Worth going lithium for a small van build or just stick with AGM?

Doing a weekend/occasional weekender conversion on a Transit Custom. Space is tight so I'm looking at 100Ah as my battery bank — no more. Running a 40L compressor fridge, a few USB charges, maybe a small inverter for a laptop. Nothing mad.

The AGM route is cheaper upfront — I can grab a decent 100Ah leisure battery for around £80-100. But obviously usable capacity is roughly half, so realistically 50Ah. A Fogstar Drift 100Ah lithium is around £260-280, which gives me the full 100Ah usable and charges faster off my 175W Renogy panel.

Has anyone actually done the maths on this for a small build where you're not running a massive system? Trying to work out if the lithium premium is genuinely worth it when the total bank size is this modest, or whether I'd be better off running two AGMs in parallel for similar usable capacity at lower cost. Weight saving matters a bit — it's a daily driver too.

@RayPowell at 100Ah usable capacity the chemistry choice really matters. AGM gives you roughly 50Ah usable before you risk premature ageing; lithium (LiFePO4) gives you 80–90Ah from that same rated 100Ah. On a compressor fridge that difference is significant overnight.

For a Transit Custom I'd seriously look at a Fogstar Drift 100Ah — compact, built-in BMS, reasonable price point. Pair it with even a modest Victron SmartSolar MPPT and you've got a properly monitored system.

Weight saving is also genuine — roughly half that of an equivalent AGM, which matters in a Custom where payload limits creep up on you.

The upfront cost stings but across 2,000+ cycles versus AGM's 400–500 at 50% DoD, the maths favours lithium particularly if this van sees regular use rather than sitting idle for months.

@RayPowell the weight saving alone is worth it on a Custom — I shifted from AGM to a Fogstar Drift 100Ah in my garden office setup and it's transformed things. More relevant to you though: that compressor fridge will cycle constantly overnight and you'll genuinely feel the difference between 50Ah usable vs 90Ah+.

Tight spaces also favour lithium because you can mount them in any orientation and they're typically a smaller footprint for equivalent capacity.

Only caveat — budget for a lithium-compatible charger/DC-DC unit if you're using the van's alternator. A Victron Orion-Tr Smart is the obvious choice but adds to the cost. Factor that in upfront rather than discovering it later like I did.

@RayPowell had almost this exact dilemma on my narrowboat a couple of years back — tight space, modest loads, wondering whether lithium was overkill.

The thing that swung it for me wasn't the usable capacity or the weight (though both matter). It was charge acceptance. When you're plugging into a site hookup for a few hours, or running the engine briefly, lithium just drinks the charge whereas AGM starts refusing it once it's past 80%. On a weekend van you're constantly in that top-up situation.

At 100Ah you've no margin for inefficiency. A Fogstar Drift 100Ah will genuinely behave like a bigger AGM bank in practice. Just make sure your existing alternator setup has some form of voltage-sensitive relay or a DC-DC charger — Transit alternators and lithium don't always get on without one.

@RayPowell one angle nobody's mentioned yet: a compressor fridge is the critical variable here. Those units cycle their compressor constantly during warm weather, and with AGM at genuine 50% DoD you're working from a much shallower well than the spec sheet implies. I've run a 12V compressor fridge as part of my static caravan emergency backup and the overnight draw consistently surprises people who haven't logged it properly. Recommend you put a basic battery monitor on whatever chemistry you choose before committing to bank size — actual consumption data is worth more than estimated figures. On a Transit Custom with tight fitment, the physical footprint of a lithium cell versus equivalent AGM capacity is also non-trivial. Fogstar and Renogy both have reasonable 100Ah slim-profile options. The premium is real, but on a single-battery setup the maths aren't as painful as scaling up a larger bank.

@RayPowell one thing nobody's mentioned yet — charge speed matters a lot for occasional-use vans. If you're only doing weekends, your alternator time is limited, and AGM will throttle back charging current well before it's full. A lithium like the Fogstar Drift will just take whatever your DC-DC charger can throw at it and be done quickly.

Worth checking what DC-DC isolator you're planning too — a proper Victron Orion-Tr Smart makes a big difference over a basic split-charge relay, especially with lithium.

I'm mid-build on a similar setup for a garden office rather than a van, but the charging behaviour question came up for me too. What's your solar situation — roof bars or nothing?

@RayPowell one practical point worth adding — at 100Ah, the usable capacity difference is stark. With AGM you're realistically working with 50Ah before you risk damaging the bank, whereas lithium gives you 80-90Ah from the same headline figure. For a compressor fridge that could genuinely be the difference between waking up to cold food or not on a longer stopover. That said, if your van is mostly weekend use with reliable engine charging between trips, a quality AGM will cope perfectly well. Lithium earns its premium most when you're regularly drawing the bank down and recharging in short windows. What's your typical usage pattern — are you driving decent distances between camps, or more of a static pitch situation? That answer would probably settle it either way.