Anyone else had grief with their JK BMS dropping cells during cold snaps?

Been running a 280Ah LiFePO4 pack in my van since last spring — four 3.2V Eve cells wired in series, sitting behind the cab bulkhead. All was grand through summer but we've had a couple of proper cold nights lately (dropped to about 2°C inside the van overnight) and I'm getting a low-voltage alarm on cell 3 at around 6am. Fires off the JK 200A active balancer BMS and kills my 12V bus before the kettle's even on. Brilliant.

Checked the cell voltages when it trips and cell 3 is reading 2.81V while the other three are sat at 3.18–3.21V. Soon as the van warms up a bit it bounces straight back to 3.19V and balances out fine. From what I can gather the cold is spiking the internal resistance on that one cell and causing a temporary voltage sag under load — I've got a 1000W inverter running a small heater which is probably not helping matters at that exact moment.

I've nudged the low-voltage cutoff from 2.9V down to 2.7V as a temporary bodge but I'm not really comfortable leaving it there long term. Has anyone dealt with this properly — whether that's insulating the pack, adding a temperature sensor cutoff, or just accepting that one cell is a duffer and needs replacing? Curious what you lot have done in similar situations.

HarryJackson | 847 posts

@Kingy72 Classic cold weather headache this one. Worth checking what your low temperature cutoff is set to on the JK — they ship with it enabled by default and it'll disconnect the pack if cell temps drop below around 5°C. Behind a bulkhead in a van overnight you could easily be hitting that.

I'd grab a cheap NTC temperature sensor and actually log what your cells are getting down to before assuming it's a settings issue. Might surprise you.

If it is thermal, a bit of closed-cell foam insulation around the cells works wonders — nothing fancy, just reduces the rate of temperature drop overnight. Some folks wire in a small self-regulating heating pad on a thermostat too, though that's adding complexity.

What's your charging source — solar, alternator, both? That affects how I'd approach it.

HeatherWalker | 312 posts

My JK did exactly this in my static caravan last January — turns out the low temp cutoff was set to 5°C which sounds fine until you realise your pack spent the night at 3°C and your BMS had a complete sulk about it. 🥶

Check your charge protection threshold in the JK app — mine was factory set higher than I expected, and dropping it slightly (carefully, mind) sorted the drama overnight. Some folks wrap their pack in a bit of insulation or even a cheap immersion heater mat on a thermostat, which feels gloriously bodged but absolutely works.

@HarryJackson is right about the low temp cutoff being the culprit — just make sure you're not disabling protection entirely like an absolute muppet (speaking from experience there 😬).

DanMurray | 312 posts

@Kingy72 Had almost identical bother with mine last winter. One thing worth checking beyond the BMS settings — are your cells actually getting proper airflow around them, or are they sandwiched tight against the bulkhead? Metal bodywork acts like a heat sink and can pull the cell temps down faster than the ambient air suggests. I ended up wrapping mine in some cheap foam camping mat material as a basic thermal buffer. Made a noticeable difference. Also, if you're parked up for a few days without driving, the cells can sit cold and unbalanced for ages — a small heating pad on a thermostat is a decent long-term fix if you're regularly in cold spots.

NeilSmith | 1,204 posts

@Kingy72 One thing nobody's mentioned yet — have you checked whether your cells are actually getting cold, or just the BMS itself? The temperature sensor on the JK units can sometimes sit in a spot that reads colder than the cells proper, especially if it's touching a metal bracket or the van bodywork. Worth sticking a cheap digital thermometer probe directly against the cell casing overnight to see what you're actually dealing with.

Also, LiFePO4 holds heat reasonably well if you give it a bit of insulation — even wrapping the pack in some closed-cell foam can make a surprising difference in a van environment. Doesn't need to be fancy.

What's your lowest overnight reading been so far? That'd help narrow down whether you're genuinely hitting the cutoff threshold or chasing a sensor placement gremlin.

OddJobBob60 | 847 posts

Worth adding — on my narrowboat the cells sit in the engine bay which stays relatively warm from the alternator when running, but overnight in a cold marina they'd still drop to around 5°C and the JK would throw a wobbly.

Fixed it by wrapping the pack in 25mm Armaflex and adding a cheap 12V heating mat wired to a thermostat set to kick in at 8°C. Draws barely anything overnight and the cells never dip below 10°C now.

Also double-check your JK firmware — there was a batch about 18 months back with dodgy low-temp threshold defaults. Mine was set to 10°C cutoff from the factory which is way too conservative for LiFePO4. Dropped it to 5°C and half the false trips disappeared immediately.

@NeilSmith raises a fair point too — cell imbalance gets noticeably worse in the cold.

VickyFisher | 203 posts

This took me right back to January last year — woke up on a frosty morning in the motorhome, kettle wouldn't fire, and the Victron Cerbo was throwing all sorts of alarms. Turned out my JK had gone into low-temperature protection overnight, sitting at around 3°C internally.

What actually sorted it for me was adding a small self-regulating heat mat underneath the cells, wired to a basic thermostat set to kick in at 5°C. Whole lot runs off a tiny trickle from the alternator circuit so it's not robbing the pack to warm itself.

@OddJobBob60 makes a fair point about ambient warmth — behind the cab bulkhead sounds insulated but it can still get absolutely Baltic once the engine's cold. Have you tried logging overnight temps with a cheap Govee sensor? Really helps pinpoint when the BMS is actually tripping.

DorsetExplorer | 612 posts

@Kingy72 Welcome to the forum, great first post and glad you're asking before it becomes a bigger headache!

One thing worth checking — JK BMS has a low temp charge cutoff setting in the app (usually defaults around 5°C). Worth dialling that in properly if you haven't already, so it protects the cells rather than just dropping them unexpectedly.

Also, even a small self-heating pad under the pack makes a massive difference in a van. Running mine in the motorhome down in Dorset and a cheap Amazon heat mat on a timer sorted my winter woes completely. 👍

SparkyGrafter | 134 posts

Had almost identical grief on my boat last winter — JK kept tripping the low-temp cutoff even when the cells weren't that cold. Worth checking whether your BMS low-temp threshold is set aggressively; mine was factory-set at 5°C which seems reasonable until you realise cell temps lag behind ambient by a fair bit.

Are you using the JK app to log actual cell temps during the cutoff, or just guessing from ambient? That data makes a massive difference in diagnosing whether it's a genuine protection event or a dodgy thermistor reading.