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

Been scratching my head with this one for a couple of weeks now. I've got a 280Ah 16S LiFePO4 pack in my shed setup, running a JK BMS (the 200A active balancer version). Since the cold snap hit in November, it's been cutting off charging from my solar and the Victron MPPT when the morning temps drop, even though my cheap probe thermometer on the cells is reading 6–8°C, which should be well above the default 5°C cutoff.

I've had a poke around in the JK app and the low temp charge protection is set to 5°C with a 2°C release, so it shouldn't be tripping. But it clearly is. My suspicion is that the BMS's own internal temperature sensor is reading colder than the cells actually are — possibly because it's mounted on the outside of the pack and the shed gets a proper draught overnight. The fault clears itself by mid-morning once things warm up a bit, which fits that theory.

Has anyone else run into this? I'm wondering whether to just nudge the BMS cutoff down to 3°C as a workaround, though I'm a bit nervous about actually charging cold cells if the probe is the one lying. The other option is adding a proper insulating wrap to the pack to keep temps more stable overnight — some folks seem to do this with Armaflex.

Would be curious what temperatures people are actually seeing inside their packs versus what the BMS reports, and whether anyone has a better fix than just fiddling with the settings.

@Rodney3 classic JK moment — the NTC sensor is probably reading colder than the actual cells because it's touching the case rather than flush against a cell, so wrap it in a bit of self-amalgamating tape and press it properly against the middle cell of your pack rather than dangling in the cold air like a sad thermometer.

@Rodney3 yeah, had exactly this in my cabin setup last winter. The JK's onboard temp sensor reads colder than the actual cell temps — it's mounted on the board, not on the cells themselves. So if your BMS is sat in cold air, it'll trip even when the cells are fine.

Fix is simple: either add an external NTC sensor directly on the cells (JK supports this), or just nudge the low temp cutoff threshold down a degree or two in the JK app.

Worth checking which sensor the BMS is actually using for the charge protection — there's a setting for that. Caught me out for ages before I bothered reading the manual properly.

Has anyone checked the actual low temp charge cutoff setting in the JK app? Mine was defaulting to 5°C but the hysteresis (recovery offset) was set to only 1°C, meaning once it tripped it wouldn't re-enable until 6°C — which in a cold garden office basically means it stays locked out half the morning even when cells are warm enough.

Worth going into the BMS parameters and bumping that recovery offset to 3-4°C so it actually latches back on properly. Also double-check your "Cell Low Temp Charge Protect" vs "MOS Low Temp Charge Protect" — they're separate settings and both need checking. Caught me out for days before I spotted it.

Following on from what @Bomber's raised about the hysteresis — that's actually a really important point that catches a lot of people out. If your cutoff is set to 5°C with say a 2°C hysteresis, the BMS won't re-enable charging until the cells read 7°C. So even if you manually warm things up slightly, it stays locked out longer than you'd expect.

Worth also double-checking whether you've accidentally got two temp sensors configured in the JK app, @Rodney3. If T2 is enabled but left dangling loose in the air near a cold surface, it'll skew the readings badly. I'd go through each sensor setting methodically and make sure any unused inputs are properly disabled. Took me a frustrating afternoon to figure that one out on my own setup last February!