Dual battery and 12V systems for touring

The purpose of a dual battery system is narrow: run your accessories from a battery that is not the one that starts the engine. Everything else is detail in service of that.

Get it wrong and the failure mode is specific and memorable. You wake up in a remote campsite to a warm fridge and a vehicle that will not turn over.

Sizing it from the fridge

The fridge is almost always the dominant load, so start there and work outward.

A typical 12V compressor fridge draws somewhere around 1 to 2 amps while the compressor runs, but it cycles rather than running continuously. The useful figure is daily consumption, commonly quoted in the range of 20 to 40 amp-hours per day depending on ambient temperature, how full it is, and how often it is opened.

Ambient temperature is the variable that dominates. The same fridge in a Kimberley afternoon consumes dramatically more than in a Victorian spring, so size for the trip you actually intend.

Add lights, phone and device charging, a water pump and any other loads, and you have a daily figure. Multiply by the number of days you want to sit without driving or sun. That is your usable capacity requirement.

Usable is the operative word, and it is where chemistry matters.

Chemistry, briefly

AGM lead-acid is the traditional choice. Cheaper up front, tolerant, widely available. The catch is depth of discharge: taking a lead-acid battery below about half its rated capacity repeatedly shortens its life considerably. So a 100 amp-hour AGM gives you roughly 50 usable amp-hours. It is also heavy.

Lithium iron phosphate costs substantially more up front and gives you most of its rated capacity as usable, holds voltage steadier as it discharges, weighs around half as much, and lasts far more cycles.

The practical comparison: a 100 amp-hour lithium battery does the work of roughly a 200 amp-hour AGM, at a third of the weight and several times the price. On a weight-constrained touring vehicle that trade is usually worth making, and it gets more compelling the longer you keep the vehicle.

Lithium has requirements of its own. It needs a charger with a lithium profile, it does not like being charged below freezing, and it needs a battery management system, which quality units have built in.

Charging

Three sources, and a good system uses more than one.

From the alternator, via a DC to DC charger. Modern vehicles with smart alternators vary their output voltage deliberately, which means a simple voltage-sensitive relay may never fully charge an auxiliary battery. A DC to DC charger takes whatever the alternator provides and delivers a proper multi-stage charge at the right profile for your chemistry. On any recent vehicle this is the right answer rather than an upgrade.

Solar. The complement to driving, because it works while you sit still, which is exactly when the alternator does not. A regulator is mandatory. Fixed panels are convenient and always compromised by wherever the vehicle is parked; a portable blanket you can angle at the sun produces far more from the same rated wattage.

Rated wattage is optimistic. Real output depends on angle, temperature, cloud and dust on the panel, and a realistic expectation is well under the number on the label.

Mains, when you have it. A quality multi-stage charger before a trip, so you leave with a genuinely full battery rather than a mostly full one.

Where it lives

Under the bonnet is hot, and heat is the main enemy of battery life. It is also where the space usually is.

In the cabin or a canopy is cooler and kinder to the battery, and it brings its own requirements: lead-acid batteries can vent gas and need to be sealed or vented externally, and any battery needs to be restrained properly. An unsecured battery in a rollover is a heavy object with terminals on it.

Cable sizing matters more than people expect over the runs involved. Voltage drop over an undersized cable from the engine bay to a rear canopy will quietly rob you of charging performance, and the fix is expensive after the fact because it means pulling the loom out again. Fuse every circuit, at the source.

This is the part of a touring build where the fit-out and the electrical system have to be designed together rather than sequentially. Work out battery location, cable runs and load layout as one job. The common bad outcome is a sound battery system awkwardly retrofitted around drawers that were already installed, which means either long cable runs or pulling the fit-out apart again.

A monitor is worth more than another battery

The single most useful addition to a 12V system is a shunt-based battery monitor that tells you actual state of charge and current draw.

Voltage alone is a poor indicator, particularly with lithium, where the voltage curve is nearly flat across most of the usable range. A monitor turns a guess into a number, and it changes behaviour: you find out that a particular load is costing more than you thought while there is still time to do something about it.

A working starting point

For a couple running a fridge, lights and device charging, wanting three or four days stationary:

• 100 amp-hour lithium auxiliary battery
• 25 amp DC to DC charger with solar input
• 150 to 200 watts of portable solar
• Shunt battery monitor
• Properly fused distribution, adequately sized cable

That covers most touring without being extravagant. Scale up for an inverter, a second fridge, or long stationary periods in the heat.