Narrowboat 12V system overhaul — worth going lithium when you're mostly on shore power?

Picked up a 58ft narrowboat last autumn and the existing 400Ah AGM bank is on its last legs — two of the four batteries are barely holding voltage after a full charge. The boat sits on a marina berth about 70% of the time with a 16A hookup, so it's not like I'm off-grid adventuring every weekend. But we do take 2–3 week continuous cruising trips each summer with no hookup at all, running a 12V compressor fridge, lighting, water pump, and occasionally a 240V inverter load for a laptop or small appliance.

Thinking about a 200Ah lithium (LiFePO4) drop-in replacement rather than like-for-like AGM. The usable capacity on 200Ah lithium at 80% DoD would actually exceed what I'm getting from the degraded AGMs, and obviously the weight saving is significant on a boat where trim actually matters. Looking at Fogstar Drift 200Ah units — two in parallel would give me 400Ah nominal with decent BMS protection. The alternator on the Beta Marine engine is only a 70A unit though, so I'd want to fit an external regulator or at minimum a DC-DC charger to avoid cooking it when the lithium bank is hungry.

Main thing I'm unsure about: does anyone run a largely shore-power-based setup on lithium and find the partial state of charge situation problematic? I've read conflicting things about whether keeping LiFePO4 cells sat at 100% SOC on a float charge damages them long-term — some say store at 50–80%, others say modern BMS units handle it fine. The Victron shore power charger I'd fit (probably a Phoenix Smart 30A) can be programmed with an absorption cutoff and a storage float, which should help.

Also wondering whether the Beta alternator protection issue is actually as serious as some forums make out, or whether a quality drop-in BMS with a gradual charge acceptance curve smooths it enough in practice

Gazza24 | 847 posts

Good timing to reconsider the whole setup, @PennineVanLifer. If you're predominantly on shore power, lithium is harder to justify purely on cost grounds — AGMs are quite happy sitting at float charge for extended periods, which is essentially what marina life demands. Lithium actually prefers not to be kept at 100% SOC long-term, so shore power becomes slightly awkward without a proper battery management setup.

That said, if you do extended cruises away from the marina — holidays, summer liveaboard stretches — lithium's usable capacity advantage becomes genuinely worthwhile.

My honest suggestion: replace the two duff batteries with matched AGMs for now, restore your 400Ah properly, and reassess in a couple of years when lithium prices drop further. Don't spend £1,500+ solving a problem that a £200 pair of batteries temporarily fixes.

What's your typical off-grid duration when you do cruise?

VanHolly | 312 posts

@PennineVanLifer Worth thinking carefully about your actual usage pattern before committing to lithium. If you're genuinely marina-based 80%+ of the time, decent AGM replacements might serve you better value-wise — lithium's advantages really shine when you're cycling deeply and regularly. That said, if you do extended cruising stints away from the pontoon, lithium's usable capacity and faster recharge from your engine alternator become genuinely compelling. One thing I'd flag specifically for narrowboats — make sure whatever BMS you go with handles low-temperature cutoff sensibly, because bilge temperatures in a UK winter can catch people out. What's your inverter situation like? That often shapes which direction makes more sense.

EwanYoung98 | 203 posts

@PennineVanLifer One angle nobody's mentioned yet — if you're mostly on shore power, have you considered just replacing with AGM again but dropping to a smaller bank? A quality 200Ah AGM set properly maintained will serve you far better than a neglected 400Ah one. Lithium genuinely shines when you're cycling hard off-grid, but if you're plugged in 80% of the time you're paying a significant premium for benefits you'll rarely use. That said, if you do fancy occasional extended cruising or mooring away from the marina, lithium starts making more sense. What's your typical summer pattern like — are you cruising at weekends much, or mainly static?

WezFisher | 1,203 posts

Lived aboard a 58-footer for three years before moving to the current boat, so I know exactly the scenario you're describing.

Here's the thing nobody mentions: shore power changes. You'll have periods — festival weekends, winter pump-out queues, summer cruising fortnights — where you're suddenly off it for days. Happened to me constantly at Aynho Wharf.

If your AGMs are shot anyway, the replacement cost calculation shifts considerably. A 200Ah Fogstar Drift lithium will genuinely outperform your dying 400Ah bank and cost less than four new AGMs once you factor in lifespan.

The real question is your existing charger. Shore power means your battery charger does most of the work — a cheap unit will absolutely murder lithium cells. Before anything else, check what you've got. If it's not lithium-compatible, budget that into your sums.

HarryWalker | 847 posts

@PennineVanLifer One practical thing worth flagging — lithium on a marina berth means your charger really matters. Most older boat chargers aren't lithium-compatible and will either undercharge or cause grief with BMS cutoffs. Budget for a decent multi-stage charger like a Victron Blue Smart alongside the cells themselves. Also, if you're on 16A shore power (common on UK marina berths), check the charger's input rating won't trip your bollard — I've seen lads trip out their neighbours at 2am doing a fast charge! If shore power reliability is genuinely your main concern, honestly even two fresh AGMs might sort you for another five years at a fraction of the cost. Lithium shines when you're cruising the canals for weeks — the Llangollen, say — rather than sitting plugged in at a berth.

BatteryEmma | 1,847 posts

Worth noting that your shore power charger compatibility matters enormously here. Most marina pedestals deliver a fairly dirty feed, and cheap onboard chargers will absolutely hammer AGM-spec absorption voltages into lithium cells if you don't swap the charger profile or the unit entirely.

When I rewired my shepherd's hut system I specified a Victron Blue Smart IP22 specifically because the lithium preset cuts absorption short and drops to float properly — on a narrowboat I'd be looking at their shore power convertor range or at minimum confirming your existing unit has a dedicated lithium programme.

Also worth checking: does your boat have a battery-to-battery charger for the engine start battery? If you swap the domestic bank to lithium you'll need to isolate the alternator charging circuit properly — a Victron Orion-TR Smart handles this well and prevents overcharging the start battery during cruising.