If you can afford to do only one single modification on your vehicle, a dual battery system must be the number one on your list.

Without electrics, a vehicle is stranded and a battery failure is something that happens in regular intervals on any car, new or old. If you get more than 3 years of life out of a unit, consider yourself lucky. Consider battery failure as a given, and unpredictable.

Batteries can fail in a number of ways, including instant death and of course by your own actions like forgetting the lights on.

If there is no battery to charge or act as buffer, the alternator may have difficulty to regulate voltage, which could under certain conditions fry vehicle electronics.

Like always, I stick to keep it simple and did not waste any money on dual battery controllers. Dual battery controllers ads little if any benefit in terms of charging the batteries and the little capacity indicator is not actually telling you that much. For the few hundred rand of a dual battery controller, rather buy yourself a third battery.

The tutorial below is not meant to be step-by step set of instructions. It is meant to give you pointers to the important things that you should know and to encourage you to do the necessary reading work in order to know enough that you can make good decisions. Their is nothing more dangerous than doing things like a parrot.

Battery Basics

In South Africa, all vehicles are using the negative earth system. This means that the body and chassis of the vehicle is connected to the negative pole of the battery and any negative wire to a device, can be connected to the body or chassis, for the current to flow.

The positive wires can under no circumstances touch the body, because that will create a short circuit. This is why wires MUST have in-line fuses. Cars indeed have whole fuse boxes dedicated for this purpose. Without a fuse when something goes wrong like when the insulation shaved through, the wire will simply heat up and will easily glow red or melt in a matter of seconds. This is a major reason for vehicles set on fire - hence the use of fuses are mandatory.

Battery capacity is expressed in Ampere Hour (Ah). This means a battery can deliver a certain current for an hour long before the battery is depleted. There is a lot of technicalities to understand about batteries and for now, lets just say that if there is a 70 Ah label on your battery, it does NOT mean that it can deliver a full 70 A for 1 hour. It usually is much less under heavy current. Under light loads however - say at 5A drain, it will last 70 Ah / 5 A = 14 hours of constant drain.

If at all you can fit a 105 Ah battery, do so. That is usually good enough to take most families through a camping holiday if you are not driving each day, and without going bonkers by investing in solar panels for example. It may need a few minutes of idling the vehicle on a day. Due to space, I could never fit anything more than a 70 Ah battery and it works, but need the vehicle to be started at least every second day.

Finding Space

Finding space for a second battery, can be quite a challenge in some vehicles. Usually with the older vehicles, there is more open space than on the modern vehicles and it is less of a headache.

Often, the preferred place for a second battery is in the engine compartment because the thick wires can be made shorter (thick wire is pretty expensive), and messy things like battery acid which may be expelled from a faulty battery, can happen in the most suitable place.

For all my vehicles, I managed to find space in the engine compartment. For the Nissan Safari I had to make a pan and bracket for the battery and for the Land Rover Discovery 1, a space is already there.

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On vehicles like the Land Rover Defender, there is huge space under the passenger seat intended for a second battery - right next to the main battery.

In some cases it may be convenient to have a second battery in a removable box, which can be plugged into the vehicle supply, when needed.

Best Practice

In order to make a dual battery system work, there are a few simple, but vital rules to follow.

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If you do not know anything about electricity and wiring, it is highly recommended that you do a bit of homework and spend a weekend reading so that you get on top of the basics. This is in any case vital knowledge to have in the bundus. Make sure you know enough that you can comfortably use a multi-meter and understands what the figures means. I use a Major-Tech MT-777 meter that can measure DC-amperage by simply clipping the tong around the wire. This is a great function to have when troubleshooting vehicle electrics. DC-current clamp on testers are quite a bit more expensive than the ones that can only measure AC current, but you can find smaller Major-Tech models for below R 1000. This is a highly recommended investment.

Next, which ever position you decide on for your battery - fixed or portable, it is essential that the battery be securely fastened. Batteries are very heavy and on rough roads where vibrations and movements are a given, a poorly fixed battery will simply detach or even launch itself and will cause considerable damage to the vehicle or even serious injury or death to passengers.

Wwires and connections for the installation, should be done properly. As a general rule, don't go any thinner than the starter cable for bridging the two batteries or the earth cable. If you need to make up cables - which is often the case, use only soldered lugs on all ends and for in-line fuses. It is often easy to find battery cables with different ends and in different lengths from spares shops.

Thick cable should be installed and fixed in plastic sprague tubing. This helps a lot to protect the wire insulation from shaving through and possibly making a short circuit. Sprague tubing usually is used standard on many vehicles and is easily recognised by the ribbed walls it has.

Make sure the wire is kept far enough from the exhaust manifold in particular. If for some reason you have no choice but to run it close to the exhaust manifold, make sure the wire is wrapped with heat shield materials like aluminium spraque or woven asbestos tube. It is highly recommended that you do regular inspection of such pieces of wire after long trips to ensure that the heat shield is effective.

Tools and Materials

In order to do the installation, you will need the following tools and materials:

  1. Cable cuter / Hack saw to cut thick wires.
  2. Carpet knife, to strip off insulation.
  3. Small MAP-gas torch - BernzOMatic is a popular, affordable and high quality make.
  4. Lead Soldering (Acid-Core).
  5. Battery Cable, Red and Black.
  6. Lugs for the above. This can be either eye-lugs or battery terminal connectors.
  7. Crimping tool for use with small wires and lugs (The blue, red and yellow ones).
  8. Cole-Hersee Solenoid.
  9. In-Line Fuses
  10. Anderson plugs if needed.

Connection

The diagram below is the most basic diagram of a dual battery system and can be modified as your need is.

For my Safari's, it was most convenient to rewire my second battery as the "main battery" that powers the fridge and vehicle when stationary, but on the Land Rover it was easier to user the second battery as the starting battery. There is no fundamental difference between the two configurations.

On the Land Rover, I also connected the main battery that will power the fridge when stationary in such a way that I can connect the battery from the trailer with the Anderson Plug at the tow bar to help when stationary. This is a simple variation with just another thick wire from the positive pole of the main battery with an in-line fuse close to the battery pole.

Dual BatteryDiagram

Regular or Deep Cycle?

This question is now debated for years on 4x4 forums. Should the second battery be a regular or a deep cycle battery?

Well, this is entirely depending on your situation. The most important thing is the voltage at which your alternator charges the battery.

Ordinary starting batteries are quite happy to charge at 14.0V and that will give them a 100% charge.

However, deep cycle batteries CANNOT be fully charged at 14.0V. They need 14.4V to get fully charged.

If a battery charging system cannot manage to fully charge a battery, the battery fluid will start to make sulphate crystals, which over time reduce the capacity of the battery until the battery is useless. This is a non reversible process.

So, if the alternator your vehicle charges at only 14.0V, you need to make a plan. On Bosch alternators, it is often possible to easily change the regulator to a 14.4V one, but on most other makes, one may need considerably more stripping work and some expertise, to change the charging voltage.

On my Land Rover which has a Magenti-Marelli Alternator for example, it will require considerable labour to change it to 14.4V charging voltage.

I therefore decided for now at least, to make my main battery a normal starting battery. I know from experience that if a deep cycle battery is not fully charged, it gives poor performance and I lost more than one after a single 3-weeks holiday of hard work - because it could not get fully charged.

There are also fancy battery management systems from companies like Bushpower, which have the ability to charge a starting battery at 14.0V and the deep cycle battery at 14.4V at the same time, but that too requires manually removing and bringing out wires from the alternator to the unit. It will set you back the price of a couple of batteries and is only worth it if you are lucky enough to have a number of tours per year.