Lithium vs AGM for caravans: the real cost per usable amp-hour

Last month a bloke at a caravan show in Adelaide told me lithium was "basically free once you factor in the resale." I asked him to show me the sums. He couldn't. Most people can't, because most people are comparing sticker price on the shelf rather than cost per usable amp-hour over the life of the battery. That's the number that actually matters, and almost nobody runs it before they buy.

I've had this conversation more times than I can count doing fault-finding on vans where the owner replaced a "dead" AGM with lithium and suddenly everything worked fine, not because lithium is magic, but because they'd been running the AGM into the ground for years without realising it. So let's do the actual comparison: same fridge, same solar, same charging behaviour, two battery chemistries, and what it really costs per amp-hour you can use.

Usable capacity is the whole game

A 200Ah AGM battery is not a 200Ah battery in practice. Lead-acid chemistry, AGM included, degrades hard if you discharge it past 50% depth of discharge on any kind of regular basis. Manufacturers rate cycle life assuming you stay above that halfway mark. Push an AGM to 80% depth of discharge repeatedly and you'll be lucky to get 300 cycles before capacity falls off a cliff. So your genuinely usable capacity from a 200Ah AGM, if you want it to last, is closer to 100Ah.

A 100Ah lithium (LiFePO4) battery, by contrast, is comfortably rated for 80-100% depth of discharge with no meaningful capacity loss over thousands of cycles. Most reputable cells on the market now, and I mean the ones with proper BMS protection and cells from known manufacturers, not the unbranded ones you find on marketplace listings, are rated 3,000-5,000 cycles at 80% DoD. So your 100Ah lithium gives you roughly 80-100Ah usable, in a box half the weight and roughly half the physical size of the AGM.

That's the first correction most people need: a 100Ah lithium and a 200Ah AGM deliver you roughly the same usable capacity day to day. They are not different capacity classes wearing different price tags. They're the same usable capacity, at very different weights, footprints and lifespans.

What each actually costs, amortised

Prices move around, so treat these as the kind of ballpark I'd use in a spreadsheet, not a quote. A quality 200Ah AGM from a known brand currently sits somewhere in the AU$400-600 range. A 100Ah drop-in lithium with a built-in BMS from a reputable supplier is typically AU$800-1,400, more if it includes Bluetooth monitoring and a low-temperature cutoff.

So on sticker price alone, AGM wins by a wide margin. But run the cycle life against real usage. If you're touring seriously, say 100-150 nights a year off grid, you'll put through roughly 100-150 cycles annually. An AGM good for 300 cycles at the discharge depth you're actually using it at will need replacing in two to three years. A lithium good for 3,000+ cycles at 80% DoD will outlast the van.

Do that math over a ten-year ownership period and the AGM owner has bought three to four batteries at $400-600 each, somewhere around $1,600-2,000 total, plus the fitting labour each time if they're not doing it themselves. The lithium owner has bought one battery for $800-1,400 and it's still going. On cost per usable amp-hour over a decade of real touring, lithium wins comfortably, and that's before you count the weight saving, which matters more than people think once you're near your GVM.

This is the number the brochures never show you, because AGM manufacturers aren't going to volunteer that their product needs replacing every two years under hard use, and lithium sellers would rather talk about weight and fast charging than admit the sticker price is confronting.

Where AGM still makes sense

I'll say something that won't make the lithium crowd happy: AGM is still the right call for a chunk of caravanners, and anyone who tells you lithium is always better is selling something. If you're doing 10-20 nights a year, mostly in caravan parks with 240V hookup, and your battery mostly sits topped up rather than being cycled hard, the cycle-life advantage of lithium barely gets exercised. You're not putting through enough cycles to reach the point where AGM degradation bites. In that use case, the cheaper AGM will probably outlast your ownership of the van anyway, and lithium is money spent on a problem you don't have.

AGM also tolerates cold better in one specific sense: standard lithium cells shouldn't be charged below 0°C, full stop, because it damages the cells permanently through lithium plating on the anode. If you're touring the high country in winter (see our piece on what you can tow into the Victorian High Country) and charging while genuinely below freezing, you need a lithium battery with a low-temperature cutoff built into the BMS, or you're better off with AGM for that specific van. Some lithium batteries handle this properly now with internal heating pads, but check the spec sheet, don't assume.

Charging behaviour is different, and it trips people up

This is where I see the real failures in the field. Lithium wants a different charge profile to AGM: a higher absorption voltage, no float stage in the traditional sense, and it can accept a much higher charge current without complaint. If you swap an AGM for lithium and keep the old AGM-profile DC-DC charger or solar regulator, you'll undercharge the lithium, meaning you never actually get that full usable capacity you paid for.

Most DC-DC chargers sold in the last five or six years have a lithium profile you select with a dip switch or button, so check yours actually has one rather than assuming. If you're running solar as well, your MPPT controller needs the same lithium-specific profile, and the two chargers need to agree with each other on absorption voltage, or you'll get weird behaviour where the battery never quite reaches 100% according to the shunt.

Speaking of the shunt, if you haven't got one, get one. A voltage reading alone will lie to you on lithium far more than it does on AGM, because lithium holds a flat voltage across most of its usable range and then drops off a cliff near empty. I've written about this in more detail in our piece on why your voltage reading is lying to you, and it matters more with lithium, not less, because you lose the gradual warning slope AGM gives you.

Weight, and why it matters more than the spec sheet suggests

A 200Ah AGM weighs roughly 55-65kg. A 100Ah lithium delivering the same usable capacity weighs roughly 12-15kg. That's a 40-50kg difference sitting right over your axle group, which matters a lot more once you've added solar panels, a second fridge, recovery gear and everything else that creeps onto a van's tare weight over a few years of ownership.

I've looked at enough weighbridge dockets from readers who were convinced they were under their ATM to know that battery weight is one of the easier places to claw back margin without touching the layout. If you're already tight on payload, that saving alone can be worth more to you than the cycle-life argument. Worth reading alongside our piece on why your van tows different full than empty if you haven't run your own numbers lately.

Sizing to your actual fridge, not the brochure fridge

None of this matters if you haven't sized the battery to what you're actually running. A compressor fridge on a 12V system typically draws somewhere around 0.5-1.5A on average once you account for duty cycle, more in a hot Kimberley wet season, less in a mild Victorian autumn. Add lighting, a water pump, maybe a CPAP, and most touring setups land somewhere between 40-80Ah of genuine daily draw. We've broken the actual sums down properly in sizing your 12V fridge to your battery, and it's worth doing that exercise before you decide on chemistry, because a correctly sized lithium setup might be smaller and cheaper than the AGM setup you were about to oversize out of habit.

The verdict, for what it's worth

If you're touring more than about 40-50 nights a year off grid, lithium wins on cost per usable amp-hour, wins on weight, and wins on charge acceptance from solar on short winter days. If you're a weekender mostly on powered sites, AGM will do the job for less money and you won't exercise the difference. The Australian Competition and Consumer Commission's general guidance on comparing warranted product life is worth applying here too: don't buy on cycle count alone, check what conditions the warranty actually requires, because plenty of lithium warranties are voided by exactly the cold-charging mistake described above.

Run your own numbers before you buy either. Bring a genuine daily load figure, not a guess, and work out what usable capacity you actually need before you get seduced by a bigger number on the box.

— Priya Raman, Gear, Power & Trip Planning