Dual battery system

Also called auxiliary battery system, second battery setup, house battery, battery isolator

A dual battery system adds a second battery wired through an isolator or a DC-DC charger, so a fridge, lights or a winch can draw on the auxiliary battery without any risk of leaving the starting battery too flat to turn the engine over.

What is a dual battery system?

One battery starts the engine. A second, the auxiliary or house battery, runs everything else: a fridge overnight, camp lighting, a winch, a radio. Wired together with nothing in between, the two act as one large battery, which sounds convenient until the fridge runs all night and the starting battery is flat along with it in the morning. The point of a dual battery system is to prevent exactly that, by putting a device between the two batteries that lets power flow one way while keeping the starting battery's charge off limits to everything else.

REDARC, a maker of the chargers this entry describes, states the function plainly: these isolators protect the primary battery from excessive discharge, while allowing the auxiliary battery to supply non-essential loads and accessories.

Isolator or DC-DC charger

The simplest version is a voltage-sensing relay: it joins the two batteries once the alternator is running and charging, and splits them again once the engine stops, so accessory loads on the auxiliary battery can never reach back into the starting battery. It is cheap and it works, and its limits show up on a long, thin wiring run or with a lithium auxiliary battery, where a relay's charging voltage and timing are wrong for the chemistry.

A DC-DC charger replaces the relay with a proper multi-stage charger built for the auxiliary side: it keeps your auxiliary battery charged when driving or with portable solar panels when parked, in REDARC's phrasing, and most models also accept a solar input for topping up at camp. It costs more and it is the right tool for a lithium battery or a run of wire too long for a relay to charge properly.

What people get wrong

Undersized wire is the most common failure: a thin cable run a long way from the alternator to the back of the vehicle drops enough voltage that the auxiliary battery never actually finishes charging, however good the charger bolted to it is. The second is battery chemistry mismatch, wiring a lithium battery to a simple isolator built for lead-acid voltage curves, which can leave it undercharged or trip its own protection circuit. The third is mounting a flooded lead-acid battery somewhere unvented inside the cabin, which is a fumes problem a sealed AGM or lithium battery does not have.