Anyone else had their JK BMS throw a low temp charge fault in winter even with cells above 5°C?

Been having a nightmare with my 280Ah LiFePO4 bank this past week. Running a JK BMS (the 200A active balancer version) and it keeps cutting charge at what feels like perfectly reasonable temperatures. I've got the low temp charge cutoff set to 5°C as recommended, but it's been tripping even when my cheap clip-on thermometer is reading 7 or 8°C on the cells. Starting to wonder if the BMS temperature sensors are just wildly out compared to my external probe.

The setup is in a timber-clad box bolted to the floor of a Sprinter. Four 70Ah cells in series (yeah I know, planning to double up in spring), 40A DC-DC charger from the alternator and a Victron SmartSolar 100/30 on a 200W panel on the roof. The van lives outside on a driveway in Yorkshire so it's been absolutely baltic lately — we're talking -3°C overnight last week.

Has anyone tried adjusting the low temp threshold down a degree or two to compensate for sensor inaccuracy, or is that just asking for trouble with the cells? Also wondering whether wrapping the bank in some closed-cell foam would be enough to keep the temps stable overnight, or if people go further than that with heat mats and a thermostat.

Yeah, @Ken1999 I had exactly this with mine last autumn. Worth checking what temperature sensor position you've actually got in the JK app — the default low temp cutoff is often set to something daft like 5°C, but the sensor itself might be sitting against the cell casing rather than between cells, so it's reading a degree or two colder than actual cell temp.

Try nudging the "Charge Low Temp Protection" threshold down to 2°C in the BMS parameters and see if that sorts it. LiFePO4 can generally handle charging down to around 0°C without issues at modest charge rates, so 5°C is quite conservative anyway.

Also double-check the hysteresis setting — sometimes that's set too narrow and it's cycling in and out of the fault repeatedly.

Hey @Ken1999, worth double-checking what your low temp charge cutoff is actually set to in the JK app rather than assuming it's at factory default. Mine came configured at 10°C from the supplier, not 5°C as I'd expected. Caught me out completely first winter.

Also check the temperature sensor placement — if it's cable-tied to the busbar or sitting near a cold aluminium enclosure wall rather than actually touching a cell, you can get readings 3-4°C lower than the true cell surface temp. Moved mine to sit flush against the middle cell and the phantom faults stopped immediately.

The JK app can also be a bit laggy updating the displayed temp, so you might be seeing a slightly delayed reading versus what the BMS actually triggered on.

My JK kept doing this until I realised the sensor was cable-tied to the outside of the cell, not flush against it — ambient air was dragging the reading down a good 3–4°C below actual cell temp, enough to trip the fault every frosty morning at the cabin. 🥶

@BatteryGill beat me to the sensor placement thing, but I'll add — mine was reading 2°C colder than actual cell temp because I'd routed the cable near a draughty van wall like an absolute muppet, so even a "correct" reading can be lying to you depending on where cold air sneaks in. 🥶

@Ken1999 mine did exactly this on the boat last January — turned out the BMS temp threshold was set to 8°C from the factory and the cells were sitting at a perfectly happy 6°C wondering why nobody wanted to charge them.

@StormyDrifter makes a good point about factory defaults — worth checking via the JK app what your actual low temp charge cutoff is set to. On mine I dropped it to 3°C which suits the motorhome fine in UK winters.

Also worth knowing: the JK measures temp in tenths of a degree but rounds oddly sometimes. I've seen mine show 5.0°C on the app while tripping a fault set at 5°C — basically sitting right on the threshold and bouncing either side with any small fluctuation.

Might be worth setting it to 2°C or even 0°C if your cells genuinely won't see freezing temps. Just don't go below 0°C or you're asking for trouble charging LiFePO4.

Slightly different angle here — has anyone looked at whether the charge current itself is the trigger rather than just raw temperature? I've noticed on my setup that when it's cold, even a modest charge rate seems to stress the cells enough that the BMS gets nervous and trips earlier than the stated threshold would suggest.

Worth logging your actual charge current alongside the temp readings when it faults. If you're running solar + EV charging simultaneously like I sometimes am, the combined input might be pushing it over an internal limit that interacts with the low-temp protection logic.