Typically in a dual battery system there is the:
Auxiliary - Used for running the fridge and external lighting etc and generally it would be suggested this would be a deep cycle.
Main/Starter - normal lead acid for running the car.
They are linked together by a solenoid, DC-DC charger and the most popular one seems to be the Redarc smart charger, which links the batteries when a preset voltage is exceeded and is the simplest to install as extra wires etc are not required. It is charged by the alternator. Im not a fan of the smart unit on the side of the Redarc solenoid because it will always pull the voltage down with the energy required to keep the solenoid powered unit the voltage drops - I would prefer to keep battery voltage at max when parked.
The drawback in the aux/main setup is that (like happened to me) you can setup in a remote location somewhere for a few days and the auxiliary is drawn down by the fridge and a power drain on the cruiser. At this point I couldn’t see why on earth the above setup is any good so considered a way to isolate the starter battery.
Dual battery setup in the Silver 105 Landcruiser.
This suits me as I tend to only stop in one place for a max of two nights and the fridge (running off the car) is a 40L Engel. The camper trailer has its own fridge with two deep cycle batteries and a solar charger.
Cruiser fitted with two normal lead acid batteries under the bonnet, which are actually the green Amarons. Individually defined as “Starter Battery” and “Main battery”, where the main battery runs everything in the vehicle when the key is turned off. The alternator is connected to the main battery.
The starter battery is isolated from any current draw when the ignition is off as it is connected:
1. To the starter motor (solenoid)
2. To the solenoid with a heavy lead
3. To the switch in the cabin to access power to close the solenoid – described later.
The two batteries are linked together with a basic solenoid, which is closed when powered from the ignition when the engine is starting, running and charging. IF the main battery is completely dead (or removed), the solenoid can be closed by taking power from the starter battery to link the solenoid.
In-cabin control over the solenoid is done using a three position switch, positions being:
Up – default position where both batteries are linked while starting or if the engine is running. Gets power from the ignition supply.
Middle – no connection which is useful is you want to isolate the starter battery such as if your alternator dies and you need to preserve the starter battery. This wire is connected to the solenoid activation.
Down – (momentary so it will release unless being held) takes power from the starter battery.
We mount the solenoid close to the positive terminal on the passenger side on a bracket off the inner guard so the leads can be moved onto the solenoid posted with a minimum of fuss. For FZJ105 or late model cruisers the fused/fusable links block should be moved around onto the solenoid and the starter wire left on the battery terminal. An extra short lead should be added to the battery terminal and go to the other side of the solenoid.
Other Considerations:
I don’t like mounting a third battery in the engine bay as the inner guards aren’t strong enough to support them long term when belting down dirt tracks. If I needed the extra power storage then this would be lithium mounted in the back of the wagon charged by an integrated solar charger/ DC-DC unit.
For solar chargers the minimum requirements should be:
1. Maximum power point tracking (MPPT), which is 15-30% more efficient than other units. It doesn’t move the panel, just adjusts to get more current when the panel voltage rises.
2. Prioritises solar input even when input from alternator is possible. Older systems would stop using solar when the engine was running.
3. The panel must not be partially covered, such as under a roof rack as this will significantly reduce its output.
There are a number of battery options. The common types are deep cycle or normal lead acid, but Lithium batteries are coming down in price so worth considering as alternatives for deep cycle as the weight is significantly lower than lead batteries.
Deep cycle batteries
PROS - are designed to be better at longer discharging whereas a normal lead acid battery likes to be kept fully charged to get a long life out of them. They are ideal for caravans and campers to run your fridge and other devices
CONS - Deep cycle batteries like to have a specific charge method and are best charged using a DC -DC regulator/controller instead of directly from the alternator. If you run a deep cycle down over a few days then to charge off the vehicle you will need to drive for a few hours unless you have an expensive high amp rateed DC-DC.
Most deep cycle batteries are not rated to effectively charge when hot (ie engine bay) and you will not get a good life unless mounted elsewhere.
If you expect to be stationary for a few days then a deep cycle system with solar chargers are a good idea.