Anyone else had grief with a JK BMS dropping cells mid-charge on a cold morning?

Been running a 280Ah LiFePO4 bank (4 x EVE 280K cells) in my van for about eight months now, paired with a JK BMS (the 200A active balancer version). Generally it's been brilliant, but over the last few weeks as the temperatures have dropped I've had it cut out twice during morning bulk charge — both times around 4–6°C ambient inside the van. The Victron MPPT just sits there blinking, no fault on its end. Pull the BMS back up via the app and everything looks fine.

Wondering if it's the low-temperature protection kicking in, but my settings show LTP set to 5°C with a 1°C hysteresis, so it should only trigger below that. Both incidents were right on that boundary, mind. One thing I did notice is cell 3 was reading about 18mV lower than the others at the point it tripped — not sure if the balancer was overwhelmed or if I've got a slightly weaker cell developing.

Has anyone actually recalibrated the temperature sensors on these JK units? I've seen mention of an offset setting in the app but I can't find solid info on whether the onboard NTC is actually accurate or just a rough guide. Also curious whether people run a separate external probe and if that's made any difference. Happy to share my BMS logs if useful.

Classic JK low-temp cutoff doing exactly what it's supposed to — those EVE 280K cells genuinely don't want charging below about 5°C or you'll plate lithium on the anodes faster than you can say "warranty void."

Check your BMS app (the JK Bluetooth one is actually decent) and look at your Cell OT/UT Charge Protection settings — bet your threshold is sitting at the default and your van floor is basically a fridge in January.

Quick fixes ranked by laziness:

1. Self-heating cable wrap around the battery
2. Stick a Fogstar self-heating cell in there (pricier but tidy)
3. Bodge a 12V heat mat on a temp-controlled relay

My narrowboat setup gets the same grief moored in a lock overnight — thermal mass is your friend but it takes ages to warm up from cold.

Good shout from @FETFan on the low-temp protection. Worth checking your actual cell temperature sensor placement though — if the sensor is reading the BMS board temperature rather than sitting flush against a cell, you can get false readings either way. Mine was reading 2-3°C warmer than the cells themselves on cold mornings, which masked the issue initially.

If you're confident the cutoff is triggering correctly and it's a genuine cold problem, a cheap self-regulating heat mat tucked under the battery box sorted mine completely. Set the low-temp charge cutoff to 5°C in the JK app and the mat kicks in before you'd ever hit that threshold. Belt and braces but it's been rock solid since October. What are your current low-temp charge settings showing in the app?

What @T5Build said about sensor placement is key — mine was tucked against the BMS board itself rather than actually touching a cell, so it was reading ambient not cell temp. Moved it under a bit of foam directly on the cell surface and the false cutoffs stopped.

Also worth checking: JK app lets you tweak the low-temp charge cutoff threshold. Default's often set quite conservatively. I dropped mine from 5°C to 0°C on my Fogstar cells after confirming they're rated for it — just don't go below what EVE's datasheet actually states.

If you're parked overnight in winter regularly, a small self-regulating heat mat wired to a thermostat is probably the proper fix though.

Had exactly this last January, parked up in the Cairngorms for a week. What saved me was adding a small self-regulating PTC heat mat (the 12V reptile tank type, embarrassingly) directly against the cell bank, wired to a cheap thermostat module set to kick in below 5°C. Draws almost nothing overnight and the cells wake up at a sensible temperature before the Victron MPPT even thinks about pushing current in.

The JK is just doing its job protecting those EVEs — can't argue with that. But giving it warmer cells to work with rather than fighting the BMS settings is the more elegant fix, especially if you're heading somewhere reliably cold rather than just the occasional frosty morning in a supermarket car park.