Springers Solar guide to 12V electrics: Dual batteries

ByRV DailyApril 26, 2020
Springers Solar guide to 12V electrics: Dual batteries

If you are towing a caravan, setting up the tow vehicle correctly is crucial for a functioning battery system in your van

One of the first modifications we do to a vehicle will often be the addition of a dual-battery system. There are three different types of dual-battery systems that are of interest to the vehicle-based traveller. Each has its unique needs and complexities. You may have a combination of different dual batteries at the same time. Let’s begin with the starter battery and the engine alternator.

Isolate that starter battery

Cold beer is great, but if your starter battery goes flat in the process, getting home could be a problem. The first thing you want to do with any dual battery system is to isolate the starter battery from any draw from your accessories. Fridges are one of the biggest draws on energy from a battery.

The easiest way to do this is with a simple isolator like the Redarc Smart Start SBI, which is basically a voltage sensitive relay that activates a contact to either link or unlink the start and auxiliary batteries. If the SBI detects a draw being made on the auxiliary battery, it will disconnect the starter battery so that it retains its charge for engine starting.

The problem with simple voltage isolators is that most modern vehicles have ‘smart’ alternators in them. In these cases, the voltage often won’t get high enough to engage the simple isolators, so we need something more sophisticated to charge the auxiliary batteries properly.

DC2DC chargers

These more sophisticated chargers are triggered by the ignition and a lower start voltage, and have the added benefit of different charging algorithms to ensure the auxiliary batteries get the correct amount and type of charge to keep them healthy. In addition to this, many DC2DC chargers have a solar input. The chargers will come with different current ratings, range from 25 amps to 40 amps, and will control how much charge is sent to your auxiliary batteries.

Cable size matters

Don’t skimp on the cable size. Cable size really matters and it is critical to have the correct size cable between the components of a dual-battery system. The number of amps and the distance of run will greatly affect what cable size you need. Basically, the heavier the draw, the thicker the cable required.

There are formulas that can be used to work out the cable size, but I find a chart a little simpler. Here are some steps to work through:

  1. First off, decide what current flow in amps is required.
  2. Is the circuit going to be a non-critical one (e.g. lighting), or will it be a critical one (e.g. fridge)?
  3. How far does the cable need to run? Remember that the cable probably won’t run in a straight line. When you have got that distance in metres, you need to double the number, as the circuit runs from the battery to the accessory and then back again.
  4. Once you have decided on these, look up the chart to find where they all intersect. Match the size/colour of the cable in the main table with the size/colour in the smaller table below. This will give you different naming standards of cable, so you will know what to look for in your local cable supply shop. Simple, right? Go back to step 1 if I have lost you.
  5. And just remember that measurements of cable diameter and cross sections does not include the insulation.

Under-bonnet dual battery systems

This is probably the simplest and easiest type of dual-battery system. Most 4WDs and tow vehicles will have room for an auxiliary battery under the bonnet. One of the issues of an under-bonnet installation is the heat of the engine, so choice of battery is important for your particular needs. It is worth checking the warranty information as certain batteries and brands may limit their warranty if placed under the bonnet.

Here we have the DC2DC charger built into the rear of the vehicle so the battery can be removed when not required

In-vehicle systems

Utes and 4WD wagons will generally have enough space for a second battery in the vehicle or on the tray. Sometimes these dual-battery systems are built into drawer systems so they are out of the way. For those not needing a dual battery all the time, there are different options for portable batteries. These can be all-in-one units or a simple battery box with connections to the charging circuit.

Space to spare

Caravans and trailers will often have multiple batteries but do have the space to carry larger systems. More and more vans and trailers are requiring larger systems. The most important thing here, is to make sure that the tow vehicle is set up correctly to provide charge for these larger systems. This is certainly not the time to skimp on cable size. Long cable runs and heavy energy draw will require heavy cables.

Headache or happiness?

Get your dual-battery system wrong and you will be in for a few headaches as beer gets warm and food goes off.

If you are intending to install a dual-battery system, spend the time and money to get it right the first time. These systems are often hidden and run in the background, but when they work well, travelling becomes a much more enjoyable experience.

Words and images Gary Tischer.

Main pic Anthony Kilner.