Swapped out my 100Ah AGM for a 100Ah LiFePO4 — was it actually worth the cost?

Finally pulled the trigger last month and replaced the ageing Banner Energy Bull AGM in my Transit-based camper with a Fogstar Drift 100Ah LiFePO4. Been running it for about six weeks now so thought I'd share some early thoughts and see if others have made the same switch.

The usable capacity difference is immediately obvious. With the AGM I was nervous going below about 50%, so effectively had 50Ah to play with. With the Fogstar I'm comfortably using down to 10–15% without any anxiety, so in practice I've gone from roughly 50Ah to around 85Ah of real-world capacity — same physical battery, same price point on paper, but a completely different experience. Pair that with the fact it charges back up so much faster from my 200W roof panel and the Victron 75/15 MPPT, and it's been a noticeable upgrade on longer off-grid stops.

The one thing I wasn't fully prepared for was needing to double-check my B2B charger compatibility. I'm running a Sterling BB1230 and it does have a lithium profile, but I had to dig through the manual and reprogram it — took a good hour of head-scratching. Worth doing properly though.

So — for those of you still on AGM, is the price premium holding you back, or have you already made the jump? Curious whether anyone's gone for a second 100Ah battery in parallel rather than upgrading chemistry, and whether that's worked out cheaper overall.

@GlenDixon Interesting timing — I'm currently weighing up exactly the same swap for my van build.

Quick question though: did you need to replace your existing B2B charger or shore power charger to accommodate the LiFePO4 chemistry? I've read conflicting things about whether a charger set to AGM profile will actually damage the cells long-term, or whether it's just suboptimal.

Also, what's your resting voltage sitting at after a typical overnight discharge? I'm trying to establish whether the claimed 80% usable capacity translates meaningfully in real-world UK conditions — particularly over winter when you're not getting the solar input to regularly top it off to 100%.

The Fogstar Drift seems to be the go-to recommendation on here but I want to understand the actual charging infrastructure changes required before committing.

@GlenDixon six weeks is about when the penny properly drops, isn't it.

The bit nobody mentions beforehand: usable capacity is the real story. Your old Banner was giving you maybe 50Ah before you started worrying about damage. That Fogstar is handing over 90-100Ah without breaking a sweat. You've effectively doubled your usable bank without adding a second battery.

I made the same swap on my boxer-based build two years ago and the thing that genuinely surprised me was how flat the discharge curve is — my 12v fridge stopped hunting and cycling so much because voltage wasn't sagging under load.

Worth noting for fellow UK folk: Fogstar's warranty support is actually decent, which matters when you're parked on a Scottish hillside wondering if your battery is the problem.

The upfront cost stings once. The AGM replacement cycle stings repeatedly. Maths eventually wins.

@GlenDixon worth noting the charging side too — AGMs want a proper absorption stage that eats time, whereas LiFePO4 just... fills up and stops. On my motorhome setup with a Victron SmartSolar I reprogrammed the profile when I switched and suddenly my panels were finishing the job an hour earlier most mornings.

One thing I'd flag: check your vehicle's B2B charger or split charge relay is actually LiFePO4 compatible. Some older VSR setups don't deliver enough voltage to properly top a lithium up, so you're potentially leaving 10-15% sitting on the table every time you drive.

Six weeks in you're probably still finding little wins. Wait until your first proper cloudy week — that's when the deeper usable capacity really proves its worth compared to the AGM.

Great thread @GlenDixon — would be keen to hear your full write-up once it's posted.

One thing worth adding to what @HO_Marine touched on: if you're running a B2B charger off the alternator (which I'd strongly recommend rather than relying on the vehicle's smart alternator directly), do check it's properly profiled for LiFePO4. I made the rookie mistake of leaving mine on an AGM profile for a fortnight before noticing it wasn't hitting full charge. The Fogstar Drift in particular seems to want a clean 14.6V absorption to really top off properly.

Also worth keeping an eye on your battery monitor if you have one — resting voltage readings mean something completely different now compared to your old AGM, so recalibrate your expectations of what "80% full" actually looks like on the gauge.

@GlenDixon one thing I'd stress from running LiFePO4 on my narrowboat (Victron MPPT into Fogstar cells) — check your alternator charging situation carefully.

Most Transit alternators will see a LiFePO4 as a near-dead short during bulk charge and run at full current continuously. AGMs naturally back-off the alternator as voltage rises; LiFePO4 doesn't behave the same way. Over time that's hard on the alternator, particularly on shorter runs where it never gets a rest.

A DC-DC charger (Sterling or Victron Orion-Tr Smart) between the alternator and the leisure battery is worth serious consideration — controls the charge current properly and protects both the alternator and the BMS.

It's the upgrade people skip to save £80-100 and then regret when the alternator goes six months later.

@GlenDixon good timing on this thread — from my years wiring up leisure setups I'd add one thing people often overlook: check your B2B charger or split charge relay is actually compatible with LiFePO4 profiles. Some older relay setups will just tick along charging at whatever voltage the alternator throws out, which isn't ideal long-term for the cells.

Also worth confirming your battery-to-battery charger has a proper LiFePO4 programme rather than just a "lithium" setting that might actually be tuned for LiPo chemistry — they're not the same thing.

Six weeks in you've probably not done a massive mileage yet, so that alternator charging question will matter more come summer touring. Fogstar's a decent choice mind you — good BMS on those units. Looking forward to your write-up @GlenDixon.

@GlenDixon six weeks in and you haven't posted a full write-up yet — classic LiFePO4 adopter behaviour, too busy actually using the extra usable capacity to come back and tell us about it 😄

Genuinely though, on my static caravan the switch felt like going from dial-up to broadband — same "100Ah" on the label, completely different experience in practice. The Fogstar Drift specifically holds voltage so well my 12V fridge motor stopped that annoying mid-afternoon grumble it always did when the AGM was sagging.

@GlenDixon worth it every time in my experience. Running a Fogstar 100Ah in my shepherd's hut and the usable capacity difference alone sold it for me — you're genuinely getting 80-90Ah versus maybe 50Ah you'd safely pull from that Banner.

One thing nobody's mentioned yet: weight. Swapping AGM for LiFePO4 in a Transit matters more than people realise, especially if you're near payload limits. Worth checking your figures if you've added other kit recently.

Also make sure your alternator charging profile is sorted — a lot of factory B2B chargers aren't optimised for lithium out of the box.

@GlenDixon curious whether you had to swap out your existing charger too, or did it work with what you already had? I'm in a similar position with my tiny house build — running a Banner AGM currently and eyeing up the Fogstar Drift, but I've got a basic Victron IP22 which I think handles LiFePO4 profiles. Did you need to adjust any charge parameters or was it fairly plug-and-play with your existing setup?

@GlenDixon great timing on this thread — been watching the Fogstar Drift get popular on here lately. @WD40Wizard23 raises a really important point actually. A lot of people overlook the charger compatibility side and it can catch you out. Worth checking your split charge relay too if you're charging from the alternator — some older VSRs aren't ideal with LiFePO4 due to the flatter voltage curve. They can disconnect prematurely thinking the battery's full when it isn't. A DC-DC charger like a Victron Orion is the proper solution there. @GlenDixon did you change anything on the charging side, or were you already set up for it?