Anyone running lithium on a narrowboat? Tempted to ditch the AGMs

I've got a 58ft narrowboat and I'm currently running four 110Ah AGM batteries wired in 12V, charged off a 110A alternator via a Sterling B2B and a small 400W solar setup on the roof. It does the job when we're moving regularly, but sit still for a few days in winter and it gets pretty grim. The AGMs are also three years old now and starting to lose capacity noticeably.

Been seriously looking at swapping to two 200Ah lithium (LiFePO4) batteries instead, which would actually give me more usable capacity than the four AGMs even though it's fewer ah on paper. Thinking Fogstar Drift or possibly the Fogstar Lite to save a bit of money — has anyone used either on a boat? My concern is the damp environment and whether the casings hold up long-term compared to something like a Battle Born or Victron.

The other thing I'm not sure about is the B2B charger situation. My Sterling unit is set up for AGM profiles at the moment — does it need replacing entirely or can you just reprogram it for lithium? I've heard mixed things. Similarly wondering if the 400W solar and my Victron MPPT 75/15 will handle lithium charging fine without changes, or whether I need to look at a larger controller given the faster acceptance rate.

Would love to hear from anyone who's made this switch on a narrowboat or similar liveaboard setup. Particularly interested in real-world winter performance and whether you hit any issues with BMS cutoffs in cold weather.

@Paddy78 made the switch on a similar setup a couple of years back — went with two Fogstar Drift 200Ah 12V in parallel and honestly it transformed things. The usable capacity difference over AGMs is night and day, especially over winter when you're running the diesel heater blower and lighting constantly.

Your Sterling B2B should be fine with lithium provided you set the charge profile correctly — worth double-checking the output voltage settings. I'd also strongly recommend a proper BMS with low-temp cutoff if you're mooring up north; lithium really doesn't like charging below 0°C.

The weight saving alone on a narrowboat is worth it — shifted about 60kg out of the bow in my case which actually improved trim noticeably.

Only caveat: budget properly for a decent Victron BMV monitor, you'll want accurate SoC readings.

Great timing on this thread — I made the switch last spring on my 60ft trad and haven't looked back. One thing worth flagging specifically for narrowboats: make sure your Sterling B2B is set up correctly for lithium absorption/float profiles, or you'll either undercharge or stress the cells. Also worth thinking about where you're mounting them — lithium doesn't like being directly over the engine while it's running due to heat. I put mine midship under a cratch board seat and it's been spot on. @MarshChild is right about the usable capacity difference being a genuine game changer for liveaboards. My inverter runs the induction hob without any drama now whereas the AGMs would sag badly under that load. What's your current inverter situation @Paddy78?

@Paddy78 done exactly this on my old widebeam a few years back. The weight saving alone is worth it — AGMs are brutal on older boats with dodgy waterlines.

One thing people don't mention enough: your Sterling B2B will need checking. Some older firmware doesn't play nicely with lithium charge profiles. Ring Sterling direct, they're decent about it.

Also worth knowing — Fogstar Drift are solid but make sure you've got a proper battery protect or BMS-aware relay on the loads side. Victron do a good one.

Main gotcha on boats vs static installs is condensation and ventilation in the battery box. Lithium is more tolerant than AGM but still don't want damp terminals.

What's your inverter situation? That often dictates what batteries actually make sense.

Great thread — lithium on a narrowboat is absolutely worth it in my opinion. One thing I'd flag that hasn't been mentioned yet: make sure your Sterling B2B is the lithium-compatible version or has the right profile set. Some of the older units default to AGM charging curves and won't properly top up a lithium bank, leaving you perpetually sat at 80-ish percent. Worth double-checking before you commit to the switch. Also, with 400W solar and a decent B2B, two 200Ah lithium cells would likely serve you better than trying to match your current 440Ah AGM capacity — usable capacity is just so much more practical with lithium. What's your typical daily consumption like, @Paddy78? That'd help dial in the right bank size.

@Paddy78 one thing worth flagging with your Sterling B2B — make sure you've got the lithium-specific firmware or profile loaded. The default AGM profile will undercharge LiFePO4 and you'll think the bank is playing up when it's actually just never reaching proper absorption. Sterling do publish updated profiles but it's buried in their support docs.

Also worth checking your alternator temperature under heavy B2B draw. Lithium will happily pull maximum current continuously in a way AGMs never did — 110A alternators on older narrowboat engines can run quite hot when the B2B is hammering them hard on a depleted bank. A cheap CHT sensor saves potentially frying the alternator mid-cruise.

Fogstar Drift cells are worth a look for the build quality at the price point if you're going the DIY route.

Really good points from everyone here. One thing I'd add that hasn't come up yet — think carefully about your low-temperature charging situation. Lithium cells (particularly LFP) really don't like being charged below about 5°C, and on a narrowboat that's sitting unused over winter, your bank could easily get cold enough to cause issues. Some BMS units have built-in low-temp cutoffs which will simply prevent charging, so you won't damage the cells, but you could find yourself stranded with a flat bank on a cold morning. Worth looking for a battery with thermal protection built into the BMS, or factoring in some insulation around your battery box. Not a dealbreaker at all, just something AGM users switching over often don't anticipate. @Paddy78 where do you moor over winter — are you liveaboard or leaving her unattended for stretches?

Great thread @Paddy78. One thing nobody's touched on yet — cell balancing and BMS communication. A decent active balancer or at minimum a quality passive BMS that can talk to your charging sources properly is worth budgeting for. On a boat you're often doing lots of shallow cycles rather than full charge/discharge, which can cause cells to drift over time. I'd also strongly recommend a battery monitor like a Victron BMV-712 if you haven't got one already — knowing your true state of charge becomes even more important with lithium since there's no voltage sag to warn you you're getting low.