Lithium vs AGM: Which Caravan Battery Setup Is Right for You

People ask me "lithium or AGM" like it's a footy allegiance, and I answer it the same boring way every time: with a spreadsheet. The right battery isn't the one with the best ad or the loudest bloke at the caravan park. It's the one whose usable watt-hours, weight and cost match how you actually camp. So let me hand you the numbers I'd use, and you can do your own sums.

The number that matters is usable capacity

A battery's headline amp-hour rating is not what you get to use, and this is where lithium quietly wins before anything else is counted. An AGM (a sealed lead-acid battery) hates being deeply discharged. Pull it below about half and you shorten its life fast, so the sensible working figure is roughly 50 per cent usable. A LiFePO4 (lithium iron phosphate) battery is happy being run down to 10 or 20 per cent, so you get roughly 80 to 100 per cent usable without hurting it.

Run that through real capacity. A 100Ah AGM at 12V is nominally 1200 watt-hours, of which about 600Wh is usable. A 100Ah lithium is the same 1200Wh nominal but around 1000Wh usable. So a single lithium does the work of nearly two AGMs before you've talked about weight, and that ratio is the heart of the whole decision.

The full comparison

Weight is a payload decision, not a footnote

This is where lithium stops being a luxury and starts being a weights argument. To match one 100Ah lithium's usable energy you need roughly two 100Ah AGMs, which is around 50 to 60kg of lead versus 10 to 13kg of lithium. On a van fighting for every kilogram of payload, that 40kg-plus swing is real, and it lands in the same budget as your water and your gear. I've moved people to lithium purely to claw back payload, and if that means nothing to you yet, read towing weights explained and it will. Weight up high and behind the axle also affects towing manners, so lighter batteries mounted low is a quiet handling win too.

Charging: where lithium pays off, and where it bites

Lithium takes charge fast and all the way, so it makes far better use of solar and of a DC-to-DC charger (a DCC, the unit that charges the van's battery from the car's alternator as you drive). An AGM's charge current tapers off as it fills, so those last percentages crawl in, which means on a short drive between camps a lithium bank recovers far more than an AGM would from the same solar and the same hour of driving. If you free-camp and move often, that's a big practical difference.

Now the catch I make everyone hear: you must not charge lithium below freezing. Charging a LiFePO4 cell below 0C damages it, so a good battery has a built-in battery management system (BMS) that blocks cold charging, or you fit a heated battery. If your touring takes you into genuinely cold country, the High Country in winter or desert nights that drop hard, this matters. AGM doesn't care about the cold nearly as much. It's the one column where the old technology wins outright.

One wiring point people miss when they switch: on a modern tow vehicle you generally can't just run a heavy cable from the start battery to a lithium bank and hope. Newer vehicles have variable-voltage ("smart") alternators that don't hold a steady charging voltage, and a lithium battery will happily try to swallow every amp the alternator can give, which can cook wiring or the alternator itself. That's why a DC-to-DC charger isn't optional with lithium; it takes whatever messy voltage the car provides and delivers a clean, current-limited charge the battery and the alternator can both live with. Budget for the DCC as part of the lithium price, not an afterthought, because a lithium bank fed by a proper DCC and a bit of solar is where the technology actually shines.

On lifespan and cost, it's worth being honest both ways. Lithium costs several times what AGM does up front, and that stings. But a quality LiFePO4 is rated for thousands of charge cycles against a few hundred for AGM used properly, and far fewer if you keep flattening the AGM past half. Spread the price over the cycles and the gap narrows a long way, especially for full-time travellers who cycle their batteries every single day. For a weekender who camps a dozen nights a year, the AGM will likely outlast their interest in the van, and the cycle-life advantage of lithium never gets a chance to pay off. Match the battery to the mileage, not the marketing.

Sizing the bank to your real daily use

The mistake I see most is people buying capacity by the biggest number they can afford instead of working out what they actually use in a day. So do the sum. Add up your loads in amp-hours over 24 hours. A 12V compressor fridge in a warm climate might pull somewhere around 30 to 60 amp-hours a day depending on size, ambient heat and how often the lid opens. Lights, phone and tablet charging, a water pump and a diesel heater fan might add another 20 to 40. Call it roughly 60 to 100 amp-hours a day for a typical touring couple who aren't running an inverter for luxuries.

Now match that to usable capacity, not headline capacity. To cover 80 amp-hours of daily use with AGM at 50 per cent usable, you'd need about 160Ah of AGM just to break even each day, and you never want to run a battery flat to empty, so realistically more like 200Ah-plus of lead. To cover the same 80 amp-hours with lithium at 90 per cent usable, you need a little over 90Ah of lithium, and 100Ah gives you a small buffer. That's the whole argument in one comparison: one 100Ah lithium roughly matches two 100Ah AGMs for daily work, at a fraction of the weight. Then size your solar to put back what you take out on an average day, because a big battery with too little charging just runs flat slower.

So which should you buy

My honest, slightly unfashionable position: not everyone needs lithium. If you mostly stay on powered sites, run modest loads, and keep a tight budget, a good AGM setup is perfectly sensible and you'll spend the saving on fuel and camp fees. AGM is not obsolete just because the forums say so.

But if you free-camp for days, run a compressor fridge plus a few extras, move between camps often enough to lean on solar and a DCC, and you're fighting for payload, lithium wins clearly on usable energy, weight and cycle life, and the higher price amortises over thousands of cycles rather than hundreds. Over a long trip that cost-per-cycle gap narrows a lot, which is worth weighing against everything else in what a lap actually costs. Size the bank to your real daily watt-hours, not to the biggest number you can afford, and match it to the rest of the build in our gear notes and to the kind of remote country covered on runs like the Gibb River Road, where the days between mains power are exactly when this decision shows its worth. It should also shape the van you buy in the first place, which is why choosing an off-road caravan starts with the electrical and the weights, not the upholstery.