Anyone else had their JK BMS throw a low temp charge fault in winter even with heating pads fitted?

Got a bit of a head-scratcher on my hands. I've got a 280Ah LiFePO4 bank (four EVE cells in a 12V configuration) sitting in an uninsulated locker on the back of my camper. Fitted a pair of 25W self-regulating heating pads direct to the cells back in October, wired them through a simple thermostat set to kick in at 5°C. Thought I had it sorted.

Fast forward to last week — we had that cold snap and overnight temps dropped to around -3°C here in the Peak District. Woke up to find the JK BMS (the 2A active balancer 200A unit) had thrown a low temperature charge protection fault and cut off the solar input. The thermostat was doing its job, the pads were warm to the touch, but the BMS sensor was still reading 4°C — just under the 5°C charge cutoff I'd set in the JK app. Cells themselves were probably fine but the BMS disagreed.

I'm wondering if it's a sensor placement issue — mine is tucked between cells 2 and 3 which might be a cold spot — or whether my thermostat setpoint needs bumping up to maybe 8 or 10°C to give a proper buffer before the BMS trips. Has anyone dialled in a reliable setup for keeping a LiFePO4 bank happy through a proper British winter? Keen to hear what's actually working out there.

CraftyGrafter | 847 posts

@ExFirefighter59 Classic one this! Check where your temperature sensor is actually positioned on the cells - I found mine had slipped slightly and was reading ambient air temp in the locker rather than the cell surface. The JK BMS takes its charge inhibit decision from that single probe reading, so if it's not making proper contact you'll get a fault even when the pads have done their job.

I'd also double-check your pad thermostat setpoint if you're using a standalone controller for them - sometimes they're cutting out earlier than you'd expect. Wrap the whole bank in a bit of closed-cell foam too, makes a massive difference to how long the pads need to run overnight. What's your low temp cutoff set to in the BMS software currently?

FormerMariner36 | 1,203 posts

@ExFirefighter59 Had almost identical grief on the boat last February. The JK's low-temp cutoff is measured at the sensor, but if your heating pads are sandwiched between cells and the sensor is clipped to the outside of the pack — facing the locker wall — you'll get a massive lag. Cell surfaces might be toasty while the BMS is still reading 4°C and throwing fits.

My fix was wrapping the whole bank in 25mm Armaflex before relying on the pads at all. Drastically reduced how hard the pads had to work. Also worth checking the JK app — you can dial back the low-temp charge threshold slightly if your cells genuinely support it. EVE's datasheet suggests 0°C minimum, so there's sometimes a degree or two of headroom built into default settings.

JackeryNerd | 412 posts

@ExFirefighter59 Worth checking whether your heating pads are actually drawing power when the BMS cuts out — bit of a chicken-and-egg problem if the BMS kills output before the pads have had chance to bring cell temps up.

I run a separate always-on fused circuit directly from the cells for my heating pads, bypassing the BMS entirely. Slightly unconventional but it means the pads can never be starved of power mid-cycle. Obviously size your fuse properly and use a decent thermostat controller.

Also — what threshold have you set for low-temp cutoff in the JK app? Factory default on some firmwares is suspiciously high (around 5°C). I'd verify that hasn't crept up after a firmware update. My garden office battery had exactly this after I updated the JK app last winter and it silently reset a couple of parameters.

ShortCircuit56 | 634 posts

@ExFirefighter59 One thing nobody's mentioned yet - what's your low temp charge cutoff actually set to in the JK app? Factory default is often 5°C which sounds reasonable but the recovery threshold (where it re-enables charging) defaults to 10°C on some firmware versions, meaning it won't kick back in until the cells are properly warm. Also worth checking your heating pad thermostat setpoint if it has one - some of the cheaper self-regulating pads don't actually kick in until ambient drops below 0°C, so you've got a window between roughly 0-5°C where the BMS has tripped but the pads aren't running yet. Classic gap. Try nudging your BMS low temp cutoff down to 2°C as a temporary measure while you sort the root cause.

RetiredChef71 | 287 posts

Had this exact issue with my cabin setup last January. The problem nobody's flagging here is sensor placement. The JK reads the thermistor stuck to the cell surface, but if that pad is sitting between the cell and the thermistor, the pad's warming the sensor before it's properly warmed the cell core.

Moved my thermistor to the coldest face (away from the pad) and suddenly the BMS was reading reality rather than a false warm spot.

Also worth noting - at sub-zero ambient, those 25W pads on a large cell mass are borderline adequate. My Fogstar cells needed a solid 40 minutes before the core temperature genuinely came up enough to accept charge safely. The BMS wasn't wrong to trip, essentially.

RetiredNurse58 | 891 posts

@ExFirefighter59 Something I learned the hard way with my static caravan setup — the BMS reads cell temperature, not the pad surface temperature. My JK was triggering faults even with the pads running warm to the touch, because the sensor sits on the end of the cell, furthest from where my pads were mounted along the side face.

Repositioned the pads to wrap around the terminal end where the BMS probe actually sits, and the problem vanished almost immediately. Worth checking with an IR thermometer exactly where your sensor is placed versus where the heat is actually reaching.

Took me a whole miserable February to figure that one out whilst trying to run 240V backup heating through the motorhome simultaneously. Simple fix in the end.

OldSailor78 | 1,247 posts

Had almost identical grief last February with my motorhome setup. Worth checking where your temperature sensor is actually physically sitting — mine had shifted and was reading ambient air temp rather than cell surface temp. Completely defeated the heating pads. Taped it flat against the cell with kapton tape and the false trips stopped immediately.

Also, EVE cells in an uninsulated locker is tough going. I ended up lining mine with 25mm PIR board — cheap as chips from Screwfix — made a massive difference to how long the pads needed to run.