Switching from lead-acid to lithium on my 28ft narrowboat — worth the hassle?

Currently running four 110Ah AGM batteries (Numax) wired in parallel giving me 440Ah usable at roughly 50%, so about 220Ah in practice. Been doing this for three years on the boat and it's... fine, but the AGMs are getting tired and I'm weighing up whether to replace like-for-like or make the jump to LiFePO4.

The obvious candidates seem to be Fogstar Drift 100Ah or 200Ah cells, or going with something pre-built like a Victron Smart Lithium. The price gap is significant — Fogstar DIY route looks like it could save £600-800 versus an equivalent Victron setup. But I've also got a Victron MultiPlus-II 12/3000 as my inverter/charger, and I'm wondering how seamlessly that actually plays with non-Victron batteries in practice.

Main concern is the charging profile. My existing Victron kit is set up for AGM and I know lithium needs a different charge curve — no absorption float nonsense. Has anyone actually reprogrammed a MultiPlus-II for a Fogstar or similar non-Victron LiFePO4 and found it reliable long-term?

Also curious whether anyone's done this on a boat specifically — the damp environment and occasional violent tilting when a widebeam flies past makes me wonder if cell-level connections need any special treatment.

ZoeBurns | 847 posts

@RetiredChef2 Worth flagging you're posting in the Motorhome forum — you might get more narrowboat-specific advice over in the Marine & Waterways section, though plenty of crossover knowledge here regardless!

On the lithium question — the humidity and temperature fluctuations on a boat can be trickier than a campervan setup, so make sure whatever BMS you go with handles low-temperature cutoff properly. Narrowboats sitting unused through a cold January can cause issues with cheaper lithium setups if they try to charge below around 5°C.

Also worth considering: with lithium you can genuinely use 80-90% of capacity, so you'd be looking at roughly 350-380Ah usable from a like-for-like 440Ah bank — that's a significant real-world difference from your current 220Ah. The weight saving on a boat matters too!

Ben1989 | 312 posts

@RetiredChef2 Agree with @ZoeBurns, you'll get much better advice over in the Waterways & Marine section — narrowboat setups have some specific quirks, particularly around your inverter charger and how you're charging off the engine (alternator protection is crucial with lithium, you'll likely need a B2B charger).

That said, the short answer to your question is yes, absolutely worth it in my opinion. I made the switch on my camper last year and the usable capacity difference is night and day. You'd be looking at genuine 80-90% DoD versus your current 50%, so effectively doubling your usable bank without adding weight.

Main headaches are the BMS compatibility with your existing charger and shore power setup. What inverter/charger are you running currently?

MXM_OffGrid | 1,204 posts

@RetiredChef2 Jumping in before you migrate to the right subforum — lithium on a narrowboat is absolutely worth it in my view, but the boat context does add a couple of considerations beyond a typical campervan swap. Mainly: are you running a BMV or similar battery monitor? Lithium's flat discharge curve means your existing voltage-based gauges will be pretty useless without one. Also worth thinking about your alternator setup — unprotected lithium banks have killed a fair few alternators on canal boats because they accept charge so aggressively. A DC-DC charger between your engine and the bank is strongly recommended. The usable capacity jump alone (roughly 80-90% vs your current 50%) makes the switch compelling though. What's your primary charging source — solar, shoreline, engine?

Curly7 | 2,341 posts

@RetiredChef2 One thing worth adding before you head over to the Waterways subforum — your charging sources will need attention too. Alternators and most shore power chargers won't play nicely with lithium without either a proper DC-DC charger (B2B) or a lithium-compatible multi-stage charger. Budget that in alongside the battery cost itself. On a narrowboat your engine alternator is often doing heavy lifting, so a decent Victron Orion or similar between alternator and lithium bank is pretty much essential rather than optional. Can make a significant difference to the overall project cost.

ThistleRunner | 876 posts

@RetiredChef2 Worth mentioning something nobody's touched on yet — check your inverter/charger compatibility before anything else. Many older units struggle with lithium's charge profile, particularly the absorption stage, and you'll potentially need a Victron or similar that handles LiFePO4 natively. Also, with a narrowboat specifically, think carefully about battery placement relative to the engine bay — lithium doesn't appreciate the temperature extremes you can get in poorly ventilated engine rooms. That said, going from your current ~220Ah usable to something like 80-90% DoD on lithium will feel transformative. Definitely head over to the Waterways subforum as others suggest — there are a few liveaboards over there who've done exactly this conversion and their threads are genuinely useful.

BoatMark | 847 posts

@RetiredChef2 Living proof this works — swapped my 28ft out of AGMs into two Fogstar Drift 200Ah LiFePO4s last spring and the weight saving alone stopped my stern sitting like a sulking labrador. Your 440Ah lead-acid prison becomes effectively 400Ah usable lithium with half the weight and none of the "why is my Victron MPPT crying at 2am" drama. Only gotcha nobody's mentioned: narrowboat bilges love to be damp and lithium BMS boards are less forgiving than your old AGMs were — get them up off the floor and ventilated.

KevWatson57 | 412 posts

@RetiredChef2 One thing I'd flag from my own boat setup — lithium is genuinely transformative but your alternator situation needs sorting first. A standard alternator will hammer itself trying to fill lithium's low internal resistance. Either fit a DC-DC charger (Victron Orion-Tr Smart is what I used) to isolate the alternator from the bank, or get a lithium-compatible external regulator. Done properly it's brilliant — I've had zero issues. Get this wrong though and you're looking at a fried alternator mid-canal somewhere inconvenient.

WheresMeWires | 1,243 posts

One thing worth flagging that nobody's mentioned — on a narrowboat specifically, your alternator situation matters massively with lithium. Lead-acid batteries naturally limit alternator charge current as they bulk up; lithium will happily pull maximum current continuously and cook your alternator.

Worth looking at a Victron Orion DC-DC charger between your engine alternator and the lithium bank. Yes, it adds cost, but I've seen two alternators fail on boats that skipped this step. The Orion also lets you programme proper charge profiles.

@BoatMark — what did you do about alternator protection with your Fogstar setup?

GlenRobinson | 634 posts

@RetiredChef2 One practical consideration specific to narrowboats — check your alternator situation carefully before committing. Lithium's low internal resistance means it'll happily flatten a tired alternator trying to bulk charge at full whack. Lots of boat owners fit a DC-DC charger (Sterling or Victron Orion) between the alternator and lithium bank rather than charging direct. Adds cost but protects your alternator and gives the BMS something predictable to work with. Also worth confirming your existing 240V charger is lithium-compatible — many AGM chargers aren't, particularly older units.