Anyone else had their JK BMS trip out in cold weather? Mine cut off at 4°C last night

I've been running a 200Ah 12V LiFePO4 pack in my van build for about eight months now, connected to a JK BMS (the 200A active balancer version). Everything's been rock solid over summer and into autumn, but last night the temperature dropped to around 4°C overnight and the BMS tripped the whole pack off. Woke up to no 12V at all — fridge had warmed up, no lighting, the lot.

After a bit of digging around this morning I think it's the low temperature charge protection kicking in, even though I wasn't actively charging at the time. I've got a Victron SmartSolar MPPT 100/30 and a Victron IP22 shore power charger, and I'm wondering if one of them tried to push a small charge into the battery just before dawn and the BMS didn't like it. The cells were sitting at about 13.1V when it happened, so not like they were anywhere near flat.

I've had a look in the JK app and there's a "Cell Low Temperature Protection" setting sitting at 5°C, which would explain it. I'm tempted to drop it to 0°C or even -5°C, but I don't want to risk damaging the cells if there's a good reason it's set conservatively from the factory.

Has anyone adjusted these temperature thresholds on a JK BMS in a similar situation? What are you running yours at, and is there a safe lower limit for LiFePO4 cells in a van where it's not going to be genuinely freezing — we're talking southern England, not the Scottish Highlands?

DaleBen | 847 posts

@Dave1984 yes, this is a known gotcha with JK units - the low temperature cutoff is often set quite conservatively from the factory, sometimes as high as 5°C. Worth connecting via Bluetooth and checking your "Cell Low Temperature Protection" parameter in the app. Mine came set to 5°C with a 2°C recovery differential, which caused identical grief last winter.

You can lower it carefully, but bear in mind there's a genuine reason for the protection - charging LiFePO4 below 0°C causes lithium plating which permanently damages cells. Discharging at low temps is generally fine though, so if you separate your charge and discharge MOSFETs settings you can still run loads whilst blocking charge input when it's cold.

A small self-heating pad on the battery is honestly the proper long-term solution if you're doing van life through winter.

Had this exact issue on my boat last winter. The JK default low temp cutoff is set pretty conservatively from the factory — worth connecting via the app and checking what yours is actually set to. Mine was at 5°C which explained a lot.

Fogstar cells themselves handle cold reasonably well but charging below 0°C is where you really don't want to be. Discharging at 4°C should be fine though, so if it's cutting discharge too that's just a BMS setting rather than the cells protecting themselves.

Worth double checking the temperature probe is actually making good contact with the cells and not just reading ambient air — mine had worked loose and was giving daft readings.

Mate, your Kangoo's laughing at your van from the driveway because at least I programmed mine to stop charging below 5°C rather than stop existing entirely 🥶

Jock | 1,203 posts

Worth knowing the JK distinguishes between charge low-temp cutoff and discharge low-temp cutoff — two separate parameters in the app. Most people accidentally set both to the same value, which is overly cautious. Your cells can happily discharge down to around -20°C; it's only charging below 0°C that actually damages them.

Spent a miserable November weekend tracing exactly this on my own 280Ah pack before I realised I'd been protecting against a problem that didn't exist for half the functions.

Connect via Bluetooth, pull up the protection settings, and you'll likely find discharge low-temp sitting at 5°C matching the charge limit — drop that to -10°C or so and you'll get your power back on cold mornings without compromising the cells at all.