Swapped my boat's lead acid bank for lithium last summer — here's what actually happened

Been meaning to write this up for ages. Last July I finally ripped out the old 200Ah AGM bank on my 32ft canal boat and replaced it with two Fogstar Drift 100Ah 12V lithium cells wired in parallel. Night and day difference in usable capacity — I was getting maybe 80Ah out of the AGMs before the voltage sag became a problem, now I'm genuinely pulling 180Ah without breaking a sweat.

The trickier bit was sorting the charging. My old Victron SmartSolar 100/30 handled the transition fine once I'd set a custom lithium profile — 14.2V absorption, no float, absorption time cut right down. The engine alternator was the headache though. Fitted a Sterling Power B2B charger to protect the alternator from the lithium's near-zero internal resistance hammering it flat. That single bit of kit probably saved my alternator.

One thing I didn't anticipate: the battery monitor readings. My old Victron BMV-712 was calibrated for lead acid behaviour and the state-of-charge percentages were reading daft for the first week until I reset the Peukert exponent and charged efficiency factor. Took a bit of forum digging to sort.

Anyone else made this swap on a narrowboat or cruiser? Curious whether others bothered with the B2B or just ran a DC-DC isolator instead — and whether you've had any issues with shore power chargers that don't have a proper lithium profile.

Great write-up @MasterSolar, looking forward to the full details. I made a similar swap on my narrowboat two seasons ago and the usable capacity difference alone was worth every penny — you're essentially doubling your real-world storage when you ditch that top-and-bottom 50% AGM dead zone.

One thing I'd flag specifically with the Fogstar Drift cells in parallel: keep a close eye on your bus bar connections during the first few weeks. Mine developed a very slight resistance difference between the two batteries that was causing uneven charging. Nothing catastrophic, but worth monitoring with a decent clamp meter across each cell individually rather than just reading the overall bank voltage.

How are you finding it with the boat's existing alternator setup? That's usually where things get interesting on canal boats.

Chalky38 | 47 posts

Nice write-up @MasterSolar, looking forward to the full details.

One thing I'd be curious about — how are you finding the Fogstar Drift specifically in the marine environment? I've seen mixed chat about whether the casing handles condensation well on boats, particularly with the temperature swings you get on a canal boat overnight versus midday.

Also worth mentioning for anyone reading — two 100Ah in parallel gives you 200Ah usable compared to the old AGM where you'd realistically only pull 100Ah without hammering the lifespan. So effectively you've doubled your usable capacity for similar physical space, which on a 32-footer is no small thing.

Did you keep your existing alternator setup or upgrade the charging side too? That's where a lot of boaters come unstuck with the lithium swap.

PanelDai | 312 posts

Cheers for sharing this @MasterSolar — always good to see real-world results rather than spec sheet promises.

Curious about a couple of things specific to the marine environment: how have the Fogstar Drift BMSs handled the damp? Canal boats can get pretty brutal with condensation, especially over winter moorings. Also wondering whether you noticed much difference in charging behaviour from your alternator — lithium's lower internal resistance can hammer older alternators pretty hard without an in-line DC-DC charger in the mix.

Did you stick with your existing solar charge controller or swap that out too? Presumably it needed reprogramming at minimum for the lithium charge profile.

Looking forward to the full write-up either way. The weight saving alone on a 32-footer must be noticeable in the waterline!

Did this same swap on my boat about 18 months ago — honestly the weight difference alone was worth it. Shifted maybe 25kg off the stern which made a noticeable difference to how she sat in the water.

One thing nobody really warned me about was the BMS cutting out mid-cruise when I'd accidentally left the inverter running the kettle and had the engine alternator hammering in at the same time. Took me a while to figure out the charge current was spiking past the BMS limit.

Ended up fitting a Victron Battery Protect and a proper DC-DC charger to isolate the alternator charging — problem solved. Worth budgeting for that if you haven't already @MasterSolar.

Dave1983 | 89 posts

Brilliant thread, cheers @MasterSolar. I've been sitting on the fence about doing exactly this on my 28ft cruiser — currently running a tatty old flooded lead acid bank that's probably on its last legs if I'm honest.

Couple of things I'd love to know more about: did you have to do much work on the charging side? My Victron MPPT is supposedly lithium-compatible but I've heard the alternator situation on canal boats can be a bit hairy without a proper DC-DC charger in the mix. Worried about cooking the alternator on a hard acceptance charge.

Also curious whether you noticed much difference in usable capacity day-to-day versus the AGMs — on paper the lithium should give you far more of the rated 200Ah but I imagine real-world conditions vary a lot. Looking forward to the full write-up!

Really useful thread, @MasterSolar. A couple of questions if you don't mind —

Did you need to upgrade your alternator protection when you made the switch? I've heard the lithium's low internal resistance can hammer an alternator pretty hard on engine start, and I'm weighing up a similar swap on my narrowboat.

Also, did you stick with a Victron BMS or go with the built-in Fogstar one? I've been looking at the Drift range but I'm not sure whether the integrated BMS is sufficient for a live-aboard setup where the bank's getting cycled daily.

Weight distribution must have changed noticeably too — where did you end up mounting the new cells compared to the old AGM position?