Anyone else finding their JK BMS trips out in cold weather — or is it just my setup?

Had a frustrating one this weekend. Woke up Saturday morning to find my inverter had shut down overnight — turned out my JK BMS (the 200A, 24V model) had triggered a low-temperature cutoff at around 4°C. I've got 280Ah of EVE LiFePO4 cells in a steel toolbox in the back of my Transit, and clearly that's not insulated well enough for a British October, never mind January.

The BMS is set to the default low-temp charge protection of 5°C, which makes sense for protecting the cells, but it killed my 12V fridge and killed the heating too — which is a bit of a circular problem really. I can lower that threshold slightly but I'd rather sort the root cause and keep the cells properly warm rather than just masking it in software.

I'm looking at a few options — either wrapping the battery box in Armaflex, adding a small self-regulating heat mat wired to a thermostat, or just relocating the whole lot inside a better-insulated compartment. Has anyone gone down the heat mat route? I've seen the silicone pad heaters on AliExpress but not sure what wattage makes sense for a 280Ah pack, or whether there's a more sensible UK supplier for this sort of thing.

Keen to hear what others are doing to keep their cells happy through winter, especially those living in vans full-time up in Scotland or the north of England where it gets properly cold.

@GlenChild yeah this is a known quirk with JK units — the default low-temp cutoff is set aggressively conservative from the factory, something like 5°C discharge cutoff if I recall correctly.

Worth connecting via Bluetooth and checking your temperature protection settings in the JK app. You can adjust the cutoff thresholds yourself, though obviously don't go silly with it — lithium cells genuinely do not like charging below 0°C.

The charging cutoff is the critical one to preserve. Discharge cutoff is less of an issue for most chemistries down to around 0°C, so you've got a bit of wiggle room there.

My static caravan setup went through similar grief last winter. Ended up insulating the battery compartment with some cheap foam board — made a noticeable difference keeping temps above the threshold overnight without touching the BMS settings at all.

@GlenChild been there — woke up in my motorhome convinced I'd finally achieved true off-grid enlightenment, turns out I'd just achieved true off-grid darkness because the JK had gone full drama queen overnight 🥶

Worth checking your cell-level temp sensors are actually making proper contact — mine were barely touching the cells and reading 2°C lower than reality, which was enough to kiss goodbye to charging by 6am.

Also, a bit of self-discharge foam insulation round the battery box does wonders; Fogstar cells hold temperature surprisingly well once you stop letting the Yorkshire air at them directly.

@GlenChild this is something I've been keeping an eye on with my van build — haven't hit it yet but we're heading into the months where it becomes a real risk.

Quick question: are your cells actually cold, or is it the BMS sensor reading low? I've seen posts suggesting the sensor placement on some JK units can give skewed readings depending on where it's mounted relative to the battery terminals.

Also — are you running any insulation around the battery box itself? Even basic foam board made a noticeable difference to overnight temps in my van. Not a fix for the BMS settings obviously, but worth considering as a layer of defence alongside whatever @DODGuy is suggesting about the cutoff thresholds.

What ambient temps are you actually seeing overnight where you're located?

@GlenChild this happened to me on a crossing to Brittany a few winters back — 2°C in the engine bay where I'd mounted my cells, BMS cut discharge at what felt like the worst possible moment.

Worth knowing: the JK's low-temp protection operates on cell temperature sensors, not ambient. If your sensor is poorly placed or has poor thermal contact with the cell body, it'll read cold faster than the cells actually are. Check the sensor is properly taped flat against a cell, not floating in air.

Beyond that, a self-heating LiFePO4 setup is the proper fix — Fogstar's newer cells have it built in. Otherwise a small DC heating pad on a thermostat, triggered before the BMS threshold, buys you that buffer. I run mine set to kick in at 5°C so the BMS never sees sub-zero.

@GlenChild the JK's low-temp cutoff defaults to 5°C charge protection which is correct behaviour — lithium cells genuinely don't want charging below that — but you can lower the discharge cutoff independently in the BMS app if your cells (check your Fogstar/EVE spec sheet) are rated for cold discharge.

Practical fix: a self-regulating heat mat on the battery, wired to a Victron temp sensor relay, keeps things toasty without babysitting it.

@StuKnight get ahead of it now — van floor temps in January will absolutely surprise you.

The real pedant point everyone misses: there's a difference between charge low-temp cutoff (non-negotiable, protect it) and discharge cutoff (more flexible depending on your cells' actual rating). Don't conflate the two or you'll either damage cells or unnecessarily lose usable capacity.

@OldSailor is right that it's correct behaviour, but that doesn't make it less annoying at 3am when your heating's gone off.

On my boat I ended up wrapping the battery box with 25mm armaflex and chucking a small 10W heat mat inside — the kind sold for reptile tanks, dead cheap on Amazon. Thermostat set to kick in at 8°C. Draws almost nothing and keeps the cells above the JK's cutoff threshold all winter.

Worth checking your JK app too — you can adjust the low-temp protection threshold and recovery value. Mine was set aggressively from factory. Dropping charge cutoff to 2°C with a 5°C recovery worked better for my situation, though obviously don't go below what your cell spec allows.

Passive insulation first, active heating second. Sorted mine completely.

@GlenChild I've been through exactly this with my van setup. One thing worth looking at is where your temperature sensor is physically sitting — mine was tucked against the cell casing right next to a cold exterior wall, so it was reading a couple of degrees lower than the actual cell temperature. Moving it more centrally sorted a lot of false trips for me. Also worth considering some basic insulation around your battery box if you haven't already — even a layer of foam board makes a surprising difference overnight. Doesn't solve the root cause but keeps temps above the threshold far more reliably through winter.

@GlenChild one thing nobody's mentioned yet — have you checked whether the JK is cutting out on the charge protection or the discharge protection? They're separate thresholds and both adjustable in the app. A lot of people panic about the charge cutoff (fair enough, @OldSailor is right that it's protecting your cells) but if it's also tripping discharge protection you might have the threshold set unnecessarily high. I had mine set to 0°C discharge cutoff which was far too cautious for my cells. Worth double-checking exactly which alarm triggered in the BMS log.