Anyone else running lithium on a narrowboat? Struggling with BMS and alternator compatibility

I've just finished swapping out the old lead acid bank on my 57ft trad narrowboat for a 200Ah lithium (LiFePO4) setup from Fogstar. Running a Victron 100/30 MPPT off two 175W panels on the roof, which has been absolutely brilliant. The problem I'm running into is with the engine alternator — a 70A Iskra unit — and getting it to play nicely with the BMS.

The BMS cuts out when the bank hits full charge, which is obviously doing the alternator no good at all — dropping the load instantly like that risks spiking the regulator and frying things. I've seen people mention fitting an external regulator like the Wakespeed WS500, but at £400+ it feels steep. Others have said a simple battery-to-battery charger (B2B) in between is the better solution and kinder on the wallet. Currently eyeing up the Victron Orion-Tr Smart 12/12-30A, but that's only 30A output and I'm not sure it's enough to make a dent when I need to top up quickly after a few days of overcast weather.

Has anyone actually done this on a narrowboat specifically? I'm conscious that canal cruising means the engine hours are already pretty limited compared to something like a sailing yacht doing longer passages, so charge efficiency really matters. Curious whether anyone's gone down the B2B route versus an alternator regulator, and what sort of real-world charge rates you're actually seeing.

@BrianKnight86 the alternator compatibility issue is the classic narrowboat lithium headache — your BMS will happily disconnect under full charge and the alternator spikes can fry your rectifier instantly.

What you need is either:

• A Victron Orion-Tr Smart DC-DC charger between the alternator and your lithium bank (isolated version ideally)
• Or a Battery to Battery charger (B2B) — Sterling make decent ones popular on the cut

The DC-DC charger protects the alternator from those nasty load-dump spikes when the BMS trips. Running directly off the alternator into LiFePO4 without protection is essentially playing Russian roulette with your engine electrics every time you cruise past a lock.

Also worth setting your Victron MPPT absorption voltage around 14.2V rather than default — Fogstar cells typically prefer that.

What alternator are you running? That'll determine which B2B rating you need.

@BrianKnight86 Worth noting that the BMS disconnect under load is what kills alternators — when the BMS trips, the alternator suddenly loses its load and the voltage spike can fry the regulator instantly.

A Sterling Power B2B charger is the proper fix for narrowboats — it isolates the alternator from the lithium bank and charges at a controlled rate. Bit of an outlay but far cheaper than a replacement alternator.

I'm running a similar Fogstar setup on my static (different application I know) and the B2B principle is the same wherever you've got an alternator in the mix.

Also double-check your Victron MPPT absorption settings — LiFePO4 wants around 14.2V absorption, 13.5V float, not the lead-acid defaults. Easy thing to overlook when you've just done a big swap.

@MuddyRigger

@BrianKnight86 Both points above are spot on. The solution I'd strongly recommend is a Sterling ProReg D alternator to battery charger — it sits between your alternator and the lithium bank and acts as a buffer, so if the BMS disconnects, the alternator sees a proper load rather than open circuit. Completely saved my setup on a 60ft cruiser stern.

Also worth checking whether your engine has an external or internal regulator. Internal regulators really struggle with lithium's flat charge curve and will run the alternator flat out for too long, which kills it through heat. The Sterling unit sorts this too.

What alternator are you running? If it's the standard Beta/Barrus fit you'll likely be fine with that solution without rewiring much.

@Trevor1967

@BrianKnight86 Good advice already from @MuddyTrekker and @MuddyRigger on the alternator protection side. One thing worth adding — make sure your engine hours justify the alternator upgrade first. If you're mainly solar-topped up and only cruising occasionally, you might get away with a decent alternator-to-battery charger like the Victron Orion-Tr Smart DC-DC instead of the Sterling ProReg. It isolates the alternator from the BMS entirely and lets you set proper lithium charge profiles. We fitted one on our 60ft semi-trad last season and it's been completely faultless. Also pairs nicely with your existing Victron kit for monitoring via VictronConnect. What alternator are you running currently — do you know the output amperage? That'll affect which Orion model makes sense for your setup.

@BlownFuse

@BrianKnight86 One thing nobody's mentioned yet — worth checking whether your alternator has an external regulator or internal. If internal, a Wakespeed WS500 is worth serious consideration; it communicates directly with the Victron ecosystem via CANBUS and will actually talk to the BMS to ramp charge current down gracefully before a disconnect rather than just reacting to it.

My setup is a static van rather than a boat, but the BMS/alternator problem is identical in principle. The sudden load dump is what fries diodes.

Also — what's your leisure-to-starter battery isolation arrangement? On a narrowboat you often see people neglecting that side and then wondering why the BMS is triggering at odd times. A Victron Orion DC-DC isolated charger between starter and leisure banks is a much cleaner solution than a VSR with lithium.