Anyone else had their JK BMS throw a low temp alarm in this cold snap?

Been getting low temp cutoffs on my JK BMS pretty much every morning this week. Shepherd's hut isn't the best insulated and the battery compartment is in a ventilated box under the floor — probably not my smartest decision in hindsight.

Temps were sitting around 3-4°C on the cells when I checked yesterday morning, which is right on the edge of the JK's default threshold. It's not cutting out completely but it's blocking charging, which means by the time the sun's up and the Victron MPPT wants to push current in, the BMS is saying no.

Few things I've tried or am considering:

• Wrapping the battery box with some 50mm PIR board (cheap off-cut from a builders merchant)
• Adding a small self-regulating heat tape on a thermostat — seen a few people do this, seems reasonable
• Adjusting the low temp charge protection threshold down slightly, though I'm not keen on that without understanding the risk better

What's everyone else doing? Particularly interested if anyone's running LiFePO4 in an unheated outbuilding or outdoor enclosure through winter. The Fogstar cells I'm using claim decent low-temp performance but I wouldn't trust marketing copy over actual experience.

Also curious whether anyone's had actual cell damage from this or if the BMS is just doing its job and protecting things properly. Presumably the latter, but would be good to hear from people who've been through a few winters with this setup.

@ExPostie same issue with my static caravan setup last winter. JK BMS was cutting out around 3–4°C which caught me off guard — I'd assumed the Fogstar cells had a bit more tolerance than that.

Ended up wrapping the battery box in 50mm PIR board and adding a small reptile heat mat on a thermostat set to kick in at 5°C. Draws barely anything overnight and solved it completely.

Worth checking what your actual low temp cutoff threshold is set to in the JK app — mine had shipped with the protection trigger at 5°C which is quite conservative. You can lower it but I wouldn't go below 0°C personally, not worth risking the cells.

Is the ventilated box a legal requirement for your setup or could you close it off a bit more?

@ExPostie had exactly this with my cabin setup a couple of winters back. Ended up wrapping the battery box in 50mm PIR board and chucking a small 12v heating mat (thermostatically controlled) underneath the cells — absolute game changer.

Worth checking your JK app too — you can adjust the low temp protection threshold slightly if your cells can handle it, though I wouldn't push it too far with LiFePO4.

One thing that helped massively was insulating under the box as well, not just the sides. Ground cold is brutal and people always forget it.

Fogstar cells have decent low temp specs but they still don't want to be charged below 0°C — protection is there for a reason really.

@ExPostie this is literally my nightmare scenario — got a shepherds hut build on the go and specifically been worrying about the under-floor battery situation for exactly this reason.

One thing I'd look at: the JK app lets you adjust the low temp protection threshold. Default is often set quite conservatively. Worth checking what yours is actually set to before assuming it's a physical temperature problem — might be triggering at 5°C when your cells are actually fine to charge at 0°C (lithium chemistry dependent obviously, LFP is generally okay to discharge at low temps, just not charge).

Also — have you got any small load running overnight? Even a tiny resistive load can keep temps up a degree or two inside a closed compartment. Some people chuck a cheap reptile heat mat in, thermostatically controlled.

@ExPostie have you considered adding a small self-regulating heating cable (the kind sold for pipe frost protection) inside the battery box? I've got my LiFePO4s in an insulated garden office and I ran a 10W/m trace heater around the base of the battery enclosure, triggered off a simple frost stat set to 5°C. Cost me about £40 all in from Screwfix.

Curious though — what temperature is the JK actually cutting out at? Mine's set to 5°C charge cutoff but I manually bumped the discharge cutoff down to 0°C because the cells can still discharge at lower temps, it's charging that'll damage them. Worth double-checking your BMS settings before spending money on insulation — might just be a threshold that needs adjusting.

@ExPostie yeah the ventilated underfloor location is really working against you there — you're essentially giving the cold air a direct route to your batteries.

One thing worth checking on your JK app is what your actual low temp cutoff threshold is set to. Some units come configured quite conservatively from the factory (around 5°C), so if your cells are genuinely fine you might have a little headroom to nudge that down slightly — though obviously don't go daft with it.

Also worth logging the actual cell temperatures over a few mornings so you know what you're actually dealing with before spending money on insulation or heating. The JK app makes this pretty straightforward. Knowing whether you're hitting 4°C or -5°C makes a big difference to which solution makes sense.

@AnneOliver's pipe heating cable idea is a good shout if the temps are genuinely getting dangerous.