Anyone else had their JK BMS throw a low-temp charge cutoff in winter even inside an insulated van?

Picked up a 200Ah LiFePO4 with a JK BMS (the 200A active balancer version) back in September and it's been brilliant — until last week when temperatures dropped and I started getting charge cutoffs at around 5°C measured at the battery terminal. The BMS is set to cut at 5°C which I know is right for lithium, but the battery is sitting in an insulated timber box under the bed in my Transit and I'd assumed it would stay warmer than ambient overnight.

Trouble is, we've had a few nights down to -3°C or so here in the Cotswolds and it seems the box isn't doing enough. I'm getting maybe 40–50Ah back in the morning from my 200W roof panel before the BMS kills the charge circuit. The rest of the day is fine once the cells warm up from a bit of discharge, but those early morning solar hours are wasted.

I've seen people mention self-heating LiFePO4 packs but mine doesn't have that feature. Was wondering whether a small 12V heat mat under the battery controlled by a simple thermostat relay would do the job, or whether anyone's got a smarter solution using the JK's own temperature probe outputs? Not keen to mess with the BMS settings and lower the cutoff — that feels like the wrong direction.

@CotswoldCamper — had exactly this with my static caravan setup last February. The JK's low-temp cutoff threshold was factory-set at 5°C, which sounds reasonable until your cells are sitting in an unheated space overnight and the BMS thermistor reads 4.8°C at 7am.

Worth diving into the JK app and checking your charge low-temp protection setting. Mine was genuinely too conservative — I nudged mine down to 2°C since LiFePO4 can technically accept a gentle charge down to around 0°C without drama.

Also worth checking where the thermistor is physically touching the cells — if it's poorly seated against the terminal end rather than the cell body, you'll get skewed readings. Repositioned mine with some thermal paste and a cable tie and the false trips mostly vanished.

Fogstar cells or someone else's pack?

Dave1983 | 47 posts

Yeah, classic JK behaviour this time of year. Worth checking your actual threshold settings in the JK app — mine shipped with the low-temp charge cutoff set at 5°C which sounds reasonable until you realise the sensor is mounted directly on the cells, not measuring ambient. If your cells are tucked against an outside wall of the van they can sit a degree or two below the air temp.

I bumped my threshold down to 2°C and added a small self-regulating heat mat (the type used for pipes) wired to a thermostat. Sorted it completely.

Also double-check the "low temp charge recovery" value — if that's set too close to the cutoff threshold it can cause the BMS to oscillate on and off rather than recovering cleanly. @StormyMechanic probably saw the same thing.

What @Dave1983 said about threshold settings is worth acting on quickly. I've got JK units on both my narrowboat and shepherd's hut and the factory low-temp charge cutoff default is often set quite conservatively — sometimes as high as 5°C.

On my hut battery I dropped it to 0°C with a 2°C recovery offset, which gives more realistic headroom for a genuinely insulated install without risking actual cold-temp charging damage.

One thing nobody's mentioned — check whether you've got a self-heating LiFePO4 or a standard cell. If it's standard, don't just lower the threshold and forget it; pair it with a small heat mat on a thermostat under the battery. Fogstar sell decent cells that handle this better if you're considering an upgrade path too.

The JK app makes threshold changes straightforward enough, just document your original settings before you touch anything.

Yeah had this on my narrowboat last winter. Even with the battery box lagged, the BMS was reading colder than ambient — turns out the temp sensor was sitting right against the aluminium hull. Moved it more central to the cells and the false cutoffs stopped.

Worth checking where your sensor is physically positioned before you start tweaking the threshold values. Mine was set at 5°C which is fairly standard for LiFePO4, didn't want to lower it too much and risk actual cold-temp charging damage.

Also — if you're running Victron kit, the DVCC low-temp cutoff in VictronConnect can sometimes conflict with the JK settings and confuse things further.

JimButler | 134 posts

Worth adding to what @EdCampbell84 mentioned about the sensor reading colder than ambient — the JK's temperature sensor is often tucked right against the cells rather than measuring the surrounding air, so you can get a genuine cold cell reading even when the van interior feels fine. Before adjusting your thresholds, I'd grab a cheap IR thermometer and check the actual cell surface temperature against what the JK app is reporting. If they match, your cells really are that cold and the BMS is doing its job correctly. A small self-regulating heat mat on a timer set to kick in overnight can sort this properly without compromising your protection settings. Don't be tempted to just lower the cutoff threshold too aggressively — LiFePO4 genuinely doesn't want charging below about 0°C.

Had this exact issue with my static caravan setup last February. The JK app lets you adjust the low-temp charge cutoff threshold — mine was factory set at 5°C which sounds reasonable until you realise the sensor placement matters enormously.

I relocated mine from the end cell (where cold air was pooling near the floor) to mid-battery, and the phantom cutoffs basically disappeared overnight. The actual cell chemistry was fine; it was just reading a cold pocket.

Worth also checking whether you've got the heating element option enabled if your pack has one — some JK units support it but it's disabled by default in firmware.

@EdCampbell84 makes a good point about lagging — I used 25mm Kingspan offcuts around the battery box and it made a noticeable difference to how stable the BMS readings were through the night.