Anyone else had issues with JK BMS dropping cells during cold nights?

Picked up a 16s 280Ah LiFePO4 pack back in spring and it's been brilliant all summer. Running a JK BMS (the 200A active balancer version) and everything looked spot on — cells staying within 5mV of each other, no alarms, happy days. Then the temperatures dropped last week and I started getting low voltage cutoffs on cell 7 and cell 12 seemingly at random. Overnight lows have been around 4°C here in South Wales.

I've checked the bus bars and they're all torqued correctly, and the cell voltages look fine on the JK app when the pack is at rest. It only seems to happen under load — I've got a 2kW inverter (Victron Multiplus-II) pulling around 80-100A at times. Wondering if cold is causing those two cells to sag more than the others, or whether I've got a dodgy cell in the batch.

Has anyone else noticed this with JK BMSs specifically, or is this more likely a cell quality issue? The cells came from a well-known AliExpress seller (EVE 280K grade A, supposedly). I'm tempted to do a proper capacity test on cells 7 and 12 individually but wanted to see if anyone had experience with this before I pull the pack apart over winter.

JakeShaw96 | 47 posts

@NeilSmith yeah had almost the exact same thing happen last October with my 8s setup. Worth checking your low temperature cutoff settings in the JK app — mine had shipped with the discharge cutoff set to 5°C which sounds reasonable until you realise the BMS is reading internal temp, not cell temp directly. Actual cells were sitting around 8°C but the sensor was reading colder due to its mounting position near the terminals.

Moved the temp probe closer to the middle of the pack and added a bit of insulation round the battery box. Sorted it completely. Also nudged the cutoff down to 0°C since LiFePO4 handles discharge at low temps reasonably well — it's charging below freezing you really want to avoid.

What are your current temp cutoff values showing in the app?

Keith1981 | 203 posts

@NeilSmith the JK BMS has a low temperature charge cutoff setting that's worth checking — mine was factory set to 5°C and was blocking charging on cold mornings even though the cells themselves were fine for discharge. Jump into the JK app and look under "Protection Parameters" — you can adjust the charge low temp protection threshold and recovery temp. Also worth insulating your battery enclosure if you haven't already; even a cheap camping mat wrapped round the pack makes a surprising difference overnight. Once temperatures drop consistently you'll want those cells staying above 0°C regardless of what the BMS is doing. What are your actual cell temps reading during these events, assuming you've got a temp probe fitted?

NotAnElectrician | 12 posts

@NeilSmith not sure if this is the same thing, but my Fogstar cells started behaving oddly last November and it turned out the BMS temp sensor had worked itself loose from the cell surface — was reading ambient air temp rather than actual cell temp. Completely threw off the low-temp cutoff logic.

Worth checking the sensor is properly taped/secured directly against a cell before diving into the settings? Kapton tape is apparently the preferred method for keeping it flush.

Also — are you monitoring individual cell temps or just relying on the BMS readings? I added a cheap Inkbird unit after my drama and it's been eye-opening how much variance there can be across a 16s pack when temps drop.

VivaroSolar | 312 posts

@NeilSmith worth also checking your cell internal resistance readings in the JK app — cold temps cause IR to spike noticeably in LiFePO4, and if one cell is already slightly weaker than the others it'll show up much more dramatically once you're below 5°C or so. That cell will sag under load and the BMS sees it as a genuine undervoltage rather than a temperature-related issue.

If you haven't already, log the actual cell voltages at the moment of disconnect rather than just the resting values — the JK keeps a fault record in the app which is really handy for this. Might tell you whether it's consistently the same cell dropping out, which would point to a duff cell rather than a settings issue.

Derek1967 | 847 posts

@NeilSmith Had almost identical with my 280Ah pack last winter. Worth checking your cable connections at the cell terminals — cold causes contraction and I found two of mine had worked slightly loose, which made individual cells look like they were dropping out under load. Gave everything a retorque and the "problem cells" disappeared overnight.

Also, are you seeing this at rest or under load? If it's only when drawing heavy current in the cold, you might be brushing against the BMS overcurrent threshold — internal resistance rises significantly once you're below 5°C and the JK can interpret that voltage sag as a fault condition.

What are your overnight low temps at the moment? That would help narrow it down.