Anyone else had their BMS cut out in cold weather? Losing power at around 5°C

I've been scratching my head over this one for a few weeks now. Running a 200Ah LiFePO4 battery bank (two 100Ah Epoch batteries in parallel) with a Daly 100A BMS, and I keep getting low-temperature cutoffs when it drops below about 5°C overnight. I'm in a converted Sprinter that lives on a rural site in the Peak District, so this is basically every night from October through to April.

The Daly is set to cut at 5°C which I gather is fairly standard for lithium, but it means I'm waking up with no 12V at all — fridge has warmed up, no lighting, and the van is stone cold before the solar even starts ticking over. I've read about self-heating LiFePO4 cells but my current batteries don't have that feature and I'm not in a position to replace them right now.

Has anyone found a practical workaround short of swapping the whole bank out? I'm wondering whether a small resistive heater mat under the batteries on a separate relay — triggered before the BMS trips — is actually feasible, or whether that's overcomplicating it. I've seen a few people mention keeping a small AGM buffer for overnight loads but I'm not sure how that plays with the lithium setup.

Would love to hear what others are doing, especially if you're living in a vehicle full-time in the colder parts of the UK. Feels like this is a gap that doesn't get talked about enough compared to all the summer solar chat.

Hey @SilverTrekker, classic cold temperature protection kicking in! Most LiFePO4 BMS units have a low-temp charge cutoff around 0-5°C to prevent lithium plating, which can permanently damage the cells. The Daly is fairly aggressive with its thresholds from what I've seen.

Worth checking whether it's cutting out on charge or discharge - they're separate protections. If it's discharge cutting out at 5°C that's quite conservative and you might be able to adjust the threshold in the Daly app.

Practical fix in the meantime - insulating your battery box makes a surprising difference. Even basic foam board around the outside keeps residual heat from previous cycles retained overnight. Some folks add a small self-regulating heating mat wired to a thermostat, draws very little power.

What's your battery location - van, shed, outside enclosure?

Mine cuts out on the narrowboat every winter like clockwork — solved it by wrapping the battery box in 50mm Kingspan and tucking a small self-regulating heat mat (the kind off Amazon, dead cheap) underneath, wired to a Victron temperature sensor so it only kicks on when needed rather than cooking your cells 24/7.

@SilverTrekker worth noting this is specifically a charge protection cutoff, not discharge — your BMS should still allow loads to draw current below 5°C, just not accept charge from solar/alternator.

On my shepherd's hut setup I ran into exactly this. The practical fix beyond insulation (as @Stu1991 mentions) is adding a self-heating battery or a small thermostatically-controlled heat mat under the cells — I use a 10W reptile mat wired through a cheap STC-1000 temperature controller set to kick in at 7°C. Costs pennies to run overnight.

Also worth checking your Daly's actual cutoff threshold — many are set at 5°C from factory but you can adjust this with the PC software if you have a USB-UART cable. Some third-party Daly configs are surprisingly aggressive. Victron's SmartShunt paired with temperature sensing gives you much better visibility of what's actually happening at the battery terminals.

Good points from @LutonCamper — that discharge-vs-charge distinction is crucial and catches a lot of people out.

Worth adding: the Daly BMS is notoriously conservative with its low-temp threshold, often triggering at 5°C when the actual LiFePO4 chemistry risk doesn't really bite until you're closer to 0°C or below. I had the same frustration on my narrowboat setup before switching to a Victron SmartShunt paired with a BMS that has adjustable temperature parameters.

If you can't replace the BMS right now, a self-regulating heat mat directly on the battery casing — thermostatically controlled — is a solid interim fix. Running mine off a small 10W draw keeps the cells above 8°C even when the bilge drops to near freezing overnight.

Also check whether your Epoch batteries have internal self-heating — some newer cells do, which would make the whole problem largely moot.

@SilverTrekker one thing worth adding to what's already been said — if you're finding the low-temp cutoff is triggering even when you're not actively charging, double-check your Daly BMS settings via the PC software (you'll need the USB dongle). The default low-temp protection threshold on some Daly units ships set quite high, sometimes 5-10°C, which is frankly useless for UK winters. You can lower the charge protection threshold to around 0°C safely with LiFePO4 chemistry, which buys you a bit more usable range before it kicks in. That said, don't disable it entirely — charging LiFePO4 below freezing genuinely does cause permanent lithium plating on the anode. Combining the software tweak with @Stu1991's insulation approach is probably your best bet overall.