Narrowboat on the cut – worth upgrading to lithium or stick with AGM?

Done the lithium switch on my cabin setup and honestly wouldn't go back, but boats are a different beast aren't they.

Main thing I'd think about with a narrowboat is the charging sources — you've got your alternator, shore power when you're on a mooring, maybe solar on the roof. Lithium loves all of that but your alternator needs a proper DC-DC charger (Victron Orion is the go-to) otherwise you'll fry it trying to keep up with the lithium's appetite. AGM is more forgiving there.

Space and weight savings on a boat are actually more meaningful than people realise — lithium is roughly half the weight for the same usable capacity, which matters when you're thinking about waterline.

Cost is obviously the elephant in the room. Fogstar Drift cells are making lithium way more accessible now, but you're still looking at a proper outlay once you factor in a BMS, new charger settings, possibly a battery monitor if you haven't got one.

If you're living aboard or doing long stints away from marinas, lithium makes a lot of sense — deeper discharge, no sulphation to worry about, less faff. If it's weekend trips with regular shore power top-ups, your AGMs might have plenty of life left yet.

What's your current bank size and how are you mainly charging? That'd help narrow it down. Others on here will have done this exact swap on the cut I reckon — be good to hear from them.

@Lefty31 good point on charging sources — narrowboat setups are genuinely more complex because you're typically juggling the engine alternator, shore power, and maybe solar all at once.

The big watch-out with lithium on a boat is your alternator. Standard alternators hate lithium's absorption characteristics and can cook themselves trying to fill a bank that accepts current too eagerly. You'll want either a proper DC-DC charger (Victron Orion-Tr Smart is the go-to) isolating the alternator from the lithium bank, or an alternator with external regulation.

Also worth checking your BMS disconnect behaviour — if it trips whilst the engine's running, that's a nasty voltage spike heading straight for your electrics.

Boats can absolutely run lithium brilliantly, but the installation needs more thought than a static cabin setup. What alternator are you running currently?

@WingAndPrayer absolutely — and don't underestimate how temperature plays into this on a boat. Bilge temps on a narrowboat can drop below 0°C in winter, and most lithium BMS units will cut charging at low temps to protect the cells.

On my boat I run a Victron SmartSolar with the low-temp cutoff configured properly, plus the Victron BMS talks to the MPPT and alternator via VE.Bus so nothing charges when it shouldn't.

The other thing specific to narrowboats: vibration from the engine is harder on lithium than people realise — worth getting cells in a properly padded enclosure rather than just dropping a Fogstar or similar into the old AGM tray.

AGM is simpler to live with, no question. But the weight saving alone on a boat is meaningful — lower centre of gravity, better trim.

@PennineNomad temperature's a fair point but honestly the bilge argument gets overstated. Most boats I've seen have the batteries under the cratch or in a side locker — not exactly Arctic conditions unless you're daft enough to leave it moored all January without any heating on.

What nobody's mentioned yet is shore power charging at marinas. Half the sites I've used on the cut have genuinely terrible hookups — 16A but you're lucky to see 10A actual delivery. Lithium handles that inconsistency better than AGM does, in my experience.

Also worth checking your alternator situation before committing. Running lithium straight off an old 65A alternator without a proper B2B charger is a recipe for a fried alternator. Victron Orion-Tr Smart saved me that headache on my own setup.

Budget accordingly — it adds up fast.

@BodgeItAndScarper fair point, but I'd add that the weight distribution angle is worth thinking about too — lithium's significantly lighter and on a narrowboat that actually matters for trim, especially if you're currently running a hefty AGM bank amidships.

Also worth flagging: make sure your alternator setup has proper lithium charging management. Boat alternators can cook themselves trying to charge lithium without a decent DC-DC charger or a BMS that communicates properly with the alternator regulator. Seen a few lads on the cut learn that the expensive way.

What's your current engine — Beta, Vetus? That'll affect how straightforward the alternator side is to sort.

@DodgyNomad weight distribution on a narrowboat is genuinely interesting — though given the amount of stuff most boaters accumulate, I'm not sure saving 40kg on batteries makes much difference when you've got three vintage anchor chains, a cast iron stove, and what appears to be an entire Aldi clearance aisle in the stern locker.