Narrowboat lithium upgrade — worth ditching the AGMs mid-season?

Running a pair of 110Ah AGMs on my narrowboat at the moment and they're on their last legs. Probably getting 60% capacity out of them now, which is painful when you're trying to run the inverter for an hour in the evening without the engine on.

Been eyeing up a Fogstar Drift 200Ah to replace both — roughly the same footprint, massively better usable capacity. Only thing putting me off is doing the swap now rather than waiting til winter. We're mid-cruise season and I don't fancy being stranded on the Trent with a half-finished install.

Main worry is the Victron SmartSolar settings — I've got a 100/30 MPPT and I know I need to reconfigure the charge profile for LiFePO4. Anyone done a swap on the water rather than on a driveway? Is it genuinely a weekend job or am I kidding myself?

Also curious whether anyone's running a Fogstar Drift specifically on a boat — any issues with damp or vibration compared to the van installs you see everywhere?

@SolarJulie mid-season is exactly the right time to swap — nothing like a Bank Holiday weekend with a flat battery to really motivate the decision. 🔋

Went Fogstar Drift 100Ah in my static and never looked back; the usable capacity jump from degraded AGMs is honestly embarrassing. Your inverter will basically feel like it's been liberated.

Only narrowboat-specific thing I'd flag: check your alternator setup — lithiums will absolutely hammer an unprotected alternator when the bank's low, so a Sterling B2B or at minimum a proper DC-DC charger is non-negotiable before you're cruising the cut.

Victron BMS talking to your Cerbo if you've got one = smug canal life sorted.

@SolarJulie I did almost exactly this swap on my cabin setup — waited too long, then ended up in a crisis moment that forced my hand anyway. Degraded AGMs are a false economy; you're essentially nursing something that's already half-dead.

The real narrowboat consideration nobody mentions is weight distribution — lithium is dramatically lighter, so factor in how that shifts your trim if you're replacing a bank tucked in the bow or stern locker.

For a live-aboard replacement I'd look seriously at Fogstar Drift cells — they've become the go-to here for value-to-quality ratio — paired with a Victron SmartShunt so you actually know your state of charge rather than guessing.

Mid-season means you've still got decent solar harvest ahead of you to properly cycle and bed them in. That's the ideal window, honestly.

@SolarJulie the 60% figure is your answer really — you're already running a degraded system, so there's no "good" time to delay it further.

Worth noting for narrowboat use specifically: lithium handles the constant partial state of charge (CPSC) that liveaboard/continuous cruiser patterns create far better than AGM ever will. AGMs hate sitting at 70-80% day after day.

Fogstar Drift 100Ah cells are popular in the boat community right now — good price point and they've held up well from what I've seen. Pair with a Victron BMS and you're sorted. Make sure your alternator charging is sorted too though — an uncontrolled alternator will kill a lithium bank faster than a dodgy Bank Holiday mooring.

The upfront cost stings but you won't look back.

@SolarJulie one thing worth factoring in specifically for narrowboat use is the weight saving — lithium will be considerably lighter than a pair of 110Ah AGMs, which actually improves your trim slightly. More practically though, lithium handles the partial state of charge that's almost inevitable on a boat far better than AGMs do. If you're regularly running the inverter and not always getting a full charge cycle in, AGMs punish you hard for that. Lithium simply doesn't care as much.

Do check your alternator situation before you commit though — lithium charges faster and your alternator may not have adequate protection against the load dropping suddenly when the BMS cuts off. A decent DC-DC charger between the alternator and your lithium bank is the sensible route. What's your charging setup currently — solar only or do you run the engine regularly?

Really good point from @DailyDream on the weight — that's often overlooked on boats. One thing I'd add specifically for mid-season: check whether your existing alternator/charger setup is lithium-compatible before you pull the trigger. A lot of narrowboat setups have older split-charge relays or basic Beta/Barrus alternator regulators that will just hammer a lithium bank trying to bulk charge it indefinitely. You'll want either a proper DC-DC charger (Victron Orion is the go-to) or a compatible alternator regulator, otherwise you risk issues with the BMS cutting out mid-charge and potentially damaging your alternator too. Budget that into the upgrade cost. The battery swap itself is straightforward, but the charging side catches people out more often than anything else on boat conversions.

@SolarJulie did this exact swap on my motorhome a couple of seasons back — pulled out two tired AGMs and dropped in a pair of Fogstar Drift 100Ah lithiums. The bit nobody warned me about was the BMS behaviour at low temperatures. Narrowboat winters can be brutal, and most lithium cells won't accept charge below around 5°C without damage. Worth checking whether the BMS on whatever you buy has low-temp cutoff and whether your charger/solar controller will handle the handshake properly — my Victron SmartSolar sorted itself out, but it needed a firmware update first.

Mid-season is honestly fine for the swap if your AGMs are already struggling. A degraded bank is a degraded bank regardless of the calendar.

Don't have narrowboat experience myself but I've run lithium in my garden office and a small cabin setup — the difference mid-discharge is night and day compared to AGMs.

One thing nobody's mentioned yet: BMS compatibility with your alternator. Narrowboat alternators can cook a lithium BMS if there's no isolation when it hits full charge. Worth looking at a Victron Cyrix or a dedicated DC-DC charger between the engine and your battery bank before you commit.

Fogstar do decent 100Ah cells at reasonable prices if budget's tight. Just don't skip the alternator protection bit.

Swapped my AGMs for a pair of Fogstar Drift 100Ah units last spring mid-season and honestly don't regret pulling the trigger early. The usable capacity jump was immediately noticeable — running the inverter without that anxious eye on the battery monitor is proper liberating.

One narrowboat-specific thing nobody's mentioned yet: check your alternator charging profile. A lot of older boat engines will hammer lithium at the wrong voltage without a proper B2B charger in between. Victron Orion-Tr Smart sorted that for me. Worth factoring into your budget before you commit.

@SolarJulie one thing nobody's mentioned yet — check your alternator charging situation before you commit. Narrowboat engines often run older alternators that'll cook themselves trying to bulk-charge lithium without a proper DC-DC charger (a Victron Orion-Tr Smart is the standard fix here). Budget that into the upgrade cost or you'll be replacing the alternator too. Also worth confirming your solar controller is lithium-compatible — a Victron SmartSolar will handle the profile fine, but some cheaper units won't. Get those two sorted and the swap mid-season makes complete sense.

@BrookRunner raises something really important actually. When I upgraded the narrowboat I nearly fried my alternator because it was happily dumping full current into the lithium bank without any current limiting. Ended up fitting a Victron Orion-Tr Smart DC-DC charger between the alternator and the lithium bank — complete game changer. Isolates everything properly and protects the alternator from cooking itself on long cruising days.

@SolarJulie mid-season is honestly fine for the swap. If anything you want it done before winter mooring when you're relying on batteries harder. Just budget for that DC-DC charger alongside the cells themselves.

@Jess1989 and @BrookRunner make a really crucial point there — a decent DC-DC charger (B2B) between the alternator and the lithium bank is non-negotiable in my view. I'd recommend looking at the Victron Orion-Tr Smart range; the 30A isolated unit is brilliant for narrowboat setups and protects both your alternator and the new batteries. Mid-season swap is absolutely fine otherwise — I did mine in July on the Shroppie and the difference was immediately noticeable. Just factor the B2B cost into your budget from the outset rather than treating it as an optional extra.