but I am still a little confused.
    I have a Hilux ute with dual batteries and a radarc SB 112 controller. Cable from the Aux battery runs to an Anderson plug on the tray to which I connect a slide on camper which in turn contains a 40 litre fridge, a small led strip light and two cigarette lighter type sockets which I occasionally use to recharge a lantern and that’s about it.
    My kids have bought me a 100W Solution X solar blanket, it comes with very little information. The separate charge regulator has three connections, Solar, Battery and Load. When camping, can I connect the solar panel (blanket) from the regulator directly to the aux battery and let everything happen from there or, do I have to make up an additional cable from the regulator load connection directly to the Anderson plug on the tray to run the fridge as well as connecting to the battery. Lastly any issue with the Redarc controller by putting a solar charger on the aux battery?
    Thanks Kevin.

    • Hey Kevin yes you can connect the solar panel to aux without worrying about doing the load connection. The problem is, if your battery is fully charged, then when your fridge kicks in the regulator will think your battery needs a recharge and will crank up the voltage, causing overcharge. After an overnight discharge, and with only a 100W panel, I don’t think that will an issue for you.

      No probs with the redarc solenoid, except that it won’t allow your solar panel to charge your starter battery.

      • Hi Joe, Now I have purchased a Projecta PC 800 charger to keep the batteries topped up over winter. Can I put the charger directly onto the starter battery thereby charging both the starter and aux batteries via the redarc solenoid?
        Would I get a full charge to both batteries?

      • yeah dude you can do that, the relay will close from the raised voltage from the charger and everything will get fully charged provided the starter and aux battery have same voltage specification. Just make sure the charger reverts to correct float voltage if you are leaving it on over winter, otherwise you may get overcharge.

  91. Hi Joe, My 200 Cruiser has a BC DC charger charging 2 x 120 AH batteries that are parallel. I would like to run a fuse block off these batteries to neaten things up a little in the wiring department. Do I run the fuse block off 1 of the batteries? If so is this still 12 volt being 2 x batteries? Or should I remove 1 battery and just put faith in 1? Cheers and thanks

    • Hey Adam if the batteries are in parallel they behave like one big 12V battery. Feeding from one battery terminal is the same as feeding from the other battery terminal or anywhere in between, since the terminals are shorted together to make them in parallel. Run your fuse block from either battery.

  92. Hi, I was told by an auto electrician that the RedArc style of voltage sensing solenoid is not compatible with most modern car and SUV alternators is this true?

    • Hey Graham refer to the section “Modern Variable Voltage Alternators”. Also the section on DC-DC converters explains what applications where they would be useful.

  93. Hi Joe, Firstly, Thank you for such an informative article.

    I am fitting a MPPT 30A solar regulator with 300W on the caravan roof and a portable 140W panel to keep 2 X 120AH batteries charged when stationary.

    When I’m driving I will have the 300W on the roof. I can also have a “12V” feed from the car.

    My question – Will a MPPT solar regulator that is used to having multiple sources in parallel just see the feed from the car as another source?

    Thank you

    • Hey Russell a rechargeable battery behaves differently to a power supply. A battery can have current flowing into it which is what happens when a charging supply is connected. Having two power supplies in parallel can cause problems if the voltages aren’t matched, since one supply will try to drive current into the other supply. See the last paragraph in the “Solar Regulators” section.

      • Joe, Yes, reread that section,but it still leaves me unsatisfied. If I have 3 solar panels feeding into a solar regulator in parallel, what would be the issues with disconnecting one of those panels and substituting a 12v feed from another source still in parallel.

        The MPPT regulator accumulates the amps irrespective of actual volts being generated from the individual panels.
        So, what are the issues of substituting a feed from the car for the portable panel when driving, instead of having a DC-DC charger. Solar panels generate different levels of current depending on angle to sun or shade status and the MPPT controller handles that, so I have been wondering how valid it is to substitute the portable panel with a feed from the car while driving. The issue I see is ” How many amps” the car is generating and whether the MPPT controller can handle all of the amps being pushed at it. BUT, are there any other issues that mean I should/must not do this.

        Sorry for being a pain. I dont see where my specific query has been covered so far.


      • ah yeah I get ya, I thought you meant having the vehicle’s 12V supply in parallel with the solar regulator’s output. Putting the 12V supply from the vehicle into the input of the solar regulator isn’t good for a couple of reasons. Firstly the voltage is vastly different to that of a solar panel (typically around 17V on a solar panel used in a 12V system). At best this will force the solar panels into an operating range where it’s very inefficient. At worst it will fry the regulator on your alternator and even fry your car’s starter battery and electronics if you leave it long enough, since the high voltage from the solar panel can backfeed via it’s connection to the alternator to the rest of the car’s 12V system. Secondly, even if you can safely put the 12V supply and the solar panel in parallel, a solar regulator is not capable of boosting voltage like a DC-DC converter does. This means there will always be some voltage drop between the solar regulator’s input and it’s output. You’d be better off connecting the 12V supply directly to the battery you want to charge.

  94. Hi Joe, just after a little advice on what i was hoping to do after reading your informative post. Ive just got a new 2016 Ford Ranger which has the fancy alternator, but I’m getting the Dual Battery mode activated by Ford to enable constant voltage from the Alternator. I currently have a 125ah AGM battery in a portable battery box which i was putting into my old car when previously going away to run my Waeco fridge for days. I would connect the battery up to an anderson with a direct power source from fuse box if i needed to charge while travelling, plus i have portable solar panels to charge it also. So with my new car am i able to just run a direct power line from a fuse out of the fuse box via a “add a circuit” the same way i previously did or do i need to have a dual VSR set up. My plans were to have a power cable running from the “add a circuit” to a location in the back seat area with an anderson connector so i could have the battery located in the car if i wanted to. I was also wanting to run a cable with anderson plugs on either end from the same back seat location to travel from the backseat cabin area out into the rear tray area where i could also have my battery located if i needed to. I was thinking that i could also connect my portable solar panels to the tray Anderson plug which would then charge the battery if i had it located in the back seat area. I was hoping that i don’t need the dc to dc charger set up as i won’t be leaving the battery permanently in the car, and only be utilising the dual battery option when i put it in the car for weekends away or shopping trips where i take the waeco to keep goods cold when not coming straight home. Thanks for you help in advance Scott

    • Hey Scott yeah you can plug the battery straight in via a plug feeding from the fuse box. VSR is only there to prevent you discharging the starter battery. You’ll need to remember to manually unplug the battery when running the fridge without the engine running.

  95. A very informative article, especially the disadvantages of the dc dc charger. Everybody wants to flog you stuff and articles like this really help you decide if you need it. Thanks.


  96. What is a great way to monitor your dual battery set up? Including what is running into it off the solar or alternator?

    • Hey Dominic there are battery monitors that measure net current flow into / out of a battery provided all the current loops are connected correctly to the monitor. Baintech do one. It gives you net current plus calculates approximate state of charge of the battery. There’s others out there. Most monitors just measure voltage. Search the net for dual battery current monitor.

  97. Hi joe,
    Im currently running a narva VSR for my dual battery set up in my 2014 sr5 hilux.
    Ive been told that it is unsafe and that i should only us a DC to DC system so I have purchased a redarc BCDC1225 battery charger a while ago but have still not swapped them over. Im unsure what is the best outcome
    what are your thoughts on this?
    should i switch it or leave it?
    will it damage my alternater?


    • Haha unsafe. Those sales dudes are awesome. Firstly you should tell all your friends and family not to deal with whoever told you that because he is a liar.

      VSR won’t damage anything. In the article under DC converter disadvantages and myths you’ll see some details to help you decide whether you want to change or not. If the voltage from your alternator is high enough my preference would be keep the VSR.

  98. Great article, fairly informative, however I would like to add a few observations, We travel extensively, usually 9 to 12 months a year, rarely stay in caravan parks, (Begrudge paying exorbitant rates to bring your own accommodation) we rely primarily on solar power (500w) with gas for cooking and hot water, we have a 200l compressor fridge in the bus and a 45l compressor fridge in the car we have 300ah of house batteries in the bus plus an extra 100ah in the back of the car (to run the drinks fridge, winch etc. over a period of 9 years we have tried different charging systems, We ( myself and several other in our group) have found that MPPT regulators do not work with compressor type of fridges, when the fridge cycles off, the reg automatically comes back to float, regardless of the voltage, and will not come back to bulk charge until the battery voltage drops below 12,5v. meaning that your battery is never fully charged, where as a simple regulator that can be set to the desired voltage will continue to input that voltage into the battery axs long as the sun shines, under normal circumstances never enough to over charge the batteries.
    The other problem I have found is with VSR (or normal relays connected from the ignition) Yes they work to a degree, however they do have a major drawback, if the car fridge has been in use over night, or longer and the battery is down to say 50% soc, then the motor is started, the batteries are connected in parallel as soon as the start battery has reached the vsr set voltage sounds great in theory, however once the batteries are paralleled then thy try to equalise, there is no limit on the rate of transfer other than what the connections can take, most sealed lead acid batteries recommend a charging rate of no more than the ten hour rate, ie, with a 100ah battery 10 amps. I have found that charging the auxiliary (fridge) battery in the car with a relay system has resulted in short battery life,(12 to 24 months compared to 5 years plus) when compared to a DC DC charger.

    • Hey John that prob with the MPPT I’ve never come across. Usually regulators are configured to go to bulk charge mode when the current is above a certain threshold. If the battery is not fully charged it will continue to sink current above the threshold when the fridge goes off. It’s not possible to use voltage dropping to 12.5 to trigger the charge, since the regulator is going to hold the battery at float voltage. This is why they always use current.

      I have had the opposite problem occur, where the fridge turns on and triggers the voltage to crank up when the battery is already fully charged, but this is resolved by connecting the fridge to the load terminals of the regulator.

      I don’t think there is really much difference between the output architecture of MPPT vs other regulators. I wouldn’t be surprised if they sometimes use exactly the same chip for the output. The difference is on the input, where the MPPT uses variations in solar panel current and voltage to try to find the maximum power.

      How did you narrow down the short battery life to high charge current? It’s not a common failure mode that I’ve ever come across. If you’re deep discharging regularly and not getting any elevated maintenance voltage to reduce sulfur build up then sulfation is more likely. With the alternator, since it can’t crank up the voltage, it’s not going to force a high charge current. The high current only comes for a short amount of time whilst the battery can sink it. A normal deep cycle battery won’t sink much current at the alternator’s voltage since it has relatively high internal resistance. The only way to sustain large current is to crank up the voltage. AGM batteries have lower internal resistance and will take a higher current but they’re typically good to charge at around 30% of the Ah rating.

      I’ve seen, on some battery datasheets, no maximum charge current specified, only maximum charge voltage. Thinking about what that means more generally for lead acid batteries, to me it indicates that charge current is not that important provided you stick to the correct voltage range. The battery will inherently regulate itself as long as you don’t force high currents with high voltages. This is consistent with my experience as well. High voltage causes grid corrosion, gas formation and puts a lot of energy into heating the electrolyte which is bad. High current, at low voltage, only occurs when the battery’s chemistry is in a position to accept that current to recharge the battery. It does warm the battery a bit but most of the energy goes into chemical storage and not into heat or unwanted reactions.

      I think the maximum specified charge current is more of a charge speed indicator to say, if you stick to the specified voltages, this battery will sink current up to the specified rating.

      • Hi Joe,
        Thanks for your reply, The problem as you stated with the MPPT regulator, was that the MPPT dropped back to float, consequently the batteries, never become fully charged, this has happened with three different makes of regulator, Australian, American and Taiwanese made. and have had the same problem with all, at which time I reverted to a simple PWMregulator with adjustable output, set at 14.5v for light use and 14.7 for heave use.
        The only thing I have used to determine the shortened life of the fridge battery is observation, the first battery I had lasted just over the 12 month warranty period, whilst the second lasted nearer two years, but cannot find the exact purchase date. (I normally write the date of purchase on the battery, and keep the receipts in a file).

      • Interesting, I wonder if there was a problem with how it was wired up or size of the regulator relative to the batteries or something. The problem wasn’t because it was MPPT, it was because it had intelligent multi-stage output.

  99. Hi Joe,
    I have two portable solar panels, 100watt and 120watt, each has its own regulator. Can I connect both to my 120Ah AGM battery? Will I get the benefit from both panels or does the battery choose the higher voltage from one or the other?
    If I get a higher amp regulator and connect both panels to it, will both panels charge the battery at the same time?
    I just heard that you have to charge a AGM battery to the manufacturer’s specifications, about 10-20% of nominal amount (120Ah battery should be charged with 12-24Amps max), so how can I charge with max Amps from the alternator?

    • Yo Klaus two regulators in parallel can cause problems since they’ll each try to control the voltage output. Most of the time they’ll find a happy balance and work ok but the outcome is unpredictable, I wouldn’t do it. If the two panels have the same voltage rating you could put them in parallel and then to a single regulator. That will be safe but the regulator will have to pick an operating point that’s a compromise between the two panels.

      Many AGMs specify 30% maximum charge current. Actually I’ve seen some that specify nothing except maximum voltages. The thing about charging direct off the alternator is, since the voltage is quite low, the current will only go high if the battery is pretty flat and has a low resistance which means it won’t sustain the high current for very long and won’t heat up much. The alternator won’t force a high current because it can’t elevate the voltage. If you go with an AGM battery I reckon you’re pretty safe connecting it straight to the alternator and letting it sink as much current as it wants.

  100. Really good info here.. Thanks a bunch for putting this together. I do have one question though which seems valid as you quoted in one part of your article that you should only discharge to 50%.

    Is there a BEC (Battery Eliminator Circuit) available for 12v car batteries?
    When I have worked with Lipo and Life batteries for electric bikes or rc stuff, we put a BEC in to protect the battery from being overly discharged.

    • Hey Troy some fridges have low voltage battery protection which turns the fridge off. Or you can use a standard VSR and ask the vendor to customize it to a lower voltage than normal. Some vendors will do it for you. Not sure if there’s any specialized BECs available for 12V systems, I’ve never looked.

      • Thanks for that. Yeah my El cheapo fridge from eBay has a low voltage cut off but the Aux lights and twin sockets in the back don’t.
        I am using a single narva vsr which is only one way.. I have found in the dual battery 4×4 segment they call them ‘low voltage disconnects’ maybe it’s excessive though..

  101. Hi Joe,
    All of the MPPT regulators were wired as per manufacturers instructions, with the load in all cases, through the “load” terminal. Two of the Regulators were all 30 amp and the other 40 amp, the solar array consists of 3, 120 A panels, the 30 A now have four panels, through a 60 A PWM controller. “The problem wasn’t because it was MPPT, it was because it had intelligent multi-stage output.” I agree with this, and believe that the problem is to do that where the battery level was taken from within the unit, which triggers the multi stage switching, Part of the problem also was related to the bulk charging rate which in all cases was well below the recommended voltage for AGM batteries. Where as the PWM has single stage has programmable voltage, Not a smart charge, and the only concession I make is when we are parked up, with minimal loads on (two Fridges only) and the batteries reach full earlier in the day, I reduce the voltage slightly to 14.5 instead of the normal 14.7.

  102. Hi Joe, wish I had read your article sooner. I have recently purchased a Red Arc 1240 DC-DC charger from Australia as the products are not available in South Africa, the only option here is the CTec 20 amp DC-DC chargers which do not work with the smart alternators. Appreciating that alternators charge at a higher rate than a DC-DC charger my choice was to select a higher amp charger to overcome this problem. Reading through your article seemingly I’m going to compound my problem by overcharging my batteries especially when using my solar panel connected to the DC-DC charger. However, my problem is a bit more complicated, in that, I generally use my Pathfinder with the smart alternator when commuting or going on soft off-roading outings to take advantage of the better fuel consumption. I use my 4.8 Patrol when bush camping and for serious off-roading utilizing the same off road trailer. Is it possible to re-wire the camper to use the DC-DC charger, solar panels and the alternator for charging when changing vehicles? I would prefer to charge directly from the alternator with the patrol but then at the same time using the dc-dc charger with the solar panels. Alternatively what would you do?

    2nd question; is ther any reason why I cannot use block terminals away from the batteries to connect my auxiliary loads and chargers to with a single cable from the block terminals connecting the batteries. At the moment my trailer batteries has a mass of terminals connected to the batteries, where the positive bolt on the battery had to be extended to accommodate all the auxiliary terminals.

    Thanks for a great article.

    • Hey Carey I’m confused with what you want to achieve with all the combinations of vehicles and charging methods.

      You can put block terminals anywhere. Make sure you have it protected by a fuse that is as close to the battery as possible.

  103. Apologies if you’ve answered this before (there are too many comment/answers here to read through this evening!), I’ve got a single VSR and so to avoid only charging the auxiliary battery if the VSR is open, I’ve hard wired an mppt controller to the main battery for connecting solar panels to so that when using solar, both batteries get charged. Any problems with this that you can see?
    Cheers (and thanks for great info!)

    • Hey Jason the problem is, if you’ve just cycled your aux battery overnight or whatever, but your starter battery is fully charged, the starter battery may experience overcharge. The solar regulator will ramp up the voltage when it sees the current that your aux battery is sinking. If you’ve just done a big cycle down overnight it means this situation will persist for many hours whilst the aux battery charges.

      Actually it will probably oscillate. Voltage rises, VSR closes, aux battery pulls voltage down, VSR opens, cycle repeats.

  104. Hi Joe.
    Great article and follow up comments. I am really trying to get my head around this topic so we can take off on our “grand tour” in comfort and safety.
    We have a 2015 Isuzu MU-X and a Coromal Silhouette camper trailer with its own battery. We need to run power through the car to the camper and I also want to add another battery in the boot of the car so if we leave the camper to go on rougher roads we still have the extra power. We have a solar panel with controller which I usually connect direct to the camper battery. The well known 4X4 firm we approached tell me I really do need to have a DCDC set up for their usual reasons and even more so because of the voltage drop due to the length of cable between the alternator through the car and the camper. Can you give some idea as to how critical this drop should be and how much the DCDC would help rather than dual VSR? You mentioned cable length a few times but I am not sure how to judge this. The battery is at the front of the camper so it would be at most 1.5 to 2 meters from the towbar, while the additional battery will be in the boot. I am not sure what cable they intend to use but they assure me it exceeds the usual 6 or 8mm standards.
    Cheers, Graeme

    • Hey Graeme a DCDC charger may be useful to combat volt drop in your scenario. But it’s possible to make volt drop as small as you want by suitable cable sizing. If we assume 30A current, 16 meters of cable (there and back) and 0.2V of volt drop, from Ohm’s law we get:

      Volts = Current x Resistivity x Distance
      0.2 = 30 x Resistivity x 0.016
      Resistivity = 0.416 Ohms / km

      This corresponds to a minimum cable size of 50mm2. Plus you’ll get a bit of volt drop through the plug that you use. So you might wanna go to 70mm2 and get the biggest plug practical. This gives you heaps of flexibility in terms of sharing large loads, jump starting, etc. But such a large cable might not be practical or too costly.

      • Joe. Thanks again for your prompt response. I think we will need ot go with DC/DC rater than thicker wiring. As I recall you suggested a cutout would be helpful, so long as it was managed properly?Cheers, Graeme

  105. Joe. Having checked over the camper wiring it appears we were wrongly advised that the exiting Andersen plug connection charges the on board battery. In fact it only powers the Dometic 3 way fridge (which we use with gas when stopped) via thinner wiring than the usual 6mm. The on board battery only gets charged through an on board charger when we have 240V (or direct by the solar panel) and then runs the camper lights, water pump and other auxiliary power sockets. If we still want a second battery in the back of the car (Isuzu MUX) can you see a problem running the fridge through the existing wiring given the different power needs from the smart alternator? Would we still need the DCDC charger or would that only be used if we decided to install heavier duty wiring to also connect up the battery in the camper? One workshop said we would need to run separate power lines for the fridge and the battery(ies). Sorry to keep asking but we are getting very different stories from three different places, all reputable. Cheers, Graeme

    • Hey Graham there’s many ways to do it all off which will work. You can keep the camper battery as an isolated system and run the fridge in parallel with whatever charging method you want for the in vehicle aux battery – dcdc charger or vsr. You could put the camper battery in parallel as well. In that case you’d probably want fatter cables especially if using vsr. Would be nice if the solar panels on your camper also top up the vehicle battery and nice if alternator charges the camper battery. To do that just put everything in parallel and fed from a single vsr or dcdc charger. Make sure batteries are of same type and have similar float and cycle voltages.

      • Thanks Joe. So reassuring to find someone with a common sense approach and no axe to grind. Regards, Graeme

  106. FINALLY! A sensible, well researched and technically accurate article! Such a refreshing change from the usual pseudo science and plain misinformation sprouted by most people on this topic. GREAT WORK!

  107. Gday Joe, Thanks for the info im very green with all of this. Ive just purchased a battery pack from arkpack looking to use that as the auxiliary option. Have you seen any of these in operation? Also im looking at using solar to keep it recharged and it also has a connection to charge via cig lighter when driving and a mains power option to recharge. Im looking at the 4wd supacentre panels that are 160w cant find any reviews do you know anything about these? Also I have a small 20 watt panel with no regulator could i wire this into the one that comes with the 160w panels if i go that way. Thanks in Advance. Travis

    • Hey Travis I dunno much about the arkpack but should do the job. No good putting different panels into the same regulator, each panel has different voltage profiles.

  108. Joe, great article!!! but I need some help….
    I am currently working in a project in campervans, I have a doubt in what will be the best solution and options in the market to charge a battery of 150 Ah VRL Gel. I’m going to have a solar panel of 255W and a Sol.Chrg.Contr. MPP430 DD. But the question is when I’m connected to 220 V or when I’m running the engine, what are the best options for this type of battery? Cheers!!!

    • Hey Pedro the options are identified in the article, there is no best option. Understand what you are trying to achieve, and read the article again, I can help if you have a more specific question.

  109. hi Joe I have a 2016 Hilux with a 20a dc to dc charger (fitted by Toyota) in the engine bay ,I am after a recommendation as to changing it to a vsr as I have a camper trailer with 2 100a agm batteries & I don’t believe the dc to dc will do the job as it gets extremely hot which in turn reduces its output (so i’m led to believe ).I will also have a 110w solar panel on the vehicle & 140 w portable panel which work great (had them on my last vehicle)
    PS i have been told by a local autoelec that a vsr will cause the main battery to boil (I think the marketing for dc to dc chargers is a little or a lot of confusing)
    I had a redarc sb12 in my last vehicle (2008 Hilux) & never had any prob’s
    cheers Greg

    • Haha how could VSR cause the main battery to boil? Never heard of that one. The DC-DC cult just keep making new shit up!

      Ramping down due to high temps is a good thing, your alternator does that too. It’s to protect the batteries. I wouldn’t make a decision based on that.

      2 x 100Ah AGM batteries can take a lot of charge current. The 20A restriction on the DC-DC charger is a big drawback. But then I dunno if volt drop would be a problem if your AGMs are hanging in the trailer when using a VSR, you’d need to do some calcs. Other than that refer to article for all the other pros and cons of VSR vs DC-DC.

  110. Great information thanks
    I have a question
    I have solar panel connected to my auxiliary battery via a 30 amp controller it charges great but when I put a load directly of the battery in this case a fridge via a Anderson plug all lights go out
    Can you help me with this problem
    As soon as I unplug fish’s it lights back up

  111. Great article. Thanks.
    I’ve put a 160w solar panel on the roof of my BT50 and connected it to my aux battery via a regulator at the battery. The aux battery is also charged by car via an isolator set to cut in/out at set voltages. When not driving, during the week, I condition the aux using a Redarc unit.
    My question… can I leave the solar panel hooked up all the time with it and the car alternator charging at the same time?

    • Yo Dayv two voltage supplies in parallel can be a problem if they don’t balance properly. On my car when I have the solar panels deployed with the engine running I get an engine check light. The fault code is exciter voltage fault or something like that, I forget exactly. No damage is caused, the fault clears when solar panels turned off.

      Will it cause damage? Unlikely. Most likely a balance will be found and no harm done. But each type of alternator and regulator will respond differently. There’s a chance of damage, if for example the solar regulator cranks up to cycle voltage and causes current to try to flow the wrong way into the alternator. So the answer to your question is maybe. Safest bet is ensure your solar panels are covered or disconnected when engine running.

  112. Many thanks again.
    I understand my main isolator does not voltage allow flow back to the alternator. In fact, there is a wire I can connect to a switch to allow just that – if I wanted to.
    Also, I have put an Anderson plug near the panel so I can easily disconnect it.
    I think I’d best do some more digging.

    • Your isolator may only be voltage sensing that is one way. Once its closed it may allow current to flow either way, that’s how they usually work. What isolator is it?

    • ah yeah interesting, it is not a relay, the output is a transistor, which means it’s possible it can only conduct one way, but I couldn’t find anywhere in the specs to indicate.

  113. Your assistance is great. I was just working on verbal advice from Jaycar. Looking at the online manual again I found these words “It does not allow the second battery to power the car electronics or feed into the
    starting of the car unless the manual Over-Ride Connection is activated”. The over-ride connection is the wire I spoke of earlier.
    Great having your help.

  114. Just a question on the solar regulator chapter it seem you have joined the wires after the regulator and they go to separate batteries is it ok to do this with with a pwm regulator? I have 2 AGM batteries and would like to run them with 1 regulator TIA

  115. Wondering if anyone has had anything like this happen with their vehicle … maybe offer some suggestions or shed so.e light on possible reasons? I have had a seies of problems with the solar/vehicle inverter and at times i have had problems with the alternator belt. I have a 24 vault system … toyota coaster. The alternator belt has broken twice and had to be tightened 3 or 4 other times. It seems to me that when the inverter isnt functioning properly the alternator belt doesnt play up and when the inverter us repaired it does! I keep getting told that the inverter will draw from the bus battery so shouldnt affect the alternator but it seems way too connected to me! Any suggestions or is it really just coincidence?

    • Hey Paula sounds like coincidence to me. Some mechanical issue with the pulleys or something. Higher electrical loads may accelerate the failure since it puts more load on the belt.

  116. Hello Joe, is it ok to run a 3,000/6,000W inverter with 400W solar and 200AH battery? I just want to run toaster and Kettle not even at the same time.. TIA

    • Yep you can do that. Make sure you have fat cable, short cable run and tight terminations. Be aware you’ll be draining the battery pretty quick at over 200 amps.

  117. hello joe, i have a new hilux i am running a bcdc 12/40 redraw battery system my first battery is the normal hilux the second is a 135 amp/hr agm battery, i am also running 2 x 130w solar panels on the roof, i am doing a trip up thru the red centre and am towing a camper trailer i have a battery in the trailer and will only using pump for water and led lights for light, but want to charge trailer battery from vehicle plus i am taking 2 x engel 40litre fridges along in rear of ute. there is no invertor or other 240v stuff just light all led
    what size cable for my front battery to redraw is needed and can i run solar panel with regulator on trailer as well and just isolate so that solar and redarc bcdc don’t conflict.

    • Hey mate the cables need to be rated to meet the input and output current rating of the charger. That’s the minimum size. Then check if you need to go bigger for voltage drop. Yes you can put all the batteries in parallel and charge them from a single solar regulator. If you want separate systems with individual regulator for car batteries and camper batteries then you need to separate panels too. Can’t run multiple solar regulators off a single bank of panels.

  118. Hi outbackjoe, firstly, great article, much appreciated.
    I currently have a duel battery setup in my toyota prado, and I am buying a caravan. I plan on having dual 100 amp hour batteries in the caravan, as well as ~400w of solar panels on the roof. The van is a Jayco Expanda, and comes with this battery management system (J35-B):

    Click to access B4sDOBE7MUOkd8xq4WFc6y3sh

    I want to maximise efficiency for extended off-grid stays.
    Regular load would be fridge, and lighting. I have a 2kw generator that I can use to charge the batteries when they are low. my question is this, should I complicate the system and have each 100ah battery with the ability to isolate and charge on the generator or should I just let the J35B manage it and just connect the regulated solar directly to the batteries?

    • Hey dude put all the batteries in parallel. That gives you a battery bank. Then decide on how you want to charge them. It’s like having one big battery. No need to separately charge batteries. I’d have a 240V charger that you hook up whenever wanting to charge off generator.

  119. Hi , i have a portable 240 solar panel . I am thinking of mounting it to roof rack . Can i start & run car with it connected or will it destroy the regulator & or solar panel ?

  120. Hey mate I’ve been researching extensively on the web to get the most of my dual battery system.
    Your article is by far the best.

    Thanks heaps for all the work

  121. Hi joe I have a rooftop tent on my flexiglass canopy on the navara I see yours over hangs at the back of your racks I have the Flexi sport flexiglass canopy mine over hangs at the front is that an issue for u. Cheers Chris.
    like reading your story’s mate.

    • Mine overhangs both front and rear but a little more on the back coz my solar panels are a bit weighty on the front so I wanted the roof platform on the cab of the car as far back as possible. Now that I have a different solar panel setup I’ve shifted it all forward a bit so that the tent is more centered over the bars.

  122. I have a 220Ah agm deep cycle battery I have a 3000-6000W pure sine wave inverter and it wont even run my toaster 700W long enough to make some toast .. the battery is quite new, all the cables are connected nice and tight but still no good any ideas? Have I just got too big size inverter, Should I get a smaller one around the 1,500 W total ?

    • Hey Marion the inverter rating is not an issue. Measure the voltage at the battery terminals and inverter input terminals when running the toaster. If both are low then battery is either not sufficiently charged or too much internal resistance to provide sufficient current (maybe sulphation damage). If battery terminal voltage is ok but inverter input is low then you have too small cables / too long cable run / dodgy terminations. Low voltage means lower than the minimum input of the inverter which is typically 10.5 volts say 11 volts to give some headroom. If voltage is ok then problem is with inverter. My old inverter had loose internal connections to the input terminals which stopped me from running big loads, I tightened it and all good.

  123. thanks Joe the inverter is fine we tried it on my son in laws van cables are fine but I will try waht you said the battery with no load was reading 13.5

  124. Good evening Joe, My sincere thanks for your time with my clarification query AND that already spent in the excellent information you’ve presented here above. Really well laid out article and that you’ve made it all so readily available is wonderful – thank you.

    I am new, but trying to be learning as fast as I can get the information into my noodle.

    I’ve aquired/inherited two commercially available ‘dual battery’ kits, one contains a ‘Fully Programable’ VSR unit. The other kit comes with a Projecta DC-DC IDC25 ( BOTH of the kits included cabling which, based upon your entirely rational reasons for potential current draw, charging efficiency and prevention of voltage drop, seems to be on the thin side of comfort (8 B&S), so I’ve lashed out on some 2AWG that I intend to place a 100amp fuse at each end of between the cranking battery and the 120ah Deep Cycle

    I completely accept that the DC/DC concept comes with compromises and has its own limitations – no argument – your comment about overcharge potential during drawdown in the presence of DC/DC solar input is concerning to me.

    Some questions:

    1. The VDC and the DC/DC are an ‘either/or’ inclusion in the circuit right? There is scant need for both in a dual system? Is there a scenario where having both present is advantageous?
    2. Assuming a DC/DC only installation, will this unit perform as a ‘dual VDC’ and isolate the cranking battery during discharge but allow for charging of the cranking battery when the DC/DC is connected to solar?

    The distributors web page has some ‘tutorial videos’ that make the DC/DC able to walk on water…including a ‘dual charging’ capability allowing Alt AND Solar at the same time.

    My thanks for your thoughts and opinions, Ken

    • Hi Ken you’re welcome glad the info is useful. You can have both VSR and DC/DC. I’ve never seen anyone do it. You’d need a couple of relays and a selector switch to choose one or the other. Then you could VSR when you want a quick top up straight from the alternator or DC/DC when you are going on longer drives. You can choose what you want based on what you need. I’ve never seen it done before but if you have both already it might be an option. I don’t think the DC/DC will backfeed the solar to the starter battery. The starter battery connection is a dedicated input to a voltage step up circuit so wont work in reverse. I’m guessing, it’s possible to design it so solar can charge starter battery too, best to check with the manufacturer.

  125. Hi OBJ and thanks for the article and advice. I was going to buy a dc-dc charger for my new vehicle until I read your article regarding dual sense VSRs. I have a few questions or thoughts.

    I have confirmed with the manufacturer that my Victron MPPT solar controller does not use the output power information from the load therminal to modify the battery charging process, so that probably impacts on the discussion of overcharging when a fridge is connected. (Actually I am waiting clarification on their answer, so will update when I get a response).

    How will the battery heat up if the solar charger keeps the battery in the absorption stage, When the battery will not be accepting any current? How does grid corrosion progress when the voltage is high but current into battery is very low?

    Do non-smart alternator regulators taper the current down when the battery reaches 14.6V or do they just abruptly drop the voltage to float voltage?

    When not using a dc-dc charger, the alternator may have to do a lot of work to charge the batteries up in the morning. Are they designed for this, or are they just designed to do regular top ups? What about the life expectancy of an alternator now charging a half empty battery every day, perhaps not what it was designed for (duty cycle-wise).

    Apologies if some of this has already been covered. I will start reading it but there is a lot!

    • Hi Duncan the battery is always accepting current when the applied voltage is greater than the cell voltage dictated by the chemistry. When the battery is fully charged that energy goes into heating it instead of into chemical storage.

      Grid corrosion happens during overcharge. It’s well documented. Read here and here for some background. If you do a tonne of research and learn the fundamental physics of how it happens please come back with an explanation.

      Non-smart regulator has a fixed voltage, usually around 13.8V. They’re always at float voltage.

      The work the alternator does is not related to whether you have a DC-DC charger or not. It’s about how much energy you put into the battery, which is the same regardless of charging method. The DC-DC charger gets its energy from the alternator. Actually DC-DC would work the alternator a bit harder because it is not 100% efficient.

      • Thanks for the replies.
        I always understood that it was the battery that controlled the acceptance of current based on the cell potential. When a three stage charger goes into flaot at the end of absorption, I thought this was because the battery accepts no more charge, not that the controller was limiting charge. If this is the case, then how can the battery heat up and corrode the plates if there is virtually no energy being dissipated across the battery?

        I think I didnn’t Explain myself well regarding the non – smart regulator. I was meaning the older type of alternator which does charge to 14.8 and float at 13.6. Just wondered if this was a step change or a taper with reducing current.

        I presumed that the stresses on an alternator outputting 100A to charge a battery in the morning would be more detrimental to its working life that outputting 20A for a dc-dc charger over a longer time. I don’t know wenough about this, maybe the only stress is on the bearings and fanbelt as the alternator filed I ceases to output all that extra current.


      • Are you suggesting that there is no valid reason that every battery in the world has a specified float voltage and that every smart charger in the world is configured to drop back float voltage? We should all just keep it at cycle voltage?

        I’ve already explained your question, read my answer again if you like, rather than me repeating myself.

        A regulator that charges at 14.8 and then drops to 13.6 when fully charged is called a smart regulator. The current would reduce as the battery becomes more charged, then there would be a step change in current when the voltage drops to float. So there is a taper and a step change.

        Yes at higher current the alternator experiences a higher load. Not sure if the higher current for shorter time would cause more wear compared to less efficient charging running for longer (more area under the curve). I don’t think either would cause a significant difference to alternator failure rate.

    • Correct it doesn’t monitor it, I don’t think any controller would monitor it. But most likely it’s internally arranged so that the current through the load terminals bypasses the sensing circuit that measures the current through the battery charging terminals.

  126. A 50A dc-dc charger with MPPT solar onboard will be available from Projecta soon. Does this change the recommendations any?

      • To consider a dual sense VSR over a dc-dc charger, all caveats considers. The 50A dc-dc would reduce some of the negatives associate with slower charging. No idea wha the price would be. I like the dual VSR ide better.

      • Haha yes according to my logic, a higher current would reduce the negative of lower current.

        I recommend always considering all options at all times, since every option is a compromise.

  127. Hey, thanks for this great writeup. It’s extremely helpful. I’m wondering if I can have a 120V (in U.S.) battery charger hooked up when I start the van or will there be problems with the alternator and charger fighting each other? I also have solar that’s hooked up without any switches (user manual says controller must be attached to battery if panels are connected to controller.) I figured the solar would switch off if it sensed the charger’s higher voltage, but I’m not sure if anything would be damaged if I forgot to unplug the charger before I started the van. If that happened I would have all three hooked up at the same time.

    • Hey Ben yeah supplies aren’t made to operate in parallel like that and it can cause problems. Usually nothing will break especially if only momentary but can’t tell for sure. I get an engine check light and an alternator fault code if I have my panels charging with the engine running but hasn’t caused damage. Depends on the design of each supply and how strong they are relative to one another.

  128. Hey Outback Joe,

    This page is awesome it so much more helpful than other pages,

    I am thinking of installing the complete system minus the inventor,

    What is the difference between the redarc solanoids ( and the dual VSR’s?

    Which Dual VSR do you recommend?

    Im finding the cable size and fuse size hard to work out is there a chart that you use?

    I am going to have both batteries and VSR under the bonnet and then leads from the secondary battery to the boot with 2 anderson plugs. (solar/240v & Fridge output)

    Can i just run one pair of leads and split it into 2 pairs in the boot or do i need to run 2 pairs?

    • Hi Scott that redarc solenoid you point to is a dual VSR. It’ll do the job.

      Cable size you gotta figure it out for current capacity and voltage drop. There’s no easy way, apart from just going with big cables. Fuse needs to be sized to protect the cable and / or whatever the cable is feeding, whatever rating is lower.

      Yes run one cable out back. Bring it to some terminals somewhere, then split off, throw some fuses in each line if you want better protection and discrimination between faults.

  129. The best and most comprehensive information I have read on the net , thank you and great work in responding to so so many questions.
    Here’s mine lol
    I have a 250watt Solar Panel (50Volt) that will be connected to a Mppt solar controller that reduces it down to the 12volts. I have a dual battery system and want to keep my 50ltr weaco running constantly and keep the batteries charged.
    I have a trailer which connects via anderson plugs to the cruiser.
    Im not sure whether the plug at the back is coming of the main or aux ? I am guessing the aux.
    With all of this gear, what would be the most efficient way to have it working at its best ?
    I also have 2 x projecta trickle charges on each battery with a simple plug to be able to maintain the batteries whenever I can.
    Am I on the right track ? Or is there something missed or potential problem ? Any input would be greatly appreciated as I have been given so much information but your write up gives me more confidence.
    Thanks again – Dave

    • Hey Dave yep sounds ok. Do you have a way for your alternator to charge your aux battery? VSR or dcdc?

      When will you use the trickle chargers? When touring whilst using your fridge and stuff or when in storage to offset self discharge?

      • Thanks for the quick response. I had a projecta dc150 linked to both batteries that was recommended to me but it keeps flashing a blue light which states overcharging?? I have now disconnected it. Not sure what to do? Suggestions?
        The trickle chargers are there for the times im home and or camping with access to power.

      • Blue light flashing means over current. It’s supplying more than 150A which is huge. What you got hanging off that? Massive bank of batteries? Or there may be a fault.

        Trickle chargers are more for when the battery is not being used for long periods of time. They probably won’t keep up with your fridge. Better to get an electronic charger with a reasonable rating, say 20A. You only need one, connect it to the starter battery and it will cause your isolator to close and charge your aux battery too.

        If you want your solar to charge your starter battery you need a dual sense vsr or you can install a second vsr.

  130. Hi Joe,
    Thanks for your article on dual batteries and solar power systems.
    I am totally green regarding 12 volt power and caravan electrics. And need some assistance.
    However, all advice we have been given seems to come with an ulterior motive, to sell us something new! So your article was really appreciated and enlightening.
    Would it be possible for you to give us some guidance with our power dilemma?
    We bought a new caravan in 2015 which came with 2 X 100 watt solar panels, a Redarc SRP0240 (20 amp) regulator and 2 X 100 amp hour AGM batteries. It also came with a 12 volt 158 litre Compressor fridge (Input power – 65 watts), Instantaneous (tankless) gas hot water system, which has a power blower or fan, Seaflow water pump. All lighting is led.
    The batteries are situated at the very back of the 21ft caravan, under the floor. While regulator is situated approx 7ft from front of van.
    None of this is adequate to power compressor fridge, as well as use hot water for dishes etc. Free camping is difficult and limited. The fridge seems to use far too much power for the rest of equipment. The solar panels, according to the monitor, only put out 6 amp when in full Kimberley sun!
    We have since installed a redarc SB112 Smart Start battery isolator. This is direct wired from starter battery to auxiliary batteries using 4B&S wiring (Ground return via chassis)
    But, this is still inadequate. Even after a days driving, the battery’s will drain and fridge stop before next morning.
    We would like to be independent, and be able to free camp whenever and wherever we like. But the problem is that I feel we need extra battery power, possibly double what we have, but, we would then need extra charging power, more solar panels? Larger regulator?
    Is there a simple answer? Do we need to completely overhaul the system we currently have, or can we add more to enable us to be independent?
    My feeling is that we need, as well as the extra battery power, to add 2 more solar panels, plus upgrade the solar regulator to charge the battery’s faster.
    Does this appear the right way to go?

    • Hey Kerry your batteries should easily last overnight with the fridge. Need to see what’s happening there first. Maybe batteries are buggered, especially if they were never charged from alternator and had inadequate solar and left long term in discharged state. Or fridge could have problem causing it to use too much power. Or some other load is flattening the batteries like a fan running continuously or inverter standby current or something. Or charge voltage is too low. Need to sort that first. Your 200W solar might be ok depending on sun and ambient temps and how good the regulator is and how much power the fridge uses and other loads. Might need another 100W with a big fridge like that to cover for less than ideal charging conditions but with 200W you should still get a few days worth of run time at least. You need a multimeter and poke around whilst charging and discharging to see what’s happening.

  131. Hi joe, I was directed here from a forum and like the way you write. Just a couple of questions.
    1. Are you aware of the 80 and 120 amp smartpass units? You went to the effort of drawing a circuit which allows alternator bulk charging before reverting back to 5 stage dc charging. To my mind that pretty much describes a smart pass’ reason for existence.
    2. You mentioned on numerous occasions about fridge and other loads affecting the charge rate. You will again be happy to know that a smart pass separates users (non critical) downstream from charging downstream.
    3. FYI the ctec 250, once having sensed the secondary battery is full, automatically brings in the start battery and 5 stage charges it off the solar.
    All of this has been on the market for a long time and since your article states you’d consider it if it was available, I’m asking how this would change your view in light of the fact that the system you imagined and drew actually exists? I have had both vsr and dc charged auxiliary systems and agree, everything is a compromise. I am finding we are getting much improved battery life from the 5 stage dc system, but we don’t ever really deeply discharge the batteries so I’m not sure if I should add a smartpass or not?

    • Hey Cam yeah I know about smartpass, I think I mention somewhere in the article that some vendors have solutions to help overcome the limitations of dcdc chargers but I try to steer clear of suggesting brands and stuff, I’m not trying to sell anything or suggest one compromise is better than any other. Smartpass you need in conjuction with dcdc so becomes pretty expensive install. I don’t want it, I got heaps of solar which gives the battery a many hour multi stage charge quite regularly. In this case I prefer the advantages and simplicity of vsr. And I prefer the close to 1k it would take for combined dcdc and smartpass to stay in my bank.

      If you’ve experienced limitations with how you’d like your system to work then consider bypass relay first, it’s cheap and simple, and also consider smartpass. Don’t just get it coz it’s “better”. If your battery is frying from overcharge or if you want cranking redundancy or if battery is going flat after short trips to fishing spot etc then consider making a change.

  132. Hi Joe, I am getting radio interference and looking for a solution.

    Interference is on FM from a Projector IDC-25 mounted on top of a plastic battery box in the cargo area of a Pathfinder R51.

    The unit is well grounded to the chassis with thick short cable, directly to the chassis under the cargo area.

    I believe the FM antenna is in both rear side windows, so it is difficult to get the box too far away from the antennas.

    Any ideas???

    • Hey Matt the problem can either be noise on power supply or noise picked up in antenna. Try a capacitor on the supply to the stereo. If no luck it’s probably in antenna. Need to keep cables as far away from antenna and antenna cables as possible. The worst interference is from cables close together running in parallel. Ensure radio antenna cable does not do this with any cables to / from charger. Perpendicular cross overs of cable are ok. Try to keep charger cables perpendicular to actual antennas too. If that fails you could try shielded cables, I dunno much about buying shielded cables for small stuff, it’s used in industry a lot for managing interference.

  133. thanks Joe, the antenna cables and stereo cables are all in the prewired manufacture cable run locations.
    i have wired in the dual battery supply through the chassis rail from the front to the back to the car and into the rear of the cabin with out running parallel with any other cables that i can see. i will try the capacitor idea prior to replacing the supply cable to the dual battery system with shielded.

    • Check in engine bay if the charger supply cable runs along side other cables for any significant length. If yes it could be adding to power supply noise, re-routing may help.

      • Not sure if its tat either. If i unplug the slupply to my battery box which is a anderson plug the interference disappears (with the supply cable still connected at the main battery). I think it may be coming from either my DC/DC charger (Projector IDC-25 ) or the battery its self.
        Could a foil sheild around the battery box potentially help?

      • It could help but unlikely, I’d put that lower on the priority of things to try. The source is more likely the input supply to the DCDC charger. That is where the most noise is generated. Usually the input current is all chopped up and spiking and falling as the charger tries to maintain a steady output, the changing current is the source of the interference. Try capacitor across stereo supply wires, or moving DCDC charger input supply cables away from everything, or shielding those cables.

        Disconnecting the battery reduces the load on the charger which reduces input current and the associated interference. Or it’s possible the interference is on the output wires and to the battery, but less likely.

  134. I live in Alaska and want to add a second battery for back up starting purposes and use a manual isolator switch to connect batteries. I also use a 1.5 amp charger plugged into AC power when temps are very cold (a fully charged battery cannot freeze!). Normally this charges the starting battery. How can I charge both batteries while the truck is in the driveway? Do I make sure the manual switch is closed to feed the AC 1.5 amp to both, or do I only worry about the starting battery being charged? Also what type manual switch, and circuit board do I need for this installation? Thank you!

    • Hey Tom charging them together will work, just close the isolator like you say. You just need any isolator or switch rated to take cranking current – say 150A. Cable should be fused at both ends to protect from damage to cable but you need to find fuses big enough for the cranking current. Or if second battery is very close to main battery I suppose you could do short cable runs out of the way of everything and take the risk without fuses.

  135. Hi Guys, apologies if this has been previously requested or there is a similar thread..I have installed a smart battery isolator between the starter battery and a second battery in a Toyota Prado 2012 model. The second battery runs a fridge in the rear cargo area. On the weekend i connected up solar panel kit to the secondary battery via the regulator. I noticed the battery isolator was tripping in and out every second or so? Is this normal and will it cause harm to the isolator? Should i be connecting these solar panels to the starter battery – as when i did this the isolator stopped tripping/switching?

    • Hey James sounds like the starter battery is pulling the voltage down when the isolator closes which reduces the voltage to below the relay lower cutoff so it opens. Then the voltage rises and the relay closes again. It’s no good for the relay. Each cycle wears it out a bit. Plus it messes up the charging sequence on the solar regulator. Try charging your starter battery with a mains charger, it could be a bit flat. Or you might need a relay with a long time delay. I think some vendors can program a long delay if you ask them. This will reduce the number of cycles or eliminate the cycling completely if voltage settles to an intermediate level. Or get more solar panels and bigger regulator which can supply more current so the voltage doesn’t fall. Or maybe starter battery is on its way out.

  136. Hi all, I’ve just started my reading before putting in a dual battery system. I have a 2016 D23 Navara NP 300. Will a dual sensing VSR work well on this vehicle?

  137. will I get voltage drop by connecting my portable fridge in the back of my ute via a 12v cable to the caravan some 5metres away?

    • Hey Gavin there is always volt drop. How much depends on current level, cable length and cable size. You’ll probably be ok coz the fridge don’t take much current but depends on the cable you are using.

  138. I currently have a dual battery system with a standard redarc isolator. i also have an AGM in the camper. The start battery charges the aux battery in the car when driving. The aux battery in the car is also connected to an AGM battery in the camper via an 175amp anderson plug. So essentially, the start battery is charging both deep cycle batteries, the one in the car and the camper when im driving. I want to change this set up and instal a 150w solar panel and fix it to the roof of the camper. This will be connected to the AGM in the camper via a BCDC1225d redarc charger. Apparently this charger has an inbuilt MPPT regulator so the solar on the camper would be connected to the battery in camper via this charger. is this right? now when I pull up at a camp site and the camper is in a shaded area, I have a 120w portable solar panel which I can connect to the anderson plug on the camper drawbar. As this cable is currently connected straight to the battery in the camper, will I need to redirect it via the bcdc1225d charger which as I said has an inbuilt MPPT regulator. Keeping in mind, I will need to isolate the controller on the portable solar panel if I took that path. Basically my question is, will this bcdc1225d charger be sufficient for this set up or will I need to instal a separate solar controller as well. If this is the case, will I connect the fixed solar panel on the camper to the solar controller, then bcdc1225d, fuse then camper battery? or Do I just go solar, bcdc, fuse, battery? The other question is if I do have either of these set ups, can I still have the anderson plug on the rear of the car connected to the anderson plug on the camper drawbar when Im driving? Because as Im driving the solar panel is injecting volts to the camper battery and If the aux battery in the car is also connected, what effect will it have? will there be mixed signals between the solar charging and alternator charging? Im not going to shy away from using larger than normal cable to consider voltage drop. Im estimating a substantial cost to this set up so to skimp on cable size is out of the question.

    • Hey Alex yes solar panel can be connected directly to BCDC1225d without any other solar regulator. Your portable panels can also connect directly to the solar input on the BCDC1225d, in parallel with the fixed panels on the camper. Should work ok when the camper panels are in the shade but if they are illuminated then you will get poor efficiency due to mismatch of panels. You can also drive with the solar panels and alternator supplying the BCDC1225d simultaneously. But don’t do it simultaneously if you use a separate solar controller, it will fight against with output from the BCDC1225d.

  139. Thanks Joe for great info and such energy in all of your responses. I now have a 2018 Hilux with smart alternator with a Trayon trayback camper. I use a wet acid 100 Ah auxiliary battery by preference (hot climate and ability to top up) under the bonnet.I have a couple of dual battery kits which worked well in my older vehicles without smart alternators. I am informed that these will not work satisfactorily in my new Hilux and will require a DC -DC charger instead. I have an 80W mobile solar which in the other vehicles I just hooked directly up to the terminals on the auxiliary battery when low and then took off when charged enough. I like to keep things simple as I do very remote 4wd in deserts as a single vehicle. My research suggests that the Projector IDC25 (25 Ah ) at $242 will do the same job as the much more expensive Redarc BCDC 1225D model at $500. My questions are: is my older Piranah DBE180S isolator and my recently purchased Baintech 160Ah VCR kit no longer useful with my smart alternator? If so should the DC-DC charger solve the problem for me? I just want to keep the charge up on the crank battery (can use jumper leads from auxilliary battery if it becomes necessary I believe) and keep a reasonable charge in the auxiliary battery when moble.
    Thanks, Greg

    • Hi Greg sometimes the blanket comment is made “it’s got a smart alternator so you need dc-dc” but that’s not always true. Monitoring the voltage over various conditions will provide some good info. Maybe the voltage is pretty high most of the time so normal isolator will do. Maybe there’s a way to make sure the alternator raises it’s voltage, like ensuring it can sense the load of the aux battery, or bridging a jumper or something into the alternator to tell it to stay at float level. If the voltage from the alternator is consistently too low and there’s no way around it then you should go for dc-dc charger to boost the voltage.

  140. Gday Joe. Love your website mate it’s been very interesting and a big learning curve for me. I have read your dual battery system article and I’m still getting my head around it. I’m fairly new to this and I’m trying to work out a system that would be best to put in my 4.2L HDJ79 cruiser. Can you please help me out here and tell me if I’m doing this right or wrong. I am very interested in the dual VSR system as I think it will best suit me to the set up I want. So I’ve opted for a blue sea VSR 65 amp continuous output with an isolator that allows me to turn a switch to combine both batteries incase the main goes flat, the auxiliary battery I plan to buy is a deep cycle 160ah lead crystal battery. I will also source a MPPT solar regulator for my 200watt solar blanket I have already bought.
    My loads will only be a 40L Engel fridge which draws 3-3.5 amps and 3 small L.E.D camp Lights which will draw about 1-2 amps each. One for the canopy and 2 outside. I roughly estimated to use abt 60-70 amps in a day so which is why I went for a 200 watt solar blanket. I’ve been told by people that the VSR won’t work with this system that I need to use a dc-dc charger because VSR can’t charge lead crystal or AGM at a required voltage of I believe it was 14.5/14.8v and that the battery bank I’ve opted to too big for it. Yeah I know that but I would love to not discharge the battery any lower than 50% just to increase the cycle life. What are your thoughts Joe am I being too unrealistic or will it work bearing in mind that I want it done correctly for reliability. After all I am touring around Australia in about 4 weeks time.

    • Hey Karl. Your setup sounds good. That big battery will take a huge charge current, running off a relay is nice coz it will dump heaps of current to allow faster recharge, compared to DC-DC charger which will be limited to its rating. Float voltage for the crystal battery is 13.6V. As long as your alternator voltage is equal to a or greater than 13.6V you should be fine, apart from when near 100% state of charge the alternator will charge quite slowly. My only suggestion would be to upgrade to a 100 amp relay. I think I’ve clocked my AGM battery charging at 35 amps. Scale it up to your size and that might be pretty close to 65 amps. Add a fridge and other loads and you might be pushing the limits of the relay’s capacity. Other than that your setup should work great. Use your solar panel regularly, that will give the battery a nice voltage boost.

      • Gday again Joe thanks for the feedback mate. I’ve been doing some research again after reading your reply. To be honest I’m still contemplating on how i’m going to set it up. I’m gona scratch the 160ah battery and use the 132ah lead crystal battery instead, saves me a bit of coin and weight too and yeah it’ll discharge a little over 50% in a day’s usage which is fine by me. The lead crystal battery does requires a 14.7 volt cycle charge. On Sunday i’ve Started up the Ute and put a multimeter on it and recorded 14.18 volts on the cranking battery terminals. Is that how I determine that if the auxiliary battery is connected as well which will receive the same voltage feed ? I keep thinking ok so i’m 0.52 volts short of what it specifies but is that really gona affect the charging side of things ? I’ve had a look at a redarc SBI12D relay it doesn’t say what max voltage output/input it does. I’ve also noticed that the BCDC1225 (25amp) or BCDC 1240 (40 amps) charger has a max 15.4 volt charge so ok, it can exceed the charge voltage required, Tmw I will ask redarc if you can pre set it to 14.7 volts.
        I’ve also been told that lead crystal battery require a 30% charge rate. I will use the solar blanket every time whenever I am camping out bush but if I am in town for awhile for work and have a place to stay I will probably not use it and disconnect the fridge to save the battery life as much as I can. What are you thinking Joe ? Tell me if I’m doing something wrong or right love to hear your input cheers.

      • Hey Karl all batteries have a cycle rating voltage. Cycle voltage is not what is required to charge. It is an elevated voltage maximum limit that allows faster charging and reduced sulphation.

        Yes, when using a relay, the voltage at the alternator is the same as the voltage you’ll have at the aux battery. Being below the cycle voltage means it will charge quite slowly as approaching 100% charge, and sulphation will accumulate. You can offset this with regular solar panel charging and / or mains charging.

        A DC-DC charger will give you the correct cycle voltage but has other compromises as identified in the article. Depends on your preferences / priorities.

  141. Hi there outback Joe, I’ve read your writings from time to time, delving into your many subjects. Makes good reading and is common sense stuff. Just a side note I’m not a fan of just “chucking” in a dc-dc charger as the go to device either. I think they have there place in a towed unit for example if wiring is poor. I have taken a side track to this whole dual battery stuff. In my hundred series which came with factory dual batteries I have kept that system and fitted a pair of underbonnet warranted AGM’s (ssb drycells), I have found a vsr namely a “blue seas” unit which can be set to operate with a combined voltage down to 12.25 volts. I know there are other units which can achieve this, but I find this unit to be quite simple. So my pair of AGMS run the accessories etc and supply power out the back to a camper. Everything is in parallel until they disconnect at 12.25 volts. the vsr is dual sensing, my solar on the camper flows forward, the alternator flows backwards and charges up great as there is equal demand (thereabouts) from all batteries. The car has no issues starting at the lower voltages. My winch gets good amps. Anyway, to me it’s nice and simple everything talks to each other. Just throw a 240 charger at the batteries when able or needed. My 2 cents worth, keep up the good work.

    • Hello Whelen, as posted just before, I have the 90-series. I am planning on -slowly- transforming it into something I can conquer the world with. I have a similar setup. Could you provide me with more details about your vsr and potentially a connection schedule. I have (some) experience with dual battery setups and I am scared of messing with the 12V/24V relay switching.
      Thanks a bunch!

  142. Thank you so much for this.
    I have an additional problem. My truck (Landcruiser KZJ95 sometimes called ‘Prado or Colorado’) from 1998 already has a dual battery system: apparently the accessories run on 12V, as per usual, but the starter is 24V, so when switching the car on, with relays, the system switches to 24V then back to 12V.
    How do I connect an auxiliary ‘home’ battery system to this or does it not affect the setup at all? It would mean alot to me if you would answer this.
    Thank you.

    • Hi Peter that would be very unusual for the vehicle to have 12V everything and 24V starter, double check there hasn’t been a misunderstanding or something. I’d have to see circuit diagram to know for sure but you should be able to hang an additional 12V system off the 12V portion of the existing system the same as any other car, provided you tap into the right spot.

      • Hello Joe.

        Thanks for your reply. So I did some more digging and apparently the 12v/24v combination was only in previous models (some 70’s and 80’s). So although they really do exist, it’s not the case with mine. I seem to have a ‘winter pack’ which just means i have an idle up – button and a secondary battery in parallel. So all’s good. Thank you for making me dubbelcheck.


  143. Great article Joe. This is a much discussed topic in 4×4 and camping circles where many many myths and misinformation is reported. This article is full of sound technical advice probably almost beyond what any layman would need to know but I always agree that the best way to approach a problem and find a solution is to begin by arming yourself with the basic principles of the system you are working on. I agree that a quality VSR is the best way to isolate batteries for the reasons you outlined however most have an override switch so the is no need to run a second one if you want to ensure that your charge can go back the opposite way. Also I do not recommend soldering terminals for vehicles used in harsh or corrosive environments as soldered joints tend to induce corrosion and make the copper around the joint stiff and susceptible to breaking. There is absolutely no disadvantage, either mechanically or electrically, in having a properly crimped and heat shrunk joint, particularly in applications where vibration and movement of the cables is likely. I have lost count of the number of times I have come across electrical failures which are attributable to soldered/ corroded/ brittle terminations. Just my two cents.

    • Hey Jade good point. I’ve had a soldered connection on my vehicle crack and open circuit. I think I’ve been told too my times by auto sparkies and stuff “soldering is better”. I might reword the article when i get a chance. Thanks for the feedback.

      Thinking back to my days getting silicosis to make mining executives rich, crimping only was the way to go even at big industrial current levels, no solder.

      I wonder how the comparison looks between a dodgy amateur crimp with pliers vs soldering?

  144. Hi Outbackjoe – I’ve just seen your excellent site for the first time and this brilliant article on dual battery systems. I’ve posted on a few other sites on the issue below and have often got a few interesting replies. As briefly as I can put it. I’ve got a 2016 Ranger with smart ECU. 2 years ago when new I had a Redarc BCDC1225 (25A) put in the rear tub by ARB.
    I only just found out that it should have been a LV version and the Aux battery has only ever been getting a few minutes Boost and Float charge as the alternator drops back to about 12.3V 5 minutes after starting.
    Aux battery would never settle to over 12.5V
    In the last week I’ve had Ford turn off smart charging to Dual Battery mode. ie 14V+ continuous now
    The dual setup also has a jump start via a solenoid in case the Starter battery goes flat. The BCDC has solar input but not priority like the new “D” model BCDC. And the Aux has a 12V Anderson outlet on the towbar. Inside the enclosed tub the standard Ford power socket is re-wired to the Aux battery. Off that I only ever run a small Engel about 2A cycling( cycles a lot cos it gets hot in the enclosed tub) – but it works fine and after 2 years the battery reads still in Good health. So I can’t really complain to ARB.
    The Aux battery is a Century starter battery 80Ah 720CCA.
    I’ve always been interested in being able to run the 3 way absorption fridge in my caravan whilst driving.
    Current draw would be 15A fridge + 2A Engel = 17A.
    Solutions suggested are commonly to wire properly with ignition cutout relay a simple and properly body earthed power circuit from the STARTER battery (charged by the alternator at 14V+) to an Anderson at the rear with proper sized cable all the way back to the fridge which will draw 15A at least. A few hours labour, Cost $500 to $800.
    I don’t want to reignite a heated discussion on this option but basically Ford wont approve it as its not wired into a Ford loom point, therefore voided Warranty is the starting point as an unapproved modification as written in black and white in the warranty wording. In Perth its nigh impossible to find an auto electrician who will do it. Reasons often cited is that there is no control over what someone might plug into the Anderson or plug into the caravan connected to the Anderson – resulting in limp mode or worse.. A fair concern I think especially if the electrical set up in a caravan could be DIY by a previous owner.
    So I went back to REDARC who said my BCDC1225 would be able to provide 25A in BOOST mode and would stay there until the Aux battery didn’t need 25A or some other control condition like ambient temperature derating. With a 17A load I can only imagine if the BCDC dropped out of Boost mode it would soon return due to battery draw down(voltage drop). Apparently staying in Boost mode isn’t a problem for the BCDC to cope. ie won’t “cook”
    In theory they said the BCDC and Aux battery should be able to provide the 15A+2A=17A supply leaving 8A to charge the battery – which may, or may not, be enough depending upon state of discharge of battery at start of journey. BUT the 17A should prevent the further discharge of the Aux battery ?
    So I’m tempted to give it a go and heavy gauge wire the Anderson outlet from the Aux battery to the 12V input to the 3 way fridge, completely disconnecting the fridge from any wiring in the van(which has its own deep cycle batteries – I want them totally left alone for use in camping). Those deep cycle batteries have a Dingo solar regulator as well as fed by the Jayco standard el-cheapo 12V feed when on 240V.
    Even a few hours of cooling on a long trip might help the “esky effect” of frozen ice-bricks in the fridge freezer compartment.
    But, here’s my question. I’m not sure what would happen if I pulled up at a fuel stop say and left the fridge Anderson plugged into the Aux battery for say 15 minutes or longer. Unplugging an Anderson (spark effect !) next to a fuel bowser is a no-no ! And its not a good idea to turn off an absorption fridge because the cooling cycle has to restart all over again – can take half an hour to restart.
    That said I could install a cutoff switch easy enough.
    Drawing down at 17 Amps per hour (like high beam headlights and spotlight all on) might not be such a good thing for a starter battery particularly, and apparently Peukerts Equation has something to say about this ??
    For about $100 I can do the wiring to the fridge so that’s my incentive. Ford warranty and reluctant auto electricians aside, its not really worth spending at least another $500 when some ice bricks and back up Engel get me by.
    I would be really interested in your opinion on this,(do you agree with Redarc re BCDC solution viability) and particularly with respect to damage to the Aux battery. ie at what point(how long in minutes/hours) would it take to wreck a starter (not deep cycle) battery, drawing down at 17A with the engine off and therefore BCDC off ?

    • Hey Cliff there’s many ways to skin a cat as always. If it was me, since you’ve already forced the alternator to be fixed voltage, I’d take out the DCDC charger and chuck in a VSR. Straight swap, nice and easy. Then you can dump as much current into the battery as it needs whilst still supplying both fridges. And your solar on the van will charge your vehicle aux battery too, and even your starter battery if you get a dual sense VSR.

      Supplying the 3 way fridge via the DCDC charger is also a reasonable solution. Depends if 8A charging is enough. This depends on your loads, how many days you stay out bush, how long you drive for and whether you have solar for your in-vehicle aux battery. I wouldn’t go that way. If I’m doing a short trip from camp site A to camp site B I want to make sure I’m dumping in as much current as possible. Slow top up means greater risk of deep discharge afterwards, which damages the battery.

      Straight from the alternator is also reasonable. You could break off from the input to the DCDC charger, that way not modifying anything in the rest of the factory arrangement. It would be hard for Ford to argue such an arrangement would cause a failure if they’ve already approved the DCDC charger, it’s no different to the load from the DCDC charger itself (how is that ok? DCDC charger is just as likely to cause limp mode and it is not fitted to a factory loom). Although from what I’ve heard about Ford customer service maybe they would argue that it caused a failure (I hate to be captain hindsight but you shoulda researched the asshole customer service that Ford has a reputation for before you bought the vehicle and bought something else!). Provided the load doesn’t have a fault that pulls down the voltage then it doesn’t really do anything to the factory system. Put in a fuse to protect from a short. If you stop 15 minutes at 15A that’s 3.75Ah. I don’t think that would impact the starter battery much, particularly if it has just come off charge from a long drive, hard to tell, maybe it would reduce its life by a month or two or something if done regularly. Bigger risk is if you forget to unplug and deplete your starter to the point where it won’t crank.

      I’m not sure if that covers your question. Let me know if you got more.

  145. Hi Outback Joe
    Many thanks for taking the time to provide me with some extra information. A bit of a story in return. I hope you don’t mind.
    To be honest its probably too big a step for me to decommission my REDARC charger. ARB installed it and the wiring is very “tidy and tight” and nigh on impossible to track wires back to connections etc. And I probably don’t do enough serious off grid camping to do a rethink and electrical rebuild.

    A bit about my set up though which might explain where I’m coming from. From the outset, my combined vehicle and caravan set up is a compromise – it kind of evolved.

    In the 12 year old Jayco when I bought it second hand two years ago even I could see that the previous owner/s had done a “dodgy” job on the single car battery and electrics – so I ripped them out and put in the portable solar and battery set up that I already had. Previously I used to tent it with a Mitsi Triton and no caravan. So I bolted the 2 x 90W unregulated solar panels that I had on to a rack on the roof of the van, re-used a Dingo solar regulator and wired that to 2 x deep cycle batteries(about 100 Ah each I think) all bought from the 12 Volt shop. All wired with heavy duty cables, fuses and manual isolation switches to keep the Setec onboard ‘power’ supply fully separated when the solar panels are in use.

    My wife and I probably don’t go off grid with the caravan all that much. Maybe 5 days would have been our longest in Karijini NP and there the solar set up kept lighting, water pump to sink and outside shower, my CPAP, devices like Tablets all comfortably charged. We put the 3 way fridge on gas and that works fine for at least 2 weeks.(twin gas bottles on van)

    Then there’s the Ranger itself.

    As noted its got a Redarc BCDC 1225. That has manual select solar feed in(not auto select like the new D models). I’ve only got a small 50W panel on the roof rack and I connect that to an Anderson input to the BCDC when needed.

    On a 5 day surfing trip away tenting it with mates (no caravan) the Aux battery(80 Ah) keeps charged fine with the solar even in cloudy conditions (the Ranger is parked and isn’t started for those days). The Aux only runs some LED lights, my CPAP and a 2A cycling Engel(the new ones are pretty efficient) – and we have a small Honda generator as well to run a lot of the fridges and heavier draw lighting at night time.

    The BCDC has a its 12V outlet wired into the standard socket in the Tub and also an Anderson outlet on the towbar.
    In addition to that its got a solenoid controlled back up jump starting arrangement which by heavy cables push button connects the Aux battery to the starter battery in the event of a flat main starter battery.

    So you can imagine the “spaghetti” maze of wiring(both skinny and fat) that for the most part cant be seen or traced because it’s all tucked behind panels.

    Generally though when we go caravanning we put the Engel in the back of the ARB canopy enclosed Tub and run it off the socket connected to the Aux battery. Even allowing for the fact that an ‘LV’ version BCDC wasn’t installed, we did a 3 month trip away with the Engel on all that time and drawing down over night and it held charge OK – probably lowest might have been 50% after a night of running.

    So you can probably guess we generally find ourselves on grid with a 240V supply 90% of the time, with off grid probably only 2 to 4 weeks a year – so the set up kind of works except for the annoying issue of the running the 3 way fridge on 12V whilst driving.

    I could install a compressor fridge but not worth $2000 when that little gas light keeps the 3 way fridge running for at least a week on a bottle of gas if off grid. I guess it’s a case of using what I’ve got and not spending any more than I already have.

    If I was to buy a new set up altogether I’d be definitely thinking differently. But I’d be very happy to buy another Ranger for the purposes we use it – mostly towing on bitumen or gravel ending up at a caravan park with occasional off road, with the van or tenting without. Mostly on 240V grid but sometimes not.

    Regarding Ford’s position on doing modifications (they call them “conversions”) their ‘bible’ on the matter tends to be their Body Equipment Manual – see link below. Its their ‘line in the sand’ and their front line customer service, and back line workshop people, and head office warranty people are all bound by it – if they didn’t preach it they wouldn’t have a job for long.

    Personally I’ve always found their people to be very helpful, but risk averse when it comes to modifications. They won’t discuss them. Page 7 and onwards really spells out that they’ll have nothing to do with unapproved ‘conversions’ and that the ‘converter’ should offer the same warranty that Ford would otherwise do if it were their product or installation. Few people ask that question when getting after market modification work done. They assume their new car warranty is fully intact no matter what they do to the vehicle after it leaves the showroom.

    Click to access Body-Equipment-Manuals_au.pdf

    Interestingly it does somewhere in the manual say that Ford supports installation of REDARC BCDCs for Aux battery set ups – providing of course that the work is done by a qualified trade person in accordance with the extensive standards stated in the Manual. Ford provides the connection points and recommended cabling route etc

    With respect to your good point “…….if they’ve already approved the DCDC charger, it’s no different to the load from the DCDC charger itself (how is that ok? DCDC charger is just as likely to cause limp mode”

    In response, I think the issue that I’ve found in speaking to many auto electricians(who wont take on putting an Anderson to directly power a 3 way fridge) is that if the power offtake is connected to a REDARC BCDC they know its hard wired to ONLY that device, that it will pull Amps up to the rating of the BCDC eg 25A or 40A and nothing more as the power supply comes off the Aux battery. The behaviour of the BCDC with Boost/Absorption/Float is very determinable. The Aux battery is like a circuit protection point. You could wreck the battery and maybe even the BCDC, but nothing upstream of that in the car/ECU etc.

    The BCDC has a “non-return” back-feed protection. Whereas the main concern that auto electricians have is that anyone could plug “anything stupid” into the Anderson directly, or via the caravan electrics and solar set up, and feed back an unacceptable or “dirty” power supply back into the vehicle.

    The vehicle ECU on sensing this might say “….I don’t recognise this, go direct to limp mode.” Electricians just don’t want the hassle of arguing about what got plugged into the Anderson because it would be so hard to prove one way or the other. Time spent arguing costs them money and business reputation. So much of what they do now is plug in the laptop and replace the faulty module.

    I guess the final point I would make – under a heading of “Disruptive Technology” – is that a lot of non Ranger owners, especially if their 4×4 is maybe pre 2011 don’t have to worry about all of the “MUST NOT DOs” stated in the Ford equipment manual or other brand equivalent. I think what has happened is that Ford jumped ahead of the market trend and put into their Ranger the kind of new technology and features that previously was only in sedans and SUVs to get an edge on market share.

    In doing so, the Rangers became more computer than mechanical with all kinds of mysterious electrics, sensors and things, many of which directly affect vehicle handling and safety features. Underneath just a diesel light truck of course but now enveloped in a cloud of technology to make everything else work.

    The age old custom of modifying to the hilt, for fun and adventure, a 4×4 in the backyard shed has really reached a point of no return I think. Companies like Ford HAD to respond with all kinds of legal disclaimers which their staff must now espouse. Customers are no longer buying a chassis with an engine and some mechanical parts. They’re buying a ‘sealed up/packaged’ piece of technology like a smart TV or laptop.

    Some 4x4ers might say, that’s OK, I’m a Toyota (or other brand ) person, I’d never buy a Ranger anyway . I think the message I would say is that within 3 to 5 years all 4x4s will have similar complexities in their technology and their warranty exclusions. They’ll have to, to get any market share. Its even been suggested that it will be illegal to modify any vehicle without express manufacturer approval when things like truly autonomous vehicles, electronically controlled steering and braking emerge (they’re half way there now in the newest releases) and even tougher emission standards being copied out of Europe. My guess is that all other 4×4 brands will mimic Ford’s equipment Manual with its exclusions and disclaimers.

    Anyway OutbackJoe, many thanks for your advice again. Its sounds like I may have some success using the BCDC Aux battery if I keep a close eye on what its charge level is doing. I take your point, its not the ideal, but for minimum cost and retaining most of what I’ve got, and given how we camp(not enough True Outback) its worth a go.

    For now I started a couple of days ago temporarily running the 3 way fridge off the Aux battery’s Anderson in the driveway to see how the battery holds up and what charging mode the BCDC ends up in.

    But funnily enough I found out that the 12V heater element in the 3 way fridge($100 ea) is burnt out and I have to replace that which is a bugger of a job to do as the fridge has to be pulled part way out !

    Thanks again and feel free to add any other thoughts.


    • Hey Cliff I haven’t had a read of the manual (it’s pretty big). But the law says they have to honor warranty unless they can prove the modification caused the failure. Any alternator within maybe the last 20 years has an electronically controlled regulated output. The Ranger isn’t special. You might just say it has more complicated software that can choose different outputs based on various inputs. I don’t think that increases the risk of any failure. The actual hardware that makes up the output module of the electronic regulator is the same. I have heard specifically though that Ford try to deny warranties a lot more often then say the Japanese or Korean brands.

      How did they approve the bypass solenoid? That can backfeed all it wants.

      Actually the bypass solenoid might be all you need. If your aux battery is depleted turn it on, that way the alternator can dump as much current in the aux battery as it wants whilst still running all your fridges with ease. Then after a couple of hours driving turn it off so your battery gets a proper boost voltage to give it the final top up and de-sulphation and your fridges will still run. And when stopped the 3 way fridge will just run off the aux battery until you pull the plug.

  146. That’s a good point about the approval of the solenoid activated jump start !

    My guess is it would (technically) be an “unapproved” modification (basically a permanently installed jump starter) as unless Ford mention something like that in their body Manual, or specific approval has been granted by them,(and they don’t) it falls automatically to the “converter” for any liability. In my case the solenoid was installed by ARB who nationwide are of course recognised as a company of repute in their installations as they do ‘thousands’ of private and fleet vehicles to a common formula or method for the various vehicle brands.I think their fitting dialogue with the big brands like Ford, Toyota, Nissan etc etc would be pretty good.

    But yes, whatever potential and current is hitting the Aux battery goes straight through to the starter battery and ECU when that circuit is opened up.

    The set up I have is a spring loaded press button switch ie you have to hold it down to open the jump circuit relay. In the REDARC instructions it says hold down the button for 10 seconds for the surface charge to equalise then start the vehicle. I’d have to install a normal On/off switch which wouldn’t be difficult.But they’ve obviously designed it with only a 10 second connection in mind.

    But I’d never thought of your solution that way. Its basically bypassing the BCDC’s 25A limitation. And as long as the ‘jump start’ cabling and fuses can handle continuously everything the alternator can throw at it, it should be fine. It would be interesting to understand current transfer magnitudes and rates as the two batteries equalise, especially if the Aux battery had coincidentally failed/dropped a cell entirely.

    Certainly something for me to consider.


    • If it’s designed for jump starting it should be right for a butt load of current. More than charging a battery and running fridge. In a jump start scenario the dead / damaged starter battery will be sinking heaps of current plus the starter motor is over 100A probably quite a bit more in the diesel.

      Take care with bypass solenoid not to leave it energized when you are parked up. Otherwise your starter battery will be depleted by the fridge and you may have a no start condition.

  147. Yes I think I’ve heard a few stories of things like that happening with both batteries connected with engine off. 😦

  148. G’day Outback Joe – shame I didn’t post this during your recent down time in Nundroo! – my dual battery ‘compromise’ is looking like, Toyota 79 series with “smart” alternator where I want to add an aux battery for normal use and start protection BUT with an added aim to be able to feed power to a slide on camper that is often on and off the vehicle. Slide on has a small solar setup which feeds to 2 x Delkor DC27 deep cycles (in parallel) in a stand alone system with regulator and a 7 stage charger for when 240V is available. I am looking at a system where I am covered if my start battery dies, ie an aux battery and remote cabin switch to connect the aux to the start battery to get me going should the need arise (SBI12D) with the aux charged from the alternator when running (BCDC1250D). Whenever I put the camper on, I want to connect the camper batteries to the vehicle aux (Anderson plug with large wires) so the BCDC1250D charges the ‘expanded’ aux battery (veh aux plus camper aux = 3 x Delkor DC27 in parallel). Cable run length from veh aux to camper aux would be 4m max and I am looking at something like 0 B&S for that run of wire. I am advised that I should try and have the same cable length from the BCDC to both the start and the aux battery, which would be simpler if both the start and aux battery were located in the engine bay. My question is – can I treat the aux battery in the same way for both the normal vehicle arrangement and the expanded arrangement with the camper? ie if I have the same length cable (say 2 B&S) between the BCDC and the veh start and aux batteries (in the engine bay) would the addition of the ‘expanded’ aux batteries connected using the large wire and anderson connection result in a working arrangement whereby the ‘expanded’ aux (3 individual batteries) get charged during driving? **extra query – for the sake of simplicity, if for no other reason, I did not intend to add the solar input into the mix for the veh aux battery, ie connect it to the BCDC when the camper was on but I am not sure that this would be OK. Your thoughts?? thanks David

    • Hi David if the batteries are all connected up in parallel it will work. The batteries further away from the charger will charge a bit slower but it will all sort itself out with time. They will all charge together from either the solar or 240v or alternator. Just make sure only one charger is on at a time (might have to cover solar panels when driving). Not sure about cable lengths having to be equal that doesn’t sound right can you ask for an explanation?

      If you want the camper batteries to charge from the alternator but not the aux battery charge from the solar you could use a voltage sensing relay to separate them when the alternator is off. Still need to cover the panels when the engine is running else the solar regulator and dc charger could fight each other and maybe get fried.

  149. Hi mate,
    I have messaged before and got some great info from your write ups, thanks very much for sharing your wealth of knowledge.
    I recently parted ways with my Landcruiser and have purchased a triton dual cab with a ute tray.
    I ripped out most of the 12 v gear from the cruiser and now want to install it into the triton.
    I have a dedicated toolbox in the rear to house the aux battery which is a 145ah brand new and I’m a bit stuck on a design guide utilising the gear I have ?
    I was hoping to share what I have and what I want to achieve and hopefully you could share your opinion on a set up that could be used for camping and my business which is repairs and home maintenance.
    I have drawn up a list of the gear I have and also a write up of what I would like to achieve ?
    Just wanted to ask if you have the time and willingness before I start rattling on lol
    Kind regards
    And thank you in advance,

    • Hey Dave yeah i should be able to help depending on time. Maybe if you split it into concise bite sized chunks of specific questions that might make it easier. You want to do it over email or on here?

      • Thanks mate,
        Via email would be good. It’s not as big as it sounds so just see what you think.

        Cheers Dave

      • Hi Mate,

        I have attached a write up on what I was looking at ? Feel free to pick at it 😊

        Thanks for your time to at least have a look ?



  150. G’day Joe,
    covering the solar panels is a bit challenging with my setup, so I will look to include a solar isolator switch which I will use to turn off the solar feed before plugging into the vehicle. (disconnecting the solar input when on alternator.) Means I won’t get the value of the solar when driving, but can’t have everything. I am extremely grateful for you mentioning isolating the solar feed in, as I had not even considered that it could be an issue! I haven’t been back in touch with the BCDC tech guys to get an explanation/reasoning behind the recommendation to have equal cable lengths but will let you know once I have. thanks again for your assistance and providing such a great resource. regards David

    • You could do it other ways to save you from having to turn off the solar. For example put a dc-dc charger with solar input into the camper and feed it off the alternator. But that won’t back charge to your aux battery. Could have an auxiliary contact off the voltage sensing relay which drives a normally closed relay in the solar feed. So it opens when the alternator is on.

      I have a similar situation in my vehicle but i cover my panels with canvas that hangs over windscreen of the car when the panels are exposed. So it’s easy for me to remember to cover the panels.

      • Thanks Joe – the relay off the voltage sensing relay sounds like a great option (avoids me mucking up and forgetting to flick the switch – canvas across the windscreen is my type of reminder! – hard to miss)

      • You need to see if you can get an auxiliary contact for your voltage sensing relay. Or get another voltage sensing relay specifically for driving the solar isolating relay.

  151. Joe,
    Thanks for the incredible blog, my new Bible for the 12v solar setup.
    I have one question that I can’t find a answer to anywhere.
    My auxillary battery is mounted on the rear of my 4×4 externally behind the body and in front of the rear tyre. If I were to make a deep river crossing and submerge the battery in water or to think of it just a really rainy day with lots of kick up, will this short the whole system/blow fuses or similar. The mounting bracket had been DIYed before I owned the car, and in my laziness just figured it was a good spot to put another battery.
    Cheers mate

    • Hey Chris with the distance between battery terminals there won’t be much current flow. Should be ok for quick submersion I reckon. If you got any nearby junction boxes or fuse boxes or whatever that could be a problem coz distance is much closer and water might not drain away quickly. Even without any electrical problems if the water hangs around it will cause corrosion.

  152. Incredibly detailed description. Thanks very much. Now all I need to do is find someone like you to sit down with, to help design and build it. 🙂

  153. Thanks for your detailed article Joe.
    I am setting up a system in my Transit van that is perhaps a little different to most. I’ll have two 140Ah AGMs as the auxilliary battery, with 650 watts of solar on the roof of the van, charging through a solar controller. I also have the requirement to run 2,000 watts of AC power to charge electric motorcycle batteries whilst driving the van. A standard alternator/VSR is obviously not up to the task, nor is a DC-DC charger. My thinking is I’ll need to install a second, dedicated alternator for charging the AGMs/running the inverter whilst charging the bikes. Should I be running an old-school alternator for this (and possibly overcharging the AGMs when not charging the bikes – you mention this in the section Charging 12V Systems with Generators), or a smart alternator?

    • Hey James a standard alternator won’t overcharge if the voltage is set correctly at around the float voltage if the batteries. It will charge quickly when the batteries are flat but will charge slowly once the batteries are nearly fully charged and won’t desulphate but your solar system should do that for you. A smart alternator I’m not sure about. They are smart in the sense they are integrated with the vehicle system to save fuel when possible and boost voltage when needed but I don’t know if there’s a smart alternator optimized for cycle charging a large bank of batteries. You’ll need something rated at maybe 200A if you want to keep up with the 2000W charging. 12V system might struggle you might need to look into 24V with a truck alternator.

      You could use a 240V generator to charge the electric bike if you’re struggling to do it on 12V. Either way you’re burning fuel to generate electricity it would be more efficient to have a petrol motorbike plus a big load off your electrical system.

  154. G’day Joe – thanks for the informative writeup. I have two 6V AGM batteries in my boat, wired in series and connected to battery isolators. Should I wire my solar panels directly to the + and – terminals on the batteries?

    • Hey Lindsay you should treat it as a 12V system – connect to negative of first battery and positive of second battery. You can’t connect the batteries separately to a single charging system because the charging system will see a short circuit where the negative and positive are connected at the batteries. You could have two separate 6V charging systems – one for each battery.

  155. Hi Joe,
    Thank you for the article, it is really useful.
    I have a problem with my current setup: I have the starter battery connected to a VSR, which is in turn connected to an AGM battery. Also, I have a solar panel connected to an MPPT charger which is in turn again connected to the AGM Battery. I also have a 600W inverter connected to the battery. My problem is that each time I connect a minimal load on the AC inverter power, the VSR starts ticking on and off. I think the inverter is surging energy which causes the voltage of the auxiliary battery to fluctuate between the cutoff and cut on rate of the VSR. Is this correct? Is there anything I can do about this?
    Thank you very much!

    • Hey Lawrence the problem is volt drop. The high load at the inverter pulls down the voltage which opens the vsr. This disconnects the load from the alternator side which allows the voltage to rise, closing the vsr. Bigger cables may solve it. Or a way of forcing the vsr closed manually whilst using high loads. Some have a switch or allow you to connect jumper wires that will close the relay when the wires are connected.

      • Hi Joe,

        Thanks for your prompt reply. I found a way to manually turn off the VSR (by turning on the headlights until the switch turns off). It solves the problem for now. Do thin cables cause a voltage drop in the battery as well? I thought voltage drop was only an issue for the receiving loads.
        I will try to get a thicker cable for the inverter, hope that solves the problem.

        Thank you!


      • You need thicker cable between primary battery and vsr and between vsr and auxiliary battery. Cable to inverter doesn’t matter as much. In my setup, even with thick cables, a load above about 1000W will cause my vsr to keep cycling. I solve it by connecting a jumper that forces the vsr closed. Gotta remember to disconnect the jumper otherwise I’ll deplete my starter battery.

        I forgot to mention, with high loads, you should have your engine running. The alternator may keep the voltage high enough to stop vsr cycling and will stop the high load from excessively depleting your batteries.

  156. Hi Joe
    Thanks for your detailed articles.
    I have bit of a problem that I’m hoping you could shed some light.
    In my campervan I have twin auxiliary battery set up that consists of,
    2 x 200W Mono Solar Panels with Controllers, connected to
    2 x 135AH 12V AGM Deep Cycle Batteries connected in Parallel.
    When we go away my wife cant go without her toast, so I got a
    3000W – 6000W Pure Sin Wave Inverter connected via a Anderson plug.
    When Toaster is turned ON, it only stays ON for few seconds and cuts out and comes back ON after a while and so on.
    Considering that the toaster is about 1700W, I thought above mentioned Inverter would be sufficient?
    Currently I only have a 100lt fridge that is running off this circuit.
    Did I get the Inverter that I ordered, as there’s no label on it confirming or could there be something else that I have overlooked?
    Much appreciated if you could assist.

    • Hey Drago sounds like you got plenty of juice with the batteries and inverter. Assuming the inverter isn’t dodgy, the most likely cause of the problem is volt drop on the input of the inverter. Check all the terminations are tight. If you got a multimeter check the voltage at the input terminals of the inverter when the toaster is running. Check the inverter specs to see what the lowest permissible input voltage is. You may need to use shorter / fatter cables and position the inverter closer to the batteries.

      Another problem could be volt drop inside the inverter. If it’s a Chinese special the quality control can be bit dodgy. Pull the inverter apart and check the terminations from the input terminals to the circuit board are tight.

      • Hi Joe

        Couldn’t reply on blog, keep getting a error message Thanks for that, Without Inverter connected my Voltmeter reading is 13V, with Inverter connected my Voltmeter and Inverter readings are 12.9V, when toaster turned ON, Inverter reading drops to 10.8V and returns to 12.9 when cuts out. My cable to Inverter is about 1mt long, might need to cut it down first if that doesn’t work hen use a thicker cable. Being a Chinese special it wouldn’t surprise me if it was the Inverter. Once again thanks for the info and prompt reply.

        Kind Regards Drago Simic


      • 10.8 isn’t too bad for such high load. Inverters cut out usually a bit less than that. Maybe it’s seeing too much volt drop internally. Try frigging around with your cable etc.

      • Hi Joe

        Thanks for that, much appreciated Will keep digging

        Kind Regards Drago Simic


  157. Hello Joe, my name is Gordon , Derynthia and I are going off camping (tenting) around OZ!!. I have decided to go Solar!! This is a first use of this alternate power, as we have normally hooked up to powered caravan park sites. Joe this is what I would like to happen so I would be pleased for your advice!!
    A. 1 Acemore 200W solar flexible panel charge 12V auxiliary battery. B.1 battery powered box + 12 volt battery. C,600W/1200W pure sine wave inverter 12V-240V. D. Power board use for min appliance for min time!!
    Now Joe I want to connect a 6 amp 7 stage ip 65 battery charger running from power board back to 12v auxiliary battery in battery power box. Joe my question is will the battery charger be able to operate a charge to allow us full use of power 24/7.
    Thank you for your time Gordon

      • Hello Joe, thanks for your quick response! The charger is to be fed from 240v power board which is being fed via inverter!!
        As I do not know how to connect it up to the Solar Panel.!! Very light loads 1 coffee maker 5 min. Lap top. toaster. light maybe.
        All cooking done by portable gas stove!
        Regards Gordon

      • Hello Joe, as I had this idea and realise the charger needs mains power.

        Thank you Gordon

      • Ha ok well if you need mains power then you will be running your toaster and laptop off mains too. You need a solar regulator rated minimum 16A that will allow you to charge your system from the solar panel.

  158. Hi Joe, Thanks for the article. I have electrical needs of 150Amp / day. I plan to buy a 200Amp AGM deep-cycle battery (I know I will discharge 75% of the battery in average but I don’t plan to use that battery for more than 2years). I don’t have enough space on my van to install 3x100W solar panels. As I gonna drive thousands of km, I liked the idea of charging my battery via the alternator! So I gonna buy a 140amp isolator that I’ll install between starting battery and house battery + in-line fuses on positive cables (as close as possible to the battery I read). My question is, is it possible to wire both solar panels and isolator to the house battery so that I can charge it while driving and via solar source when parked? I can’t find the answer as all websites propose either way but not both at the same time.. If I can wire that, do I need a switch to say when the battery shoul be charged via the alternator/solar panel?
    thanks for your help

    • Hey Loucho yes you can do both. Yes you do need a way of turning the solar panels off when the engine is running. I’ve got a canvas cover that goes over my panels. It turns them off and protects from stone chips. A switch would do the same.

  159. Hi outback joe, big thank you for all the great and practical information its been my lifeline as i start to set up my 2014 Hilux. Mine came with a dual battery system set up under bonnet that I will stick with for now. The question I have is my 300W solar feeds into the aux battery via a regulator that I mounted on the panel frame and a long lead with Anderson plugs each end.

    Can I use this same feed in lead and connection to aux battery at night to power my LED light system?? Ive added a fuse to lighter outlet housing for protection.??

    Thanks Oliver

  160. Hi Joe, great article, definitely improved my knowledge about how to go about setting up my 12V system!

    I’ve decided to go with the dual sensing VSR option, so I have a question regarding the correct sized fuses to place between the batteries and VSR. I’ve seen 100A fuses used in 6 AWG/B&S cable runs for this application, but the table in the blog says the current amp rating for this size cable is only 72 Amps? So the 100A fuse is correct here despite the cable being rater lower?


    • Hey Lee it shouldn’t matter much. The cable won’t be operating continuously at close to its limit so it doesn’t need accurate protection against low level overloads. When a serious fault occurs lots of current will flow and the fuse will blow rapidly. Ideally yes the fuse should be sized to protect the cable. But cables are able to resist short term overloads better than fuses. So if for example you want to be able to crank the engine via the cable in question then with a 70A fuse it may blow even though the cable was not in any danger. A compromise is to use a bigger fuse to allow for short term overloads. Then the risk is a low level fault won’t trip the fuse and the cable will slowly get hot, the insulation will eventually fail and cause a serious short after which the fuse blows. In that case the cable is permanently damaged but could have been saved if a smaller fuse was installed. So take your pick, better ability to provide short term overloads or better protection against long term low level overloads.

      • Hi Joe, thanks for the super quick reply!
        That’s what I figured, but just wanted to clarify.

  161. Hi Joe,
    Thanks for the fantastic article. Its really informative and user friendly.
    I’ve got a 2009 Prado with dual batteries and a Piranha DBE180S isolator. I’m planning on getting a 160W solar panel and 20A MPPT or 15A PWM solar controller installed so I can permanently setup a 60L Fridge in the back. As the panel will exceed the solar input limit of the Piranha isolator, would it work if I connect the solar controller straight to the auxiliary battery?

    • Hi Mat yeah that will work, only potential negative is it may not back feed through the isolator to your starter battery if the isolator isn’t dual sensing, so starter battery may not get charged from the solar, which isn’t a big deal.

  162. Hello Joe, this is an awesome article. Not sure if you can help but in my boat I have a 240v charger which has two outputs to charge the house and start batteries simultaneously. This is permanently wired in.

    I want to also put in a vsr between the start and house batteries so that when the alternator is running it will charge both batteries but protect the start battery from running flat when we a running the fridge etc.

    My concern is the vsr will muck up the 249 charging system as it will send the voltage increase from the charger and basically connect the house and start batteries together and not allow the 240v charger to independently charge the batteries properly – is this correct?

    • Hi Luke yeah the VSR might stuff up your charger, shorting out its outputs. It might work, charger might just think it’s got two identical batteries on each of its outputs and set the outputs to match each other. Might not work, could blow up, or could go into an error mode. You could disconnect the 240V charger from the starter battery, get a dual sense VSR and rely on the VSR closing to recharge the starter battery, you’ll be limited to the capacity of the single output from the charger but might not matter if starter battery is not being used and doesn’t require much charge. Another option would be to have a relay that automatically disconnects the VSR whenever 240V is connected.

  163. Hi,
    Great information thank you.
    I have a motor home with 2 aux batteries and 160w solar system factory fitted. I’m building a custom trailer with 1 Aux battery and 160w solar system. Question, can I join these two systems together?

    • Hi Ian yeah they can be joined. You can put the trailer 160W panel in series / parallel with the motor home panels then they both feed the same solar regulator and the output distributes to both motor home and trailer. You can’t put the output of two different solar regulators together since they may be trying to operate at different voltages. If you need the trailer to operate independently as well with its own regulator you need a way of switching between independent mode and motor home connected mode.

  164. Hi Joe, great info thanks.

    I understand that you’re not a Lithium fan but I think I am going to go that route for a number of reasons.
    However I have a question about the alternator charging that I haven’t been able to get a satisfactory answer.
    Given that normal “dumb” alternators were originally designed with lead acid batteries in mind and that the lead acid chemistry only allows moderate charge rate absorption, I am wondering will the alternator be able or capable of delivering the massive currents that top quality LiFePO4 can absorb without the smoke leaking out? I’m mainly concerned about this scenario when the vehicle is at idle.
    ie: I’ve depleted my 200A Lithium overnight and in the morning I decide to start the car and let it idle for an hour because the sun hasn’t come out.
    Will the alternator try to push more amps than it was originally expected to deliver and overheat?

    I watched a video on the Victron site regarding the alternator overheating at low rpm, made me abit concerned.

    My Dmax has a dumb 90A alternator.

    Look forward to your thoughts