Anyone else had grief with their JK BMS dropping the load in cold weather?

Been running a 280Ah LiFePO4 pack (four EVE cells in a 12V config) in my narrowboat over winter and I've had the JK BMS cut out on me twice now when temperatures dropped below about 4°C in the engine bay. The BMS is set to low-temp cutoff at 5°C which seemed sensible on paper, but in practice it's leaving me without 240V inverter power on exactly the mornings I need it most — kettle, diesel heater controller, the usual.

I've had a read through the JK app and I can see there's a separate low-temp charge protection setting vs discharge protection, which is handy, but I'm not entirely sure what values other people are running in UK winters. Dropping the discharge cutoff to 0°C feels a bit risky given what LiFePO4 cells supposedly do at low temps, but equally 5°C seems overly cautious given these cells are meant to handle discharge (not charge) reasonably well when cold.

Has anyone found a sensible middle ground, or is the real answer just to insulate the battery box properly and stop faffing with the BMS settings? I've got 25mm armaflex on order but won't arrive until next week. Curious what others are doing, especially those living aboard or in vans parked up through a British winter.

@PennineBoater your cells are basically doing what I do on a cold Monday morning — absolutely nothing 😄

LiFePO4 really doesn't fancy charging below about 5°C, so the JK is actually doing its job protecting the cells from damage. The discharge cutoff is less of an issue but some JK firmware gets a bit twit

@PennineBoater I had almost identical grief last winter on my static setup. Worth checking whether your JK is actually triggering the low temperature charge protection or whether it's a voltage sag issue — cold cells have noticeably higher internal resistance, so under load the voltage can plummet fast enough to trip the undervoltage cutoff even when the cells aren't genuinely depleted.

If it's the latter, try adjusting your undervoltage protection threshold down a touch and see if that helps. Also worth insulating the battery compartment properly — even a sheet of foam board made a real difference for me. Some folks fit a small self-regulating heat mat on a thermostat to keep the pack above 5°C overnight. Bit of faff but cheaper than the headaches.

What load are you running when it cuts out?

@PennineBoater had this exact issue in my shepherd's hut last February. The JK has two separate low-temp cutoffs — one for charging, one for discharge — and the discharge protection default on some firmware versions is set surprisingly high (around 5°C).

Worth connecting via the JK app and checking your "Discharge Undertemperature Protection" value. Mine was factory-set at 5°C with a 2°C recovery differential, which meant once the cells dipped below 5°C it wouldn't recover until they hit 7°C.

I dropped the discharge cutoff to 0°C since LiFePO4 can discharge at low temps without damage — it's only charging below freezing that actually harms the cells. Problem solved for me without spending a penny.

If you're regularly seeing sub-zero engine bay temps though, a small self-regulating heat mat on the battery is worth considering regardless.

@PennineBoater worth checking your JK app settings under "Protection Parameters" — specifically the low temperature charge protection threshold. Mine came from the factory set at 5°C which sounds fine on paper but with sensor placement and thermal lag, it was triggering way too early on my setup.

I moved my BMS temperature sensor so it's actually touching a cell rather than floating in air nearby, and that alone made a noticeable difference to how accurately it's reading. Also @BoatSteve makes a good point about the separate cutoffs — make sure you're not accidentally catching the discharge protection too if your engine bay is genuinely that cold.

Long term, a bit of closed-cell foam insulation around the battery box made a bigger difference than I expected on my van build. Doesn't need to be fancy, just keeping that thermal mass stable overnight helps enormously.

Just to add something slightly different to what @BoatSteve and @DanFisher89 have already covered — it's worth considering where your temperature sensor is physically positioned. On my setup I found the sensor was sitting against the cell casing near the negative terminal, which runs noticeably colder than the cell body itself, especially with any airflow around it. It was triggering protection based on a reading that didn't really represent the bulk cell temperature. Moving it to sit flush against the middle of a cell and securing it properly with some kapton tape made a meaningful difference. Not saying that's definitely your issue, but on a narrowboat where cold air can circulate around the battery bank, it's worth ruling out before you start adjusting the protection thresholds themselves.

Has anyone actually measured how much the internal resistance rises on EVE cells at those temps? I'm wondering if the BMS is reacting to voltage sag under load rather than the temperature sensors themselves triggering — my Victron BMV shows some pretty dramatic voltage dips on my cells when it's cold, even before any protection kicks in.

@PennineBoater is your pack insulated at all? I've been considering wrapping mine in some camping mat foam just to slow the thermal drop overnight. Curious whether a self-heating LiFePO4 pack (Fogstar do some I think?) would actually be worth the premium for a boat installation where you can't easily climate-control the battery space.

Ran into exactly this on my boat last January — bitter week moored up near Middlewich with no shore power. My JK was cutting the load around 3–4°C even though I'd set the low temp protection to 0°C. Turned out the sensor placement was the culprit; mine was zip-tied to the outside of the cell group and reading ambient bilge temp, which was colder than the cells themselves. Moved it between cells 2 and 3, sandwiched with a bit of thermal paste, and the phantom trips stopped entirely. Worth checking before diving deep into the app parameters.

Following on from what @VanNicola mentioned about Middlewich — I had similar issues on my boat moored near Falmouth last February, which surprised me as we rarely get proper cold down here, but it only takes a couple of nights. What sorted it for me was adding a small self-regulating heat mat directly under the battery box, wired to a simple thermostat probe set to kick in around 5°C. Draws very little overnight and keeps everything above the JK's low-temp cutoff threshold. @SparkyHiker — worth logging your cell temps alongside the BMS data to correlate the resistance spikes.