a camping trip of ridiculous proportions

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

last updated 16/03/2015

Need help deciding how you’re going to power your beer fridge? This is a design guide for 12V systems used in vehicle setups for touring and camping. I have an electrical background and heaps of experience camping and touring with 12V systems so this guide is a product of that background and experience. It is not a guide on how to actually wire up equipment, terminate cables, use a soldering iron etc. The purpose is to help 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 or 4WD vehicle. The arrangement provides additional battery capacity to allow appliances like fridges and lighting to be powered, and reduces the load on the starting battery, preventing it from becoming depleted so that it will be able to crank your engine over whenever you need it. 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 or trailer, does not affect 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 of the same type and the same capacity, so that they have similar voltage profiles and charge / discharge at similar rates, but this is not absolutely necessary. All lead acids behave fairly similarly. When connected in parallel, batteries do not need to be of the same capacity, as charge and discharge current will naturally distribute between them according to their capacity. A larger battery will sink more current when charging as it pulls down the charging voltage. A larger battery will supply more current 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, almost identical. So it can be done without any significant 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 cycling 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

–        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 C 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 http://batteryuniversity.com

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. Ideally every cable feed should be individually protected, but this introduces complexity. The design is a trade-off between complexity and risk mitigation.

–        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 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 etc is adequate. 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 they 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 to DC current, divide power by 12 and multiply by 1.1 to take into account inverter efficiency. 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? 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 conservative 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 on 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 closed, 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. 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 also gives your starting battery an occassional higher charge voltage above the 13.8V 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) are 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 perfect. It does not charge according to optimized charging profiles that reduce sulfation and improve battery life. It does not provide an adjustable topping charge voltage, and will take a long time to charge the final few % of a battery’s capacity. It does not allow adequate charging of a battery with unusual float and cycle voltage ratings. A solution is to use a DC-DC converter. This is an electronic charger that is powered by your alternator and is set to charge your auxiliary battery usually to around 14.5V. This charges your battery faster in the final stages which may offer slightly higher state of charge. The higher voltage also reduces sulfation. A further advantage is that it separates the charging between your starter and auxiliary batteries, so you can use batteries with completely different charging profiles.

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 sink 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 into the battery 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 you don’t need it, for example when going on long drives where your battery becomes fully charged or when using your vehicle as a daily driver. This increases grid corrosion and reduces battery life. When not cycling the battery, it should be kept at a reduced voltage. This is indicated on battery datasheets as float charge, and is usually specified at 13.8V. The alternator voltage is chosen for a reason – it is the best float charge voltage for most batteries. A charge to 13.8V will minimize grid corrosion but allow some sulfation to occur whereas charging to 14.5V reduces sulfation but increases grid corrosion. Sulfation can be partially reversed by charging to an elevated voltage, grid corrosion cannot be reversed. Overcharge is made worse if you have loads supplied by the DC-DC converter. 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 be slowly depleted 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.
  • 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 the hilux since I use VSRs.
  • Too slow 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 as an aside from the main guide. 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. It’s not true. I think it originated from DC-DC converter vendors and perpetuates with those who have one in their setup. 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. 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, the alternator will be charging over twice as fast as a DC-DC converter – 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.

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 practise 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

Modern Variable Voltage Alternators

Some modern vehicles have engine management controlled alternators. The alternator output voltage is controlled by the vehicle’s engine management to help reduce fuel consumption. Basically they’re designed to charge less whilst idling or accelerating and charge more whilst decelerating (recovering the kinetic energy of the vehicle to charge the battery). Overall the net current between charging more and charging less should be about the same as a traditional fixed voltage alternator, keeping the starter battery fully charged. If it keeps the starter battery fully charged then it should also keep the auxiliary battery fully charged.

If the alternator is spending too much time in the “charging less” phase then it’s possible it will not fully charge your auxiliary battery or it may take too long to fully charge it. However this scenario is circumvented by load sensing – if the system senses load above a certain threshold it will permanently remain in the charging more phase. This is to keep the starter battery charged if say the headlights are on. Note that auxiliary loads must be connected correctly to ensure they pass through the current sensing instrument.

Assuming that the alternator does actually spend too much time in the charging less phase, then it would be useful to have a DC-DC converter which will boost the voltage and always attempt to quickly charge the auxiliary and keep it fully charged. I have no experience with variable voltage alternators so I cannot say how much extra capacity will be garnered through using a DC-DC converter, if any. Obviously people who make money from selling DC-DC converters will claim that the benefit is significant but I have not seen any substantiated claims backed by experimental evidence or that adequately explain why the factory system would not maintain charge. The factory system has to cater for high loads for when the headlights are on and the air-con fan is cranking and the stereo is turned up and the demister is on. It can’t tell whether a high load is from your headlights or from charging an auxiliary battery – it’s always going to cater to the high load regardless.

If you are having problems with variable voltage alternators, the variable voltage mode can be disabled so it becomes a fixed voltage alternator. Dealerships or auto-electricians should be able to do this. If a variable voltage alternator is producing problems with your dual battery system this is a good solution rather than forking out for a DC-DC converter that may not fix the problem.

Charging 12V Systems from Mains

There are many 12V battery chargers available. Less sophisticated chargers are simply fixed unregulated voltage power supplies. These are ok, but do not offer optimized charging profiles to minimize sulfation and grid corrosion, 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.

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 sulphur 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 via a switch 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.

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 its ice cold when you leave.

Charging 12V Systems with Generators

Many generators have a 12V output for charging 12V batteries. Usually the current capacity is quite low – around 8 amps. So charging from a generator using it’s 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 240V AC charger.  Don’t use the generator’s 12V supply. Get a regulated charger and plug it into the generator’s 240V 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. These are specialized 12V generators. Capable of delivering around 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 http://batteryuniversity.com

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 (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 regulator is not 100% efficient? What about the variation in load from the fridges 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. Assuming you run your system 24 hours a day 365 days a year, if your fudge factor is 1, then you might run out of power most of the time. If it’s 1.2 you might run out of power 50 days a year. If it’s 1.5 you might run out of power 10 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 1.2. So in our example it changes:

Solar Panel Rating = 1589 / 7 x 1.2 = 272W

For touring with vehicles, a good sized solar panel is around 80W. 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. Assuming 80W panels, we need:

Number of solar panels = 272 / 80 = 3.4

So you’d need 4 x 80W panels. You could also use  3 x 100W panels or you’d almost get away with 2 x 130W 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 1.2 fudge factor, the calculation would be:

1589 / 1.5 * 1.2 = 1271W

So you would need around 16 x 80W 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 38 degrees and the night time minimum 30 degrees. It might stay above 35 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 1.6.

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 isolator 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 state of charge.

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 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.


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 5 for electric motors: 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 offer a smoother output closer in profile to the waveform on a mains system. 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 a few 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 will max out at around 1100W. Slightly higher peak power could be attainable.

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 or slightly more. Let’s say 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 starting to become 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!

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 insulator resistance to heat. 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 – this yields a 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

Acceptable 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 touching according to Australian Standards), typical resistivity, and a volt drop calculation to provide a guide for cable sizing. 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 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. Terminals should be soldered rather than just crimped. This provides a better connection and more tolerance to vibration from driving a vehicle, especially when over rough surfaces. 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, 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. When you are not cycling the battery, there is no point charging at 14.5 volts. It causes 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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111 thoughts on “Design Guide for 12V Systems – Dual Batteries, Solar Panels and Inverters

  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.

  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 agms..you 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 no electrical loads then you might be using say 5 amps or about 60W running the engine electrics. 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. When you first start on a discharged battery the current will be higher for a bit whilst charging and you might save another percent but that diminishes once the charge rate drops off. 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. This might save you a couple of % 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 ;-)

    • Yo Michel nice one 6 months around oz ay. I’ll shoot you an email mate. Hiking is a really good way to go, me and the wife want to get into that a bit more.

      • 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 :D

  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).

  40. 300w

    • 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.

  41. 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.


  42. What i meant about the charger as rectifier was, can a charger supply DC direct to loads, bypassing the battery altogether?

    Would a product like this do the job:


    Seems pretty pricey.. is it worth it for what it does?

    It’s been said before, but cheers for the great site and clear explanations

    • 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.

  43. 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.


  44. 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 http://outbackjoe.com/macho-divertissement/our-hilux-setup/hilux-after-market-components/. 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.


  45. 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 rod67c@gmail.com



  46. 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,


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