Anyone else finding their LiFePO4 BMS cutting out under heavy load in cold weather?

Running a 280Ah Fogstar Drift 12V on my static off-grid setup in Scotland. Over the last few weeks as temperatures have dropped to around 2–4°C in the battery shed, I've had the BMS tripping on what looks like an overcurrent fault — but only when I fire up the inverter to run something substantial (1.2kW kettle, for instance). The Victron BMV-712 is showing the battery is healthy and sitting at around 60–70% SoC when it happens, so it's not a low-voltage cutout.

My understanding is that internal resistance climbs significantly below 5°C, which would mean the cell voltage sag under load is more dramatic — possibly enough to spook the BMS even if the resting SoC looks fine. I've seen figures suggesting IR can double or even triple at low temps on LiFePO4 chemistry. Has anyone actually measured this on Fogstar cells specifically, or found a reliable way to log the voltage sag in real time during a load event?

I'm considering adding some basic insulation around the battery enclosure and maybe a small thermostatically-controlled heat mat (the type used in reptile tanks, oddly enough). Curious whether others have gone down this route or found a better fix — and whether Fogstar's low-temp cutoff threshold is documented anywhere, because I can't find it in the spec sheet.

@SueThompson yeah had exactly this on the boat last winter. Cold slows the lithium chemistry down so internal resistance spikes, BMS sees a voltage sag under load and trips on low voltage protection even though the battery isn't actually flat.

Few things that helped me:

• Insulate the battery compartment - even a bit of closed-cell foam made a difference
• Check your BMS low voltage cutoff setting if it's adjustable - might be worth nudging it down slightly
• Spread your loads if possible rather than hitting it all at once

The Fogstar Drift BMS is fairly conservative from what I've seen which isn't a bad thing but can be frustrating in cold snaps. Once temps creep back up to 8-10°C it should settle down. Heating the space even passively helps massively.

Hey @SueThompson, worth checking whether your BMS has a low-temperature discharge cutoff threshold set — some units from Fogstar come configured quite conservatively from the factory. At 2–4°C you're not in dangerous territory for discharge, but the BMS might be tripping on an overcurrent condition because, as @LDVWanderer touched on, that increased internal resistance causes a bigger voltage sag under heavy load, which the BMS can misread as a fault condition.

A few things I'd try: stagger your heavy loads rather than running them simultaneously, and consider adding some basic insulation around the battery enclosure — even a cheap camping mat wrapped around it makes a surprising difference in Scotland's winters. Also worth logging your actual load current if you can; might reveal you're briefly spiking higher than you realise. What's your biggest draw when it trips?

Hey @SueThompson, something worth looking at alongside what @SolarTom mentioned — even if your BMS low-temp cutoff is set correctly, at 2–4°C your cells will have noticeably higher internal resistance, which means voltage sag under heavy loads will be more pronounced than in summer. The BMS may be seeing a legitimate undervoltage condition rather than a fault as such.

A practical fix I used on my setup was adding a small thermostatically controlled panel heater in the battery enclosure, set to kick in around 5°C. Keeps things just above that threshold where the chemistry starts struggling. Insulating the shed walls made a bigger difference than I expected too — Scottish winters are brutal and a poorly insulated space loses heat fast overnight.

What loads are triggering the cutout? That might help narrow it down further.

Had the same thing hit me last autumn with my cabin setup. What sorted it for me was wrapping the battery in a bit of closed-cell foam insulation — dead cheap from B&Q — and chucking a small thermostat-controlled heat mat underneath. Keeps it above 8°C even on nasty nights.

Also worth knowing: Fogstar Drift cells are decent but the BMS on those does seem to trip a bit eagerly on over-current when cold. If you're pulling big loads (heating element, inverter surge etc.) you might want to pre-warm before hitting anything heavy.

@SolarTom's point about the threshold is spot on too — log into your BMS app if you can and see what it's actually set to. Some ship with surprisingly conservative defaults.

Good shout from @SaltyMaker on the insulation front. One thing nobody's mentioned yet — have you checked your cell-level voltages under load? At 2–4°C the internal resistance of LiFePO4 cells increases noticeably, so even a moderately healthy pack can see individual cells sag and trigger an undervoltage protection cut before the overall pack voltage looks alarming. Worth grabbing a Bluetooth BMS monitor app (if your Fogstar supports it) and watching what happens to the weakest cell during a heavy draw. If one cell is consistently sagging well below the others, you may have a mild capacity imbalance that's being masked at normal temperatures but exposed by the cold. Not necessarily a dud cell — could just need a good top-balance. What sort of loads are you running when it trips?