Motorhome lithium upgrade — ditching the AGMs

Finally bit the bullet and swapped out the pair of 100Ah AGMs for a single Fogstar 200Ah LiFePO4. Absolute game-changer for the motorhome setup.

The AGMs were killing me — constant voltage drop under load, and I was getting maybe 60% usable capacity before they sulked. Plus the weight was ridiculous; those two batteries alone were pushing 60kg combined.

Current setup:

• Fogstar 200Ah 12V LiFePO4 (integrated BMS, ~27kg)
• Victron SmartSolar MPPT 100/20 controller
• 400W rigid solar on the roof (split east/west orientation)
• 2000W pure sine Victron inverter
• Separate 100Ah AGM still handling the starter battery circuit (didn't want to risk the van not starting)

The lithium gives me proper usable capacity — I'm confidently drawing 3-4kW for the kettle without watching the voltage collapse. Charge times are mental too; went from half-flat to full in about 4 hours of decent sun, whereas the AGMs would take a full day.

Weight saving alone means I've got more payload capacity for longer trips. The integrated BMS handles all the low-temp cutoffs automatically, so I'm not paranoid about winter charging like I was before.

Only downside is the upfront cost — wasn't cheap — but the lifespan should be 10+ years versus maybe 4-5 for the AGMs. Already doing the maths and it tracks out.

Would absolutely recommend if you've got the budget. The difference in daily usability is genuinely significant.

That's a solid upgrade, @VoltAlison. Did you keep the same charger setup or swap that out too? I'm wrestling with a similar decision for my own van — got twin 110Ah AGMs that are approaching end of life, and the Fogstar 200Ah keeps cropping up in my research.

Question though: how's your BMS integration working with whatever charge controller you're running? I'm particularly interested in whether you've noticed any difference in how quickly the lithium charges compared to the AGMs, especially if you're relying on alternator charging whilst mobile.

Also, what's your solar array looking like? Trying to work out if I'd need to beef up my panels to properly utilise the faster charge acceptance of LiFePO4, or if my existing 400W would suffice for emergency backup scenarios.

@VoltAlison, brilliant move! The voltage stability with LiFePO4 is night and day compared to AGMs, especially when you've got multiple devices running.

@ExTrucker73, worth mentioning that most standard chargers will work fine with LiFePO4, but you'll want to check the charge profile. Some older three-stage chargers can be a bit aggressive on the absorption phase. If the Fogstar came with specs, check if your current charger suits them—many folks find they need a dedicated LiFePO4 charger or a programmable one to get the best longevity.

@VoltAlison, did you upgrade your BMS setup as well, or is the battery's internal management handling everything? That's often the overlooked bit when people swap over. Also curious whether you're running solar, mains, or alternator charging primarily?

Have you had to adjust your charger settings for the Fogstar? I'm looking at something similar for my garden office setup and I'm a bit uncertain about whether my existing MPPT needs reconfiguring.

The voltage stability sounds perfect for what I need — I've got an EV charger that gets a bit temperamental when the input voltage dips. Currently running AGMs as well and they're just not holding steady enough under load.

Also wondering about BMS integration — does the Fogstar play nicely with monitoring systems, or are you just relying on the built-in battery management? I've seen some folk mention compatibility issues with older Victron gear.

Been eyeing the Fogstar units meself for the shepherd's hut — they're meant to be proper solid. Question though: are you running a dedicated BMS with it, or relying on the battery's internal protection? I've heard mixed reports about integrating LiFePO4 with older leisure chargers.

Also curious about your charge profile — did you need to dial back the amperage compared to what you were pushing through the AGMs? I'm worried about compatibility with my current Victron MPPT, as I gather some lithium batteries can be finicky about how quickly they'll accept charge.

Did you notice much difference in how your inverter behaves? The AGMs in my boat seem to sag annoyingly when the kettle's on.

@VoltAlison, this is exactly what I've been pondering for the van. How's the weight saving treating you? Those AGMs were an absolute brick to haul about.

Quick question on the Fogstar — did you need to swap out your existing solar controller, or did it play nice with what you had? I'm running a Victron MPPT but heard some folks mention they had to tweak absorption voltage settings for lithium. Also, what's the real-world usable capacity you're getting? I know the spec says 200Ah, but curious if you're actually cycling the full depth or being more conservative with it.

The voltage stability under load is what's got me convinced though. Tired of dropping to 11V when the kettle's on and the fridge is running.

Solid upgrade. I went Fogstar 200Ah for the garden office last year and the difference is mental — no more voltage sag when the kettle and charger are running simultaneously.

@AnneOliver, yeah you'll need to dial in the charger. Fogstar's pretty forgiving but I had to tweak the absorption voltage on mine (MPPT controller). Their manual's decent though, walks you through it.

Weight saving is noticeable too. The AGMs in my old setup were an absolute pig to lug around, and lithium means you're actually using the full capacity without tanking the voltage. With AGMs you're really only getting 50% usable if you want them to last.

Only thing — keep an eye on cell balancing if you're running a basic charger. The integrated BMS handles most of it, but I picked up a cheap Victron battery balancer just for peace of mind. Probably overkill but worth the £30 for the motorhome application.

The voltage stability you're getting is the real win here. I've got a pair of Fogstar 200Ah units in the camper (paired for redundancy, bit overkill but peace of mind), and the difference under load is night and day compared to AGM chemistry.

Where you'll notice the biggest practical change: microwave, kettle, or any inductive load won't cause your inverter to throttle anymore. The LiFePO4 can actually sustain high discharge rates without the sag collapsing your system voltage. AGMs effectively halve their usable capacity under high drain, whereas the Fogstar will sit at a flat 13.2V until it's genuinely depleted.

One thing worth verifying though — what charger are you running? The Fogstar benefits massively from a smart three-stage charger that recognises LiFePO4 chemistry. If you're still using whatever was floating about with the AGMs, you might not be getting the full charge cycle efficiency. Victron MPPT or a decent ICharger makes a tangible difference.

Also curious how you're managing the thermal side in

Quick question about the pairing setup — @LDVCamper, are you running those two 200Ah units in series or parallel? I'm looking at similar for a shepherds hut build and trying to work out if I should go dual Fogstars or stack the voltage differently.

@VoltAlison, what's your charge controller handling? I'm wondering if a single 200Ah needs beefier kit than the old AGM setup, or does the LiFePO4 chemistry actually play nicer with standard Victron/Epever regulators? The voltage stability is definitely the draw for me — those AGMs were a nightmare in winter.

Also curious whether you've had to adjust your inverter settings at all, or did it just slot in without faffing about?

Parallel, definitely. Series would be a nightmare for balancing and you'd lose all the usable capacity benefits — defeats the purpose of upgrading in the first place.

The thing most folk don't realise is that LiFePO4 cells are fussy about voltage matching when you're stacking them. With parallel, you're just doubling your amp-hour capacity at the same nominal voltage, which your BMS handles without breaking a sweat. Each battery monitors itself independently.

I've got a single 200Ah Fogstar in my narrowboat setup paired with a Victron Multiplus II, and the voltage stability is genuinely brilliant compared to the AGMs I ran before. You're sitting at a flat 13.2–13.4V under normal discharge rather than sagging down to 11.8V by mid-afternoon like lead-acid does.

One caveat though — if you're going parallel, make sure your cabling is properly sized and your battery interconnects are dead short (proper busbars, not thin automotive cable). Voltage drop across dodgy connections will cause the batteries to fight each other during balancing. Seen that go sidew

Swapped AGMs for a single Fogstar on the narrowboat and can confirm the voltage stability is chef's kiss — no more watching the kettle struggle at 11.2V when you're trying to make a brew at anchor.

The real trick nobody mentions is how much space you free up; those AGMs were like having a particularly heavy, moody guest who constantly complains. One 200Ah lithium sits there quietly doing its job whilst you've got room for, well, more lithium if you're that way inclined.

Fair warning though — your charger will need a firmware update to play nicely with LiFePO4, or you'll be babysitting voltage limits like a neurotic parent. Checked mine three times before realising the Victron settings were the culprit, not a dodgy battery.

Went LiFePO4 in the van last year and never looked back. The voltage stability alone is worth it — AGMs would sag to 11.8V under kettle load, the Fogstar stays flat until it's genuinely low.

Fair warning though: if you're running solar charge controller, make sure it's lithium-aware. My old MPPT kept bulk charging way too long until I updated the settings. Victron ones handle it automatically which is nice but pricey.

Only gotcha I've had is remembering the BMS disconnect for winter storage — bit different ritual than just leaving the AGM isolator on. Worth the tradeoff for that extra usable capacity though.

The voltage stability is only half the story though — what really matters is the usable capacity. Those AGMs you're ditching were probably only giving you 50% DoD safely, so your effective capacity was closer to 50Ah each. The Fogstar 200Ah at 80% DoD gets you 160Ah usable, which is genuinely 1.6x the real-world capacity of your old setup.

Worth checking your BMS settings if you haven't already. The Fogstar's built-in management is decent, but I'd recommend configuring the low-voltage cutoff around 10.5V rather than the factory default — gives you that extra safety margin without leaving capacity on the table.

One thing I'd flag: lithium charging profile in winter needs attention. If you're running solar through the motorhome, make sure your MPPT or charger has temperature compensation enabled. LiFePO4 gets grumpy below 0°C on charge, and I've seen folk get mysterious cutoffs because the BMS throttles the input when it's cold.

The voltage sag from the old AGMs must've been brutal

Curious about your charging setup, @VoltAlison — are you running a Victron MPPT into that Fogstar, or something else? I'm looking at doing similar in my van but I'm a bit concerned about the BMS compatibility with my existing solar controller.

Also, how's the weight saving treating your fuel consumption? I know lithium's considerably lighter than lead-acid, but I haven't seen many folk actually quantify the difference over a longer trip.

One thing I'd flag though — make sure you've got decent low-temperature protection sorted if you're planning winter trips. The Fogstar's decent but you'll want to disable charging below about 0°C if you're heading anywhere chilly. Learned that the hard way with a Renogy system last year.

Great upgrade, @VoltAlison! The move from AGMs to LiFePO4 really is transformative for motorhome living.

One thing worth mentioning alongside the voltage stability @DownsExplorer highlighted — make sure your charger is LiFePO4-compatible. If you're still using an old AGM-oriented charger, you'll want to swap it out. The charging profile is quite different, and pushing AGM charge curves into lithium can cause issues down the line.

Also worth considering: have you got a proper BMS (Battery Management System) monitoring that Fogstar? Most decent LiFePO4 units come with one built-in, but if you're integrating it with existing solar or a leisure battery management system, just double-check the comms are talking to each other properly.

The single 200Ah versus your pair of 100Ahs is also clever — less wiring complexity and better thermal management generally. Just watch your charging rate; aim for around 0.5C (so 100A max for a 200Ah) to keep the cells happy long-term.

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