How Does a Dual Battery System Work? A Guide for 4WD and Touring

A dual battery system adds a second battery to your vehicle to run your camping gear, such as a fridge,
lights and phone chargers, while your main battery stays dedicated to starting the engine. When you
drive, the alternator charges both batteries. When the engine's off, the system keeps them separated,
so you can't flatten the battery you need to get home.

What is a dual battery system?

Your vehicle's starting battery is built to deliver a big burst of power for a few seconds to crank the
engine. It isn't built to run a fridge all night. Drain it repeatedly and it wears out, or worse, leaves you
with a car that won't start at camp.

A dual battery system solves this by adding an auxiliary battery, also called a second or house battery,
which is designed to be drained and recharged over and over. Your camping gear runs off the auxiliary
battery, and the starting battery is left alone.

How does a dual battery system work?

A dual battery setup has four main parts.

1. The auxiliary battery. Usually lithium (LiFePO4) or AGM. It can be mounted under the bonnet, in
the tub, behind a seat or in a canopy. Slimline batteries are made for tight spaces where a
standard battery won't fit.

2. The charging device. Either a battery isolator or a DC-DC charger. With the engine running, it
lets the alternator charge the auxiliary battery. With the engine off, it separates the two
batteries.

3. Cabling and protection. Heavy gauge cable carries current between the batteries, with fuses
fitted close to each battery. Anderson plugs give you a simple, secure connection for your fridge
and other gear.

4. Monitoring. A voltmeter or Bluetooth battery monitor shows how much charge is left, so you're
not guessing at camp.

Many systems also add solar. Some DC-DC chargers include a solar input, so a panel or blanket can top
up the auxiliary battery while you're parked.