Anyone else had their JK BMS throw a low-temp cutoff in a cold snap? How did you sort it?

Woke up on Saturday morning to find my whole system dead — no loads, inverter offline, the lot. Temperatures had dropped to about -4°C overnight here in the Lakes and my JK BMS (the 200A 8S model running a 24V 280Ah LiFePO4 bank) had triggered a low-temperature protection cutoff. Fair enough, it's doing its job, but I hadn't actually set a heating solution up yet so I was a bit stuffed.

I've read that LiFePO4 can be damaged if you try to charge below 0°C, which makes sense, but I'm wondering whether the discharge cutoff is really necessary at those temps too — or whether that's more about protecting the charge side of things. My bank is four 280Ah EVE cells in parallel then series for the 24V setup, housed in an insulated timber box in the back of my Transit. It wasn't that cold inside the box but clearly cold enough to trip the BMS.

Has anyone fitted a self-heating battery blanket or a small thermostatically controlled heat mat inside their battery enclosure? I've seen a few people mention the 12V silicone heating pads but I'm not sure what wattage makes sense for a box that size, or whether to power it from a separate small lead-acid buffer so the BMS lockout doesn't kill the heater itself. Bit of a chicken-and-egg problem there.

BazLewis72 | 847 posts | Yorkshire Dales

@CumbrianExplorer Yep, been there mate. JK defaults to 0°C low temp cutoff on most firmware versions which is absolutely useless for us lot up north.

Two things worth doing: first, log into the BMS via the app and drop that low temp charge protection threshold — I've got mine set to -10°C but obviously you're then charging at your own risk on the cells. Second, and the better long-term fix, is wrapping the battery bank in decent insulation. I used 50mm rigid PIR board around mine in the shed and it holds temperature surprisingly well overnight just from residual heat.

The cells themselves are usually fine to discharge at -4°C, it's charging that's the real danger. Check which protection triggered — charge or discharge cutoff — as that'll tell you where to focus.

Spud17 | 312 posts | Aberdeenshire

@CumbrianExplorer Painful way to start a weekend! As @BazLewis72 says, that 0°C default will catch you out sharpish once autumn hits properly up north.

Worth knowing you can adjust the low temp charge protection in the JK app — I've got mine set to -5°C for discharge and kept charge cutoff at 0°C, since charging cold LiFePO4 is the real killer for the cells, not discharging. Just connect via Bluetooth, dig into the protection settings and tweak accordingly.

Also worth chucking a bit of cheap self-regulating heat tape around the battery box if it's in an uninsulated space. Mine's in an outbuilding and that combination has seen me through two Scottish winters without a single cutoff. Belt and braces approach but it works a treat.

ValleyAmy | 1,203 posts | Mid Wales

@CumbrianExplorer Sympathies — nothing worse than a dead system on a cold morning! Worth knowing that the JK app lets you adjust the low-temp cutoff threshold and recovery value, so if you're confident your cells can handle it, you can drop the cutoff a degree or two below zero. That said, I'd actually recommend tackling the root cause rather than just moving the threshold — some basic insulation around the battery enclosure makes a surprising difference. I used a couple of layers of rigid foam board around my bank and it rarely drops below 5°C inside even when it's bitter outside. A small self-regulating heat mat on a thermostat is another option if you're in a particularly exposed location like yours. Keeps the BMS happy without fiddling with settings that are there for good reason.

HelenLewis | 1,203 posts | Peak District

@CumbrianExplorer Worth noting that even once you've adjusted the low-temp threshold in JK's app, you're really just telling the BMS to permit charging at low temps — your cells still won't like it. Lithium iron phosphate is reasonably tolerant of discharging in the cold, but charging below about 5°C causes lithium plating on the anodes, which is permanent damage that compounds over time.

If your battery location isn't insulated, that's probably the longer-term fix to think about. A simple insulated enclosure with a small self-regulating heat mat on a thermostat makes a massive difference and keeps the BMS happy without you having to override the protection entirely. Mine's been fine through some proper Peak District winters since I sorted that.

GrumpySkipper | 847 posts | Scottish Borders

@CumbrianExplorer Been there mate — miserable experience. One thing nobody's mentioned yet: once you've got your cells warmed up and the system back online, stick a self-regulating heat tape (the kind sold for pipe frost protection) around your battery bank. Runs off 12V with a small relay, costs next to nothing, and kicks in automatically below about 5°C. Keeps everything just above the danger threshold without babysitting it.

I wired mine through a spare switched output on the JK itself actually, so it cuts out when charging starts — belts and braces. Took an afternoon but I've not had a cold-temp cutoff since, and we've had some proper winters up here on the Borders.

Worth checking your cell internal resistance readings too once everything's warmed through — a cold event can sometimes highlight a weaker cell that wasn't obvious before.

CrispyWelder | 312 posts | Array

Had exactly this in my garden office setup last winter — brutal. What sorted it for me was wrapping the battery box with some cheap 50mm PIR insulation board from B&Q and chucking a small self-regulating heat mat underneath the cells (the kind you'd use for a reptile tank). Dead cheap, runs off a tiny amount of power and keeps temps above 5°C even when it's properly grim outside.

@HelenLewis makes a fair point about recovery — worth logging your cell temps via the JK app before attempting any charge. I now run a Victron temperature sensor wired into my MPPT so it automatically blocks solar charging below 5°C. Takes the guesswork out completely.

The PIR + heat mat combo cost me maybe £35 total. Hasn't tripped since. 🔥

Deano13 | 578 posts | North Yorkshire

Had this exact scenario two winters back with my 150A JK on a narrowboat. What nobody seems to mention is that the temp sensors on these units can read a degree or two lower than actual cell temp if they're not properly taped flat against the cells — so you might be triggering cutoff earlier than necessary. Worth checking the sensor contact before you start adjusting thresholds. I sorted mine with a bit of self-amalgamating tape pressing the sensor firmly against the cell body. Made a noticeable difference to the readings overnight.