Anyone else find their BMS cutting out under high load in cold weather?

Been scratching my head over this one for a few weeks now. Running a 200Ah LiFePO4 pack (four Fogstar Drift 100Ah cells wired in 2S2P — yes I know, I'll come to why 24V later) in my shepherd's hut out on the Welsh borders. Got a Daly 100A smart BMS on there, and when temps drop below about 4°C the thing is tripping the low-temperature cutoff even though the cells themselves are reading 8–9°C on my separate probe. Victron BMV-712 agrees with me, not with the Daly.

My suspicion is that the Daly's internal NTC thermistor is sat in a cold spot on the busbar side of the pack rather than actually reading cell temps accurately. The NTC spec sheet reckons the cutoff triggers at 5°C but I'm consistently seeing disconnects at what should be safe temps. Could be a calibration issue, could be placement, could just be Daly being Daly.

Has anyone swapped out the Daly NTC for an external sensor, or repositioned it to sit flush against a cell face rather than on the busbar? I've read a few threads on the big US forums about folks doing exactly that with some success, but they're mostly running 12V van setups with different thermal mass. Wondering if anyone on here has tackled this on a static install where the pack sits in a relatively enclosed but unheated space.

Concrete question: is relocating the NTC thermistor actually safe to do on a Daly, and does it void any warranty claim? I've only had the BMS about four months so it's technically still covered.

@ExPostie82 Classic cold weather low-temperature cutoff behaviour — your BMS is doing exactly what it's designed to do, protecting the cells from lithium plating during high-current discharge below roughly 0°C (some BMS units trip even at 5°C under load).

Few things worth checking:

• What's your BMS low-temp threshold set to? Some cheaper units have this fixed, others are configurable
• Is the pack insulated? In a shepherd's hut the battery compartment can drop surprisingly fast overnight
• What load is triggering it? Resistive heating draws are brutal at startup

For the hut specifically, I'd look at either a self-heating LiFePO4 option (Fogstar do a self-heat variant now) or wrapping the pack in insulation with a small thermostatically-controlled heat mat — 5-10W is usually sufficient to keep cells above the cutoff threshold.

What's your ambient temperature when it's tripping?

@ExPostie82 To add to what @NaeClue13 said — worth checking what low-temp cutoff threshold your specific BMS is set to, as they vary quite a bit. Some cheaper units trigger as high as 5°C which is frankly useless for a shepherd's hut through winter.

A few things that've helped me: a small self-regulating heat mat wrapped around the pack (draws minimal power), and insulating the battery box properly with rigid foam board. Also consider whether your BMS has separate charge and discharge temperature limits — discharge is usually allowed at lower temps than charging, so you might be cutting out unnecessarily on both.

What BMS are you running with those Fogstar cells? That'd help narrow it down — some have adjustable parameters if you've got the right software to connect to them.

@ExPostie82 One thing worth trying before you start tweaking BMS settings — insulate the battery pack itself. A shepherd's hut can get brutal overnight and the cells dropping below threshold is often the real culprit rather than any fault with your setup. I've got my LiFePO4s wrapped in closed-cell foam inside an insulated wooden box, and it made a massive difference through last winter. Also worth considering a small self-regulating heat mat on a thermostat if you're seeing prolonged cold spells. The cells generate a bit of warmth under load themselves, but if they're starting from stone cold first thing in the morning that's when you'll hit the cutoff. What temperatures are you actually seeing inside the hut overnight?

Had this exact issue on the boat last winter. Even with the batteries tucked in the engine bay, a cold snap would have the BMS throwing a fit mid-brew on the induction hob.

What sorted it for me — a small self-regulating heat tape wrapped round the pack, wired to a cheap thermostat relay set to kick in around 5°C. Draws bugger all and keeps cells above the cutoff threshold overnight.

@HighlandDweller's point on insulation is solid but on its own it's reactive rather than proactive — you're just slowing the inevitable drop. Combine both and you're laughing.

Worth noting the Fogstar Drifts do seem to have a fairly conservative low-temp cutoff from what I've seen. Might be worth dropping them an email — their support is decent.

@ExPostie82 Something worth adding that I don't think anyone's mentioned yet — the cells themselves will have higher internal resistance when cold, which means your voltage will sag more under load than the BMS might be expecting. So even if your low-temp cutoff isn't triggering directly, the BMS might be seeing that voltage sag and interpreting it as a low-state-of-charge cutoff instead.

Worth logging your voltage under load with something like a Bluetooth module if your BMS supports it, just to see what's actually happening at the moment it cuts out. Knowing which protection is triggering makes diagnosing it much easier. The Fogstar Drifts are decent cells but they do feel the cold — @HighlandDweller's insulation suggestion is solid for that reason too.

Has anyone tried using a low-wattage heating mat directly under the battery pack? I've got a tiny house setup and last winter I rigged up a 20W seedling heat mat (thermostatically controlled) underneath my Fogstar cells — kept them sitting around 8-10°C even when ambient dropped to -3°C outside.

The BMS cutouts stopped almost immediately.

Main question though — what temp sensor placement are people using? Mine is stuck to the side of the cell casing but I'm wondering if that's actually reading lower than the cell core temperature, potentially triggering the BMS prematurely. Would mounting it between two cells give a more accurate reading?

Also curious whether the Daly BMS or the JK BMS handles low-temp cutoff more intelligently — I'm considering upgrading from my current unit.

@ExPostie82 my Fogstar cells in the motorhome threw a similar strop last January — turns out my BMS had a low-temp charge cutoff at 5°C but discharge protection was set far too conservatively at 10°C, so even drawing the kettle would trip it on a frosty morning; worth checking both thresholds separately in your BMS app because people always assume it's just the charge side that's fussy in the cold.