Dual battery systems explained

A dual battery system is essential for anyone who wants to power their gear – without the risk of running a battery flat – while out in the scrub.

Dual battery systems explained

For most of us who enjoy camping and fourwheel drive touring, it’s essential to have a portable fridge.

To keep the fridge running while the vehicle is parked, you need plenty of reserve battery power. However, for many people, adequate electrical capacity is not just a convenience that helps keep your beer and food cool. It is also an essential requirement for work: it powers equipment such as safety lights and radios when a vehicle is stationary.

So a dual battery system is a must. There are a few ways the second battery can be wired into the vehicle, depending on its intended use and the system flexibility required. But no matter what, the second battery must be installed and wired – in the engine bay, the passenger compartment or load area – so that it can be isolated and doesn’t flatten the vehicle’s starter battery.

Dual battery systems
SOME engine bays are designed to host a secondary battery, like this Toyota. Others are more tight for space, so you’ll need to figure out a spot that’ll take its weight.

The final requirement of a dual battery system is for the second (or accessory) battery to be easily charged by the vehicle when you’re driving or the engine is running. This will ensure it’s charged and eady to use when you arrive at the next campsite or job.

Installing a second battery

Most vehicles are built with one battery, so installing a second battery to power requires extra hardware – this usually includes a strong fabricated steel tray fitted in the vehicle’s engine bay. To carry 15kg (or more) of battery in tough off-road conditions, the tray must be heavy-duty to prevent vibrations – especially vibrations that come via corrugations – from destroying it.

Many Aussie equipment manufacturers, such as ARB, TJM, and Piranha design and manufacture trays for popular 4x4s. These companies have extensive experience, and they make tough and durable products that place the battery’s weight into strong sections of the vehicle body shell to reduce the chance of sheet metal cracking or tearing. Most will use factory-provided installation points and several stays to provide plenty of stability and support.

Battery in traysSome 4x4 vehicles have pre-installed but unused locations ideal for second battery installation. For instance, most diesel Toyota Prados have two batteries, while petrol Prados have one plus a strong factory-engineered foundation for a second battery. However, many vehicles – especially more recent models – don’t have spare room in the engine bay, so the second battery must be located elsewhere in the vehicle, such as in or under a load tray or in the passenger compartment. This means you need to pay extra attention to the wiring and battery type chosen.

The fitment of an accessory battery tray can often be achieved by a competent handyman, although vehicle components – such as a power steering reservoir or an air-conditioner pipe – may require tweaking or relocation to provide space for a second battery tray. No matter what, batteries hate heat, so batteries should be installed as far away as possible from hot engine components; preferably on the cold side of the engine, opposite the exhaust.

No vehicle will endure a 15kg battery hanging from an inner mudguard with two small bolts or screws. Fasteners must be adequately sized and have large washers to spread the load across the panel work (such as an inner guard). You should also regularly inspect the area for cracks or loosening.

Damage of this type, usually resulting from cheap, poor quality products have wrecked many outback treks.

What battery does what?

In most dual-battery installations, the second battery’s task is to power the accessories (such as fridges), and it’s for this reason that a deep-cycle or dual-purpose battery is often installed. The vehicle’s starting battery and its wiring remain largely unmodified for starting and powering the vehicle and its equipment (such as radios and interior lights), while the second battery happily powers the fridge when the vehicle is parked.

Because of the lack of space in some engine bays, the second battery may have to be physically smaller than the vehicle’s original – too small to be useful for powering a fridge.

fridges can drain battery
A FRIDGE can drain a lot of power. So having a deap-cycle battery nearby makes sense... The only thing worse than a warm beer is a car that won’t start.

The genuine accessory dual battery system for the Mazda BT-50 mounts the auxilliary battery in the tray and uses a Redarc solenoid in the engine bay for charging managment.

Situations like this require some lateral thinking from the installer or operator; the aftermarket battery tray may need to be configured to carry a small back-up/ starting battery, with the vehicle’s original battery (or a replacement dual-purpose/deepcycle battery installed on the original tray) powering the fridge.

Vehicles with factory-fit twin batteries – such as many diesel Land Cruisers, Prados and later D22 Nissans – can be easily re-configured to have a starter and accessory battery, too, rather than twin starters. Some people wrongly believe they’re looking at a factory-fitted, trek-ready dual-battery system when they lift the bonnet of these vehicles. They’re not. But the original wiring of these vehicles can usually be easily reconfigured to accommodate a proper dual-battery system.

Many vehicles’ standard battery positions will house a larger-than-standard battery. Kilo for kilo, a larger lead-acid battery will provide power for longer than a smaller one, and a vehicle’s factory battery location is often stronger than even the best aftermarket tray.

Keeping them charged

At its simplest, a second battery can be wired into the vehicle with some appropriately sized wire (and a circuit breaker or fuse for safety) and a simple high-amperage on/off switch. This allows the second battery to be charged while you’re driving, and disconnected (isolated) when it’s time to set up camp and operate the fridge and camp lights – without running the vehicle’s main battery flat.

The next day, after the vehicle is started, the switch connects the second battery to the vehicle’s electrical system, allowing it to be charged from the alternator. This provides a dead-set simple and reliable way of doing things and has worked for generations of outback travellers.

Projecta -chargerBut there are drawbacks to this method, the biggest being its susceptibility to operator error: the driver needs to remember to flick the switch to either isolate or charge the accessory battery. That’s no biggie if you have the discipline to lift the bonnet to check oil and fluid levels each morning – as you should!

Another drawback is that the entertainment system and interior lights continue to operate from the main battery, which exposes it to the risk of being run flat with excess or careless use.

Automatic solenoid switching

Wired to the vehicle’s ignition system, a solenoid will automatically disconnect the accessory battery when the ignition is turned off. It reconnects the second battery to the vehicle’s main battery (to allow charging) when the engine is running. This way, you don’t need to remember to switch-out the accessory battery at each stop-over, and you won’t risk flattening both batteries.

4x4-grade solenoids generally have higher-amperage wiring to the second battery (often of a similar size to the vehicle’s starter cables – in fact, many solenoids used for dual-battery tasks are the same tech as some older-vehicle starting solenoids), so there’s little risk of frying wires. The second battery can be used to jumpstart the vehicle, if required, by overriding the solenoid.

Piranha dual battery management system Piranha’s DBE 180-SX is a 180Amp isolator that charges the auxiliary battery.

One minor drawback of this system – at its most simple – is the accessory battery is switched on whenever the ignition is on, including during engine cranking, and this may allow a seriously flat accessory battery to pull down voltages at the ignition coil or engine control unit to a level below the point at which they can fire the motor.

One way around this problem is to wire the solenoid to the generator light circuit (or a simple electronic timer circuit), so that it doesn’t connect the accessory battery – and therefore pull down system voltage – until after the engine has come to life.

Manual four-way switching

A four-way switch allows the vehicle’s first or second battery to perform either of two roles: starting the vehicle, or powering equipment while totally isolating the other battery. That’s great if at camp you wish to operate your vehicle’s radio or other accessories, without risking flattening both batteries.

As with the simple switch system, this full-manual system relies on the driver. But it offers the extra flexibility of allowing the fridge and other accessories to be operated with just one battery in the vehicle – not always possible with other layouts – which means part-time travellers don’t have to lumber around town all year with a battery doing nothing on board.

4x4 driving in the dirtRotating the switch to the position where the batteries are connected (1+2 or A+B) allows both batteries to be charged. This paralleling of the batteries also provides plenty of grunt for operating a winch – an important consideration for many people.

Electronic control

Electronic battery management systems have been available for a while, with varying levels of sophistication and flexibility. All provide automatic isolation and charging of the accessory battery – in that regard, they operate like a simple solenoid or relay. Some of these systems claim priority charging of the vehicle starting battery. However, this is often not true for all systems. Instead, the second battery remains switched-out until the system voltage exceeds a pre-set level; usually just after the vehicle fires into life and the alternator pushes system voltage above 14V.

Some electronic battery management systems provide extra useable power capacity at camp by allowing the accessories to be powered from the vehicle starting battery for a while before automatically switching to the second/ accessory battery when the starting battery reaches a partially discharged but safe-for-starting level. As previously mentioned, vehicle equipment such as entertainment systems, interior lights and central locking continue to be powered by the vehicle’s own battery, so accessory battery isolation won’t prevent these accessories from draining your vehicle’s main battery.

Redarc -electronic -control
Redarc isolator protects the start battery.

The adoption by most vehicle manufacturers of variable voltage charging (to help vehicle emission targets) in the past decade means that a little black box is almost essential for second battery charging. That’s because, for a variety of reasons, a second battery usually can’t be effectively charged by simple ‘old-school’ linking in parallel with the vehicle’s main battery.

Batteries in caravans and camper trailers may also need electronic assistance. Their distance from the charge source means actual voltage available (which reduces over distance, like water pressure does in a long garden hose) may be too low to effectively charge the batteries for best performance or life. As all batteries rely on the push of a higher voltage (typically 14-14.4V from a vehicle alternator) to be charged, these batteries may suffer at the end of a long cable run, requiring the step-up – and constant – voltage that’s provided by a DC/DC charger.

Taking charge

No matter what system is being used, charging time and rate needs to be factored into the setting-up of a 4x4 vehicle or camper/caravan battery bank. Batteries take time to charge and don’t charge in a linear manner. A welldepleted (flat) battery will accept lots of current at first but far less current as it approaches full charge. Think of the 80/20 guideline – a battery will be 80 per cent charged in 20 per cent of the time it takes to be fully charged. The last 20 per cent of battery capacity takes a lot longer to achieve.

4wd touringThe typical touring day of six to eight hours’ driving (and therefore charging) time may not be enough to fully charge a depleted battery bank. Having plenty of charging capacity – for example, a 200A alternator in your vehicle – will not charge things any quicker than an 80A unit if the battery (or batteries) only takes 40A.

A trick to reduce charging time is to use multiple smaller-capacity batteries in parallel. They will charge in less time (of course, while drawing more current) than one large battery of the same capacity (e.g. two 50AH batteries versus one 100AH).

When relying on accessory power in remote areas, this is an important consideration, as is the fact that multiple batteries can be separated due to damage, or loaned for other uses – getting a stranded vehicle running, for example.

Packin' power

PORTABLE battery packs are another way of powering accessories. Many vehicles simply aren’t used often enough for touring to justify a permanently installed accessory battery, or they have insufficient room under the bonnet for a traditional second battery installation.

Portable power packs are available in several shapes and sizes (and, of course, levels of quality). The better ones exceed the performance of a vehicle-mounted accessory battery, but in a relatively easy-to-move package that can be removed from the vehicle (or used elsewhere) when it’s not required.

That way, you can keep a fridge running at camp while you’re out exploring in your 4x4, or you (and your mates) can use them in other vehicles. They are also handy around the home or garage.


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