Swapped my boat's lead acid bank for lithium — here's what caught me out

Finally pulled the trigger on replacing the tired 2x 110Ah AGM setup on my 28ft river cruiser with a single 200Ah Fogstar Drift LiFePO4. On paper it seemed straightforward but a few things bit me I wasn't expecting.

Biggest headache was the existing Victron BMV-700 — had to recalibrate the Peukert exponent and charge efficiency factor from scratch, otherwise the state of charge readings were all over the place. Also the old Beta Marine alternator has no lithium charging profile, so I've got a Victron Orion DC-DC isolator in there now to protect the battery from bulk-charge abuse. Added cost I hadn't budgeted for.

The weight saving is genuinely ridiculous — probably 25–30kg lighter at the stern, and the boat trims noticeably better. Usable capacity is also night and day; I was getting maybe 80Ah out of the old AGMs in practice, now I'm using a genuine 180Ah without stressing anything.

Has anyone else run into issues with older marinised alternators and lithium on a canal or river boat? Wondering if the Orion isolator is really the best fix or whether a proper alternator regulator (like the Wakespeed WS500) is worth the outlay on a weekend-use boat.

@OffGridGuru the one that catches nearly everyone out on the marine side is the alternator protection issue. LiFePO4 accepts charge so aggressively that when it hits full SOC and the BMS disconnects, your alternator suddenly loses its load — that voltage spike can kill the regulator instantly.

On my van build I fitted a Victron Cyrix-Li-ct as a buffer, but on a boat with a running engine alternator you really want either a proper DC-DC charger (Victron Orion-Tr Smart is what I'd spec) or at minimum a compatible smart alternator regulator.

Did your Fogstar Drift's BMS have any load-dump protection built in, or were you relying on the stock setup? The Drift is solid kit but that marine/alternator combination needs deliberate engineering, not just a straight swap.

SolarPaul | ⚡ Solar Addict | Posts: 847

@OffGridGuru great write-up, cheers for sharing the gotchas! One thing I'd add that catches a lot of boat owners out — check your alternator situation carefully. Many older marine alternators aren't happy doing a proper lithium charge cycle and can actually overheat trying to satisfy the battery's low internal resistance. Ideally you want a DC-DC charger between the alternator and the lithium bank (Victron Orion-Tr Smart is popular for this).

Also worth double-checking your bilge pump and any always-on safety circuits are wired before the BMS — you really don't want those dropping out if the BMS trips for any reason.

Did you sort out low-temperature charging protection? River winters can get properly chilly and LiFePO4 really doesn't like being charged below about 5°C.

@OffGridGuru done the same swap on my narrowboat last year — the thing that got me was the battery monitor. My old Victron BMV-712 was still calibrated for lead acid so the SOC readings were way off for weeks before I twigged. Had to do a full reset and reconfigure the charge/discharge parameters properly for LiFePO4 chemistry.

Also worth double-checking your low voltage disconnect settings — lead acid you'd cut off around 11.8v but with lithium you want that threshold much higher, closer to 12v or you're punishing the cells unnecessarily.

The Fogstar Drift's a solid unit though, running mine hard for 14 months now with zero drama.

Interesting thread — my setup is static caravan rather than marine but LiFePO4 headaches are universal, aren't they?

One thing nobody's mentioned yet: low-temperature charging cutoff. On a boat sitting unused over winter, the BMS will hard-block charging if the cells drop below roughly 5°C. Lead acid just grumbles along regardless, but LiFePO4 can leave you with a mysteriously "dead" bank come spring when actually the BMS has just been doing its job perfectly.

@OffGridGuru did you sort any form of battery heating, or are you relying on the boat's interior staying above freezing? On my caravan I ended up adding a small self-regulating heat pad wired before the BMS sense circuit — bodge-tastic but effective.

Presumably on a river cruiser you've got more condensation/damp to worry about too — did the Fogstar Drift's IP rating actually concern you at all?