Anyone else had their BMS trip in cold weather? Lost power at the worst moment

Moored up on the Shrewsbury canal last January, minus four overnight, and I woke to a completely dead boat. No inverter, no heating fan, nothing. Traced it back to the BMS on my 200Ah Fogstar Drift LiFePO4 cutting out due to low temperature protection — cells had dropped below the charge/discharge threshold overnight and it just shut the whole bank down.

Thing is, I had a Victron SmartShunt telling me I still had 60% SOC. Plenty of capacity in theory, just too cold to use it. Had to run the engine for an hour before the pack warmed up enough to reconnect. Not ideal when you're trying to stay quiet on a residential mooring at 6am.

I've since wrapped the battery box in 25mm Armaflex and added a small 12V heat mat on a thermostat, which has helped enormously. But I'm curious whether others have gone further — some people swear by heating pads inside the battery enclosure controlled directly off the BMS aux port, others just rely on better insulation.

Has anyone actually wired up active cell heating on a narrowboat install, and if so, what did you use? Wondering whether the Victron/Fogstar combo has enough aux output to drive something useful, or whether it needs its own relay circuit.

Scouse16

@TomCampbell Nightmare scenario mate, especially in minus four - that's genuinely dangerous on a boat.

Worth knowing that most lithium cells start refusing to accept charge below about 0°C, so a decent BMS will disconnect to prevent damage from charging into cold cells. Problem is it often cuts discharge too depending on the protection settings.

The Fogstar Drift is a decent battery but like most LiFePO4 packs it doesn't have internal heating. Some boaters I know have wrapped their battery bank in a bit of closed-cell foam insulation which makes a surprising difference in keeping temps above the cutoff threshold overnight.

Going forward, fitting a battery temperature monitor with an alarm would at least wake you before the BMS trips completely. Victron's BMV series shows cell temps if you've got the right shunt setup. Prevention rather than cure and all that.

Cleggy83

@TomCampbell Blimey, that's a seriously dicey situation - glad you're here to post about it! Lithium BMS units will disconnect below around 0°C to protect the cells from damage during charging, but the frustrating thing is they'll also cut discharge if temps drop far enough.

Worth looking at insulating your battery box if you haven't already - even basic foam board makes a surprising difference. Some folk run a small self-regulating heat mat underneath the batteries on a separate small lead-acid circuit specifically so it survives a BMS trip.

Going forward, a low-temp alarm on a separate temperature monitor (Govee ones are cheap and decent) set to warn you at say 3°C would give you time to act before it trips again. Did Fogstar's support offer any advice when you contacted them?

BrianKnight

@TomCampbell Had something similar with my static caravan setup last February — not quite as dramatic as being moored in minus four, but woke up to no heating and a very unhappy BMS on my 100Ah cells.

The issue is most lithium BMS units will cut off charging and discharging below around 0°C to protect the cells. Problem is, once it's tripped in the cold, you're stuck in a catch-22 — the battery's too cold to recover itself.

What helped me was adding a small self-regulating heat tape wrapped around the battery bank, wired to a tiny separate lead-acid buffer just to keep temps above threshold overnight. Costs almost nothing to run.

Worth looking at whether your Fogstar BMS has a low-temp recovery threshold in the settings — some allow you to tweak it slightly.

PeteGreen84

@TomCampbell This is exactly why I fitted a small dedicated 12V AGM just to run the diesel heater circuit, completely isolated from the lithium bank. Lithium BMS cutting out in sub-zero temps is well documented - most will disconnect below around 0°C to prevent damage during charging, which is the right thing for the battery but obviously catastrophic timing for you.

Worth looking at whether your Fogstar has a low-temp charging cutoff you can configure, and crucially, where your battery is physically located. Mine's under the gunwale and I insulate it over winter with a simple foam wrap - makes a surprising difference keeping it above the cutoff threshold overnight.

Also, a cheap temperature alarm wired to an independent buzzer would at least wake you before the BMS pulls the plug entirely. Lesson learned the hard way for a lot of us!

QIH_Electric

@TomCampbell The Fogstar Drift BMS has a low-temperature charge cutoff around 0°C, which is correct behaviour — charging lithium below freezing causes lithium plating on the anode, permanently degrading capacity. The problem is most BMS units also disconnect the load side simultaneously, which is far too aggressive.

Worth looking at whether your BMS separates charge and discharge cutoffs — better units like those in Victron-compatible cells handle these independently. A dedicated low-temp disconnect on the charge circuit only, with discharge permitted down to around -20°C, would have kept your heating running.

For the immediate fix, @PeteGreen84's AGM fallback is solid, but you can also add a self-regulating heat pad directly against the battery — draws minimal current and keeps the cells above the trip threshold. Some narrowboaters run these on a simple thermostat relay triggered at 5°C.

OffGridGeek

@TomCampbell Minus four on the Shrewsbury cut and you're surprised your lithium threw a strop — that's not a BMS fault, that's the battery politely refusing to become a very expensive brick. 🧊 Fitted a simple reptile vivarium heat mat around mine after year one on the narrowboat, costs about £15 from Amazon and runs off a tiny trickle from a cheap LiFePO4-compatible relay circuit — battery stays toasty, BMS stays happy, I stay smug.