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Workshop: Dual-battery systems

Auxiliary batteries are used to operate appliances while you are away from mains 240V power, without running the risk of compromising your vehicle’s main starting battery.

How to install a dual battery system in a vehicle?

People have been using and charging auxiliary batteries in their cars and caravans with varying degrees of success for over a century. For most of us, the days are long gone when all we needed from our auxiliary battery was a small amount of power for a bit of lighting, and maybe a radio.

These days, we expect much more. We like to run lights, TVs, DVDs, pumps, fridges, and winches, charge camera batteries and mobile phones, and even power microwaves. The fact is that some of us like all the luxuries of home when we go ‘camping’.

So, over the years, our demands on the humble auxiliary battery have increased to the point that we need much more usable energy and over a longer period of time − and we need fast and safe ways to recharge it.

Many of the ways that were used to charge these batteries are now antiquated and they almost never did the job well, anyway; and neither were they conducive to long battery life.

So, let’s look at what we need to build a system that will deliver the best performance for our hard-earned rands.

What is needed for a 4×4 dual battery setup?

I guess a good place to start would be to have an auxiliary battery; then we need an efficient way of charging this battery using the car’s alternator (or other sources if necessary), and, most importantly, a way of isolating it from your starting battery so that it is not compromised. To do this, we use a battery isolator; and then all that’s left is some cable to connect it all up. Simple, if you use the right bits and put it together the right way…

Auxiliary Batteries

Basically, you should be thinking more about how big a battery you can fit, and less about how small and cheap a battery you can get away with. Fact is, the larger the battery, the less percentage-wise you will be draining it, and the longer it will last both in power usage and battery life. And this comes with the extra advantage of more capacity when needed. So, that means looking at the bigger and heavier 140Ah options, rather than settling for a default 105Ah deep-cycle or high-cycle battery.

This is a complex topic, and the best solutions will depend on your typical usage pattern. One vital fact is that batteries are at their optimum when fully charged; they DON’T like being discharged at all; they prefer never to be discharged below 50% of their capacity, and they thrive on being topped up soon after. They will lose capacity and die quickly if left discharged for long periods of time.


The battery isolator

The battery isolator is a device that allows the car’s alternator to recharge the auxiliary battery while protecting the starting battery from discharging. If these batteries are simply linked together, you run the very real risk of draining them both to a level at which you cannot start the engine.

Hence we have the battery isolator. There are many different methods used to isolate the batteries; most use some form of solenoid, which is an electronic switch that can handle larger (charging) currents than a standard switch can. They are operated by applying a small amount of power to them which actuates a solenoid that links the batteries together.

Manual isolator: The cheapest way is to turn the isolator on and off manually. I call these IDIOT switches, because sooner or later someone will forget to turn the switch on (and then no charge goes into the auxiliary battery), or off at the right time, (and then the starting battery gets drained as well). This has, in the past, been the cheap way of doing it.

Smart isolator: I believe the best way for most situations is to use a VSR (voltage sensitive relay) activated solenoid. The one I like best, and use, is one of the most inexpensive ones – it does the job very well.

When you start your engine, the VSR works by first letting your battery recover its charge − up to, say, 13.2 volts. At this point, the battery has about as much charge in it as it’s going to get. Once the VSR sees this 13.2 volts, it closes the contacts on the solenoid, linking both batteries together for charging purposes

The next thing the VSR looks for is 12.5 volts; in other words, you have shut down your engine and started to draw power from the batteries, so very quickly the voltage across both batteries will fall to 12.5 volts. As soon as the VSR sees this voltage, it will open the solenoid contact and isolate the batteries, leaving your starting battery for starting and your auxiliary battery for other duties.

You will be looking for a smart isolator with a high surge capacity (four times rating), silver contacts (better than copper) and spike protection to avoid damage to your car’s sensitive electronics.

One rider to the use of VSR units is that they are not all sophisticated enough to maintain charge in vehicles equipped with ECU-controlled variable voltage and temperature controlled alternators, which are increasingly found in many modern 4x4s. Not all of the modern DC-to-DC controllers are able to deal with these variables, either − so research both the vehicle’s alternator type and the abilities of the VCR before installing.

Wiring and cabling

This is, without a doubt, where many people get it wrong and end up with a system that may work, but that is nowhere near as efficient as it could be, and should be. Doing it right can double the usable capacity of your auxiliary batteries over and above what many people normally have, and help them to charge much faster and last longer, too.

Understand that when you charge from the vehicle alternator, one of your biggest enemies is voltage drop. You need to have the highest possible voltage being supplied to the auxiliary battery, allowing it to charge as quickly and as fully as possible. (Obviously you want to have the alternator supplying a decent charge too: above 13.7V, and preferable closer to 14.0V – again determined by the manufacturer’s rating on the car’s electronics.)

I regularly see set-ups fitted by so-called ‘professionals’ where twin 6mm auto cable (really 4.59mm²) is used. This is next to useless for fast, deep charging.

A very basic explanation would be that the larger the current draw, the larger the wire (cable) needs to be. And the further the distance the wire needs to run, the thicker the wire needs to be.

Water and power analogies work well here. If, for example, you want to fill a bucket of water fast, do you use a hose the size of a straw, or a 25mm garden hose?

I always suggest a minimum of 6 B&S (13.5mm²) cable for linking batteries. Larger is even better, but could be overkill on smaller setups, and it is not cheap.



Putting in fuses is necessary because of the potential of a short-circuit. Because we are installing heavy cable able to transfer large amounts of current quickly, the downside is that any short-circuit between chafed cables or to the vehicle’s bodywork will produce huge amounts of flame and spark. 100A fuses are purely to protect the cable − more correctly, you and your vehicle. We need one at each battery because each battery will be connected to the same cable.

As I’ve had a few failures of the 100A Maxi low amperage automotive fuses (melting because of the high resistance), I now prefer to use the Mega (bolt-down high-amperage) fuse which has been used for many years when connecting inverters to batteries.

Battery placement

A very important thing to consider is where you are going to place your batteries. Standard open wet-cell batteries give off hydrogen gas when they are charging and discharging. This is a very explosive gas − remember the Hindenburg. You won’t have a problem if you are putting the auxiliary battery under the bonnet of your vehicle, or mounting it somewhere on the chassis with good ventilation to the open air.

But, pushing the battery under a seat or in a cupboard is not only far from ideal, it’s downright dangerous. Even worse, some people place battery chargers and even inverters next to these batteries. Just think about that for a moment: a hydrogen-producing device next to a possible spark producing device…

Wet cell batteries must be housed in a properly vented area, with the venting externally, away from any ignition source. Hydrogen gas rises, so sealed battery enclosures should be vented externally top and bottom.

If you need to place batteries inside the vehicle, then you should seriously consider sealed AGM (Absorbed Glass Mat), or lead-crystal or gel batteries. These batteries, if placed in an enclosure, only need to be vented to atmosphere, and not necessarily vented externally.

Remember that gel batteries do not handle heat very well. Putting AGM and lead-crystal batteries under the car bonnet is fine as far as safety goes. The only consideration here is that they are fully sealed batteries, and as such should be kept away from the extreme temperatures of turbochargers.

How to wire it all up

This is much simpler than most people think; and, as long as you take care, almost anyone can do it.

When running the cables between the batteries, you should be very careful about where you place the cable, and make sure that it won’t rub or be cut through. Also be careful to keep it clear of sharp corners. If the auxiliary battery is going to be placed anywhere other than under the bonnet, you should seriously consider using a protective covering like split convoluted tubing, and secure it well with clips or cable ties at regular intervals.

If your auxiliary battery is in a caravan or trailer, then the standard trailer light connectors are way too small for the 6 B&S cable and can’t even come close to handling the high currents for effective battery charging. Here one must use 50A Anderson plugs.

All cable lugs used with the 6 B&S cable are far too big to be crimped with standard crimping tools; they should be crimped by an auto electrician. Any soldering should be done after this, using a gas torch; charging generates a lot of heat, enough to melt solder.

Do not install the MEGA fuses until the last, when you have completely finished all the cable runs and tightened up all other connections first.

Lastly, note that some people use the chassis for earth return, but my advice is always to run the earth/negative cable all the way between the start and auxiliary batteries. It really is worth the extra few rand in cable, and you have to do it only once, so do it right the first time.

Which battery?

Cranking Batteries

Cranking battery

  1. Are typical automotive use
  2. Can supply large current
  3. Can accept large charge current (can be charged quickly)
  4. Do not like to be discharged deeply (life is shortened)

Deep-cycle Batteries

Deep-cycle Batteries

  1. Are more suited to deep discharging (more cycles)
  2. Cannot supply very high current (not suited to winching); made for low current draw over a long period
  3. Cannot be charged as quickly
  4. Should not be discharged more than about 50% (not below 50% state of charge)

High-cycle Batteries

High-cycle Batteries

  1. Are typically used for standby applications (UPS)
  2. Can supply more current than deep cycle
  3. Can be recharged more quickly than deep cycle
  4. Should not be discharged more than about 25% (not below 75% of charge)
  5. Withstand deep discharging (up to 50% capacity) better than cranking batteries

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