Anyone running lithium on a narrowboat — how are you managing BMS cutoffs mid-cruise?

Fitted a 200Ah Fogstar Drift lithium to my static van last year and the BMS behaviour there's pretty predictable — loads are stable, no real surprises. But I'm now helping a mate sort his narrowboat setup and it's a completely different animal. Engine alternator charging, 12V fridge running constantly, inverter for the kettle — the BMS seems to be tripping out at awkward moments and I can't work out if it's a current spike issue or a cell imbalance thing.

He's got a basic Renogy 200Ah drop-in at the moment, no external BMS, no Victron MPPT — just a basic split charge relay off the engine. The relay's probably shovelling too many amps in without any regulation, which is my main suspicion. Would a Victron Orion DC-DC charger sort this, or is the BMS cutoff happening on the discharge side?

Also wondering — do narrowboat installs need to treat the alternator load completely differently to solar? On my van setup the MPPT handles everything gracefully but I've got a feeling raw alternator output is a different beast entirely.

Has anyone actually scoped the current coming off a narrowboat alternator into a lithium bank? Curious what numbers people are seeing.

@SmartSolarNerd the mid-cruise cutoff scenario is genuinely trickier than people expect, and it's the same headache I get on the motorhome when the EV charger kicks in and hammers the bank unexpectedly.

What's biting narrowboaters specifically is the combination of loads — inverter, bilge pump, bow thruster — all stacking simultaneously. The BMS sees a spike and pulls the plug at the worst possible moment.

The fix that worked for me was adding a Victron SmartShunt and dialling in proper low-voltage warnings well before BMS cutoff territory — gives you a fighting chance to shed load manually rather than letting the BMS decide for you.

Also worth checking whether the Fogstar Drift's discharge cutoff threshold is adjustable via their app. Some variants let you nudge it down slightly, giving more headroom before the hard disconnect bites.

@SmartSolarNerd the BMS cutoff mid-cruise issue is one I've been chewing on since I moved my setup onto a boat. The real problem is voltage sag under sudden high loads — throttle up, bow thruster kicks in, and your BMS sees a momentary dip and panics.

What actually helped me was setting a proper low-voltage disconnect threshold rather than relying on BMS defaults, and adding a Victron SmartShunt so I could actually see what was happening in real time rather than guessing.

Also worth checking whether the Fogstar Drift's BMS has adjustable reconnect voltage — some don't and that's half the headache right there.

One thing I'd push back on slightly: people blame the BMS when it's often undersized cabling causing the sag in the first place. Worth ruling that out first.

Great thread — one thing I'd add that nobody's touched on yet is the value of a low voltage pre-alarm if your BMS or a separate monitor supports it. Set it a good 0.2–0.3V above the actual cutoff threshold so you get warning before the BMS acts. On a narrowboat that gives you time to shed non-essential loads manually — fridges, inverters, bow thruster if it's on the same bank — rather than having the BMS make that decision for you at the worst possible moment (mid-lock, anyone?).

@DODGuy is right that it's a different beast to a static setup. I'd also suggest keeping a small AGM buffer just for engine start and windlass if budget allows — keeps those high-current spikes completely separate from your lithium bank and the BMS never even sees them.

Really good point from @OffGridAlan on the pre-alarm — that's saved me a few times. One thing I'd chuck in that hasn't been mentioned: think carefully about which loads stay live if the BMS does trip. On a boat you ideally want your bilge pump and nav lights on a separate small AGM or supercapacitor buffer that bypasses the lithium entirely. BMS cuts out, you're not suddenly running dark or flooding. Sounds over-engineered but it's genuinely cheap insurance. A small 20Ah AGM just for safety-critical stuff costs buttons compared to the grief of losing steering electrics mid-lock. Belt and braces approach, proper boaty mindset.

@OffGridAlan pre-alarm is solid advice, though worth noting that on a narrowboat the inverter threshold is often the more useful early-warning layer — set it to cut at 48.0V (assuming 48V system, obviously adjust) before the BMS hard floor kicks in, and you've got a graceful degradation rather than a cliff edge.

Running a similar setup on my own boat — Victron Multiplus as the arbiter between the BMS and anything that matters. The key thing nobody mentions: coulomb counting drift. After a few weeks of partial cycles the SOC display lies to you confidently. Force a proper full charge every week or two so the BMS can re-sync, otherwise your "30% remaining" is basically astrology.

Fogstar Drift cells specifically have fairly tight protection thresholds — not a criticism, just means sloppy charge profiles punish you faster than budget cells sometimes do.

Different context to a narrowboat but running Fogstar Drift in my motorhome and the BMS cutoff issue definitely caught me out early on.

What sorted it for me was setting my Victron MPPT load output as a pre-warning trigger rather than relying on the BMS itself to be the first line of defence. By the time the BMS cuts, it's already too late for a clean shutdown.

On a moving vessel I'd imagine the alternator charging side adds another layer of complexity — if the BMS trips mid-cruise you could be pushing the alternator hard with nowhere to dump current. Worth looking at a proper DC-DC charger (Victron Orion-Tr Smart) rather than a direct split charge setup, gives you much better control over that relationship.

Bit of an odd one to weigh in on from a cabin perspective but BMS behaviour is BMS behaviour really...

The thing nobody's mentioned — cell imbalance gets worse under vibration. Engine running for hours, propeller shaft wobble, all of it. Your Fogstar might be perfectly balanced on a static setup but that same pack on a moving boat could see one cell lag behind and trip the BMS earlier than you'd expect.

Worth checking your BMS logs (if it has them) after a long cruise rather than just assuming cutoff = low SOC. Might actually be a balancing issue. Victron Cerbo makes that dead easy to monitor.