Fridge, winch or lights: sizing a dual-battery setup for real off-road use

I had a bloke call me from a rest stop on the Oodnadatta Track last year, dead flat on his second battery, engine running, absolutely certain his "200-amp system" had failed him. It hadn't failed. It was never big enough for what he was asking it to do. That's the pattern I see in about eight out of ten dual-battery complaints — not a faulty component, just a system sized against a sticker on a box rather than an actual load.

A dual-battery setup for 4WD touring is meant to do one job well: keep a fridge, some lighting, maybe a winch or a UHF running off a second battery that charges from the vehicle's alternator while you drive, without ever touching your engine-start battery. Simple in principle. The bit that trips people up is working out how big that second battery needs to be, and what has to sit between it and the alternator to charge it properly. Get either wrong and you're the bloke on the Oodnadatta with a full tank of diesel and a dead fridge.

Start with the load, not the battery

Before you buy anything, add up what you're actually running off the auxiliary battery when the engine's off overnight. This is the step almost everyone skips, and it's the one that actually matters.

A 40-litre compressor fridge draws roughly 40-50Wh per hour on average in mild weather, more like 60-70Wh in 35°C ambient because the compressor cycles harder. Multiply by however many hours it's unpowered — say 14 hours overnight plus a few hours of driving where the fridge is still drawing even though it's technically being charged. LED strip lighting is close to nothing, a few watts an hour. A 12V fan for the kids' bunk might pull 5-10W. Add a UHF on standby, maybe a phone charging. None of that is dramatic on its own, but it adds up, and it's cumulative overnight when nothing is putting energy back in.

For most single-fridge touring rigs I'm seeing total overnight draw land somewhere between 400Wh and 900Wh, depending on fridge size, ambient temperature and whether there's a second fridge or freezer in the back. That number is the one you should be sizing your battery against, not "what's on special at the auto shop this month".

Lithium versus AGM for the second battery

A 100Ah AGM gives you roughly 50Ah of genuinely usable capacity if you're not comfortable running it below 50% state of charge regularly, which you shouldn't be if you want it to last. A 100Ah lithium (LiFePO4) gives you closer to 80-90Ah usable, because it tolerates deeper discharge without the same cycle-life penalty. That difference matters when you're trying to squeeze a fridge and lighting through a long, hot, stationary evening at a free camp with no solar top-up.

Lithium costs more upfront, no argument there, and it's not a like-for-like swap — most lithium batteries want a charging profile the older AGM-era isolators and dumb solid-state isolators weren't built to deliver properly. That's where the DC-DC charger question comes in, and I've written about that in detail elsewhere on this site because it deserves its own explanation rather than a paragraph here.

My honestly held view, and I'll cop pushback on this: for anyone doing more than the occasional weekend away, lithium is worth the extra spend now. The usable capacity gap alone often means the difference between a 100Ah lithium and a 120Ah AGM doing the same job, and the AGM route ends up costing you in physical space and weight you don't get back.

Why a smart alternator changes the calculation

If your vehicle is a newer diesel with a variable-voltage "smart" alternator (common across most Toyota, Isuzu and Ford 4WD platforms from roughly the mid-2010s onward), a basic solid-state isolator will not charge a second battery properly, full stop. These alternators drop their output voltage to save fuel once the engine-start battery reads as charged, and a dumb isolator just passes through whatever the alternator's giving out, which by the time you're an hour down the highway might be nowhere near enough to push charge into your auxiliary battery.

This is the single most common cause of "my dual battery system doesn't charge" calls I get, and it's rarely the battery's fault. A proper DC-DC charger — I'd suggest look for one rated at 25A minimum for a single lithium battery under 100Ah, 40A-plus if you're running a larger bank or want faster bulk charging on shorter drives — manages that voltage variation and gives the battery the multi-stage charge profile it actually needs. It's not an optional extra bolted on for peace of mind. On a smart-alternator vehicle it is the thing that makes the whole system work at all.

Solar isn't a substitute for correct sizing

A 150W panel on the roof or a folding 200W panel thrown out at camp both sound like they're doing plenty, and on a clear day in Central Australia they might genuinely deliver 80-120Ah into the battery across the day. But solar is a top-up, not the base of your sizing maths. If you've undersized the battery for the load, solar just means you go flat a bit slower and recover a bit faster; it doesn't fix a battery that was never big enough for the job in the first place.

I'd size the battery and DC-DC charger first, against the actual overnight load, and treat solar as the thing that extends how many consecutive overcast or low-drive days you can survive without plugging into mains or running the vehicle. That order matters more than the wattage number stamped on the panel.

Fitting it to the vehicle, not just the battery box

Where the second battery physically lives matters more on genuine off-road tracks than it does on the highway. Under-bonnet mounting is common on utes and simplifies wiring, but it exposes the battery to serious heat cycling, which shortens both AGM and lithium life. A tray or canopy-mounted setup runs cooler but means longer cable runs, and voltage drop over a long, thin cable run will quietly rob your charging performance even if every other component is correctly specified.

On genuinely rough tracks — the sort of corrugated, washboard sections you'll hit on the Birdsville Track or the sandier stretches heading into the Simpson Desert via the French Line — battery boxes need to be properly restrained, not just sitting in a cargo net. I've seen terminals shear clean off from vibration on a trip up the Gibb, and that's a battery box that was "secure enough" for the Hume Highway and nowhere near secure enough for what it actually copped.

A worked example

Take a fairly typical setup: 50-litre single fridge, some LED lighting, a phone and a UHF charging overnight, no aircon or CPAP in the mix. That's roughly 500-600Wh of overnight draw in warm weather. A 100Ah lithium battery gives you 1,200-1,280Wh total capacity and, at a sensible 80% usable depth of discharge, around 960-1,000Wh usable. That covers the overnight load with a comfortable margin for an overcast morning before solar or driving tops it back up.

Add a second fridge, a CPAP running off 12V, or genuinely hot conditions pushing the compressor harder, and that same 100Ah battery is tight, maybe uncomfortably so on a second still night without recharge. That's the point where people either step up to 200Ah of lithium or add a small inverter generator as backup for the odd flat, still, overcast run — a topic worth its own read if you're running multiple 12V appliances.

What I'd actually check before buying

Work out your overnight Wh draw honestly, including the fridge running through the hottest likely conditions you'll camp in, not the mild afternoon it was tested in. Confirm whether your vehicle has a smart alternator, because that decides whether a DC-DC charger is optional or mandatory. Size the battery to that load with real usable capacity, not the number printed on the case. Then treat solar as insurance, not the foundation. None of this is complicated maths, it's just maths most people skip because the bloke at the shop had a package deal that sounded about right.

Systems sized this way don't need luck to get you through a week at Bullara Station Stay with no mains power, or a stretch along the Gibb River Road where the next powered site is a long way off. They just work, quietly, which is honestly the whole point of doing the sums properly in the first place.

— Priya Raman, Gear, Power & Trip Planning