Dual battery systems

Modern touring relies on 12-volt power. Fridges, lights, water pumps, phone and camera chargers, radios, compressors and sometimes winches all draw on the vehicle's electrical system. Running these from the starting battery overnight risks waking up to a vehicle that won't start, potentially in a remote place. A dual battery system solves this by adding a separate auxiliary battery for accessories.

This guide explains the components of a dual battery system, the main choices and the safety basics.

How a dual battery system works

The system has two batteries: the vehicle's starting battery and an auxiliary (or house) battery. While the engine is running, the alternator charges both. When the engine stops, the batteries are separated, so accessories draw only from the auxiliary battery and the starting battery stays charged for starting.

The key components are:

• Auxiliary battery: a deep-cycle battery designed to be discharged and recharged repeatedly.
• Charging device: an isolator or DC-DC charger that manages charging from the vehicle.
• Solar input: panels and a regulator to charge at camp.
• Cabling and protection: heavy-gauge cables, fuses or circuit breakers and a good earth.
• Outlets and monitoring: sockets for fridges and accessories, and a battery monitor.

Isolators versus DC-DC chargers

Voltage-sensitive relays (VSRs) or isolators connect the batteries when the starting battery's voltage shows the alternator is charging, and disconnect them when it drops. They are simple and inexpensive, but they rely on the alternator's voltage being high enough to fully charge the auxiliary battery.

DC-DC chargers take power from the vehicle and deliver a controlled, multi-stage charge suited to the auxiliary battery's chemistry. Many modern vehicles have smart alternators that vary their voltage to save fuel, which can leave a simple isolator unable to charge an auxiliary battery properly. DC-DC chargers compensate for this and for voltage drop over long cable runs, such as to a battery in the rear of a wagon or canopy. Many include a solar regulator.

Battery chemistries

AGM (absorbent glass mat) batteries are sealed lead-acid batteries that are robust, tolerant of vibration and relatively affordable. Their usable capacity is limited if you want them to last, as repeated deep discharge shortens their life.

Lithium iron phosphate (LiFePO4) batteries are lighter, allow much more of their capacity to be used, hold voltage well and typically last many more charge cycles. They cost more upfront, need compatible charging, include a battery management system and may have temperature limits for charging. Lithium iron phosphate is generally considered more thermally stable than some other lithium chemistries, but quality varies, so buy from reputable suppliers.

Flooded lead-acid batteries are cheap but need ventilation and maintenance, and are less common in modern touring setups.

Sizing your system

Start by listing your loads and how long each runs each day. A compressor fridge running around the clock in hot weather is usually the largest. Add lights, pumps and charging. This gives your daily energy use. Choose a battery capacity, considering how much of it can be used for its chemistry, that will last between charges, with a margin for cloudy days. Then size your charging, from driving and solar, to replace what you use.

Solar

Solar panels recharge the auxiliary battery at camp. Options include fixed panels on the roof or canopy, which charge whenever the sun shines, and portable panels or blankets, which can be placed in the sun while the vehicle is parked in shade. Use a solar regulator matched to your battery, often built into DC-DC chargers. Panels deliver less than their rated output in heat, at poor angles and in cloud.

Cabling and protection

Dual battery systems carry high currents, especially when charging or running a winch. Undersized cables cause voltage drop, poor charging and overheating. Every positive cable must be protected by a correctly rated fuse or circuit breaker close to the battery, so a short circuit can't start a fire. Use proper lugs and crimps, protect cables from chafing with conduit and grommets, and secure batteries in rated trays or boxes.

Electrical work in vehicles can be dangerous. Many owners use a qualified auto electrician, particularly for fixed installations, integration with vehicle electronics and winch wiring. Poor installation can cause fires, damage vehicle electronics and affect warranty and insurance.

Portable power stations

Portable power stations, which combine a lithium battery, charger and outlets in one box, are an alternative for simpler setups. They can be charged from the vehicle, solar or mains and moved between vehicles. They must be secured while driving and should be charged through a suitable DC-DC input for best results.

Monitoring

A battery monitor shows the auxiliary battery's state of charge, current draw and charging. It helps you understand your power use and avoid running the battery flat. Many fridges also have low-voltage cut-outs to protect batteries.

Maintenance

Check terminals for corrosion and tightness, inspect cables and fuses, keep batteries secured and test the system before big trips. Store batteries charged when not in use.

Common mistakes

• Running the fridge off the starting battery.
• Simple isolators on vehicles with smart alternators.
• Undersized cables to rear batteries.
• Missing fuses near batteries.
• Loose batteries or power stations in the cargo area.
• Buying an oversized battery but undersized charging.

Inverters

An inverter converts 12-volt DC power to 240-volt AC for devices such as laptop chargers and small appliances. Inverters draw heavily from the battery, so choose one sized to the devices you actually use, with pure sine wave output for sensitive electronics. Mount it close to the battery with short, heavy cables and a fuse, keep it ventilated, and switch it off when not in use. Many devices can charge directly from 12V or USB outlets more efficiently than through an inverter.

Charging from mains power

At powered sites or at home, a quality 240V smart charger keeps the auxiliary battery topped up between trips.

Key points

A dual battery system keeps the starting battery for starting while an auxiliary battery runs accessories. Size it to your loads, use a DC-DC charger with smart alternators, add solar, use heavy cable with fuses near the battery, secure everything and use a qualified auto electrician for fixed installations.