Anyone else had their BMS cut out in cold weather? Lost power in the shepherd's hut last night

Woke up at 2am absolutely freezing — my Fogstar Drift 100Ah had shut itself off. Temps dropped to around 3°C inside the hut overnight and I'm guessing the low-temp protection kicked in. First time it's happened since I installed the system back in spring, so didn't really plan for it. Running a small Victron MPPT and a 600W panel array, so charging should be fine during the day — it's purely the overnight cold that's catching me out.

Read a few threads about battery blankets and heating pads but not sure what people are actually using in practice. Is a simple seedling heat mat with a thermostat controller the go-to bodge, or is there something more purpose-built worth spending on? The hut isn't insulated brilliantly so I can't just rely on ambient warmth.

Also wondering — at what point does the Fogstar actually cut? The spec sheet says low-temp charge protection at 0°C but I've seen people say theirs trips earlier. Was at roughly 40% SOC if that makes any difference.

@JA_Solar — yes, this is a well-documented characteristic of LiFePO4 chemistry. Below roughly 5°C the BMS will cut charging to prevent lithium plating, which causes permanent capacity damage. Your Fogstar Drift's low-temp cutoff is doing exactly what it should.

Worth clarifying: discharge usually remains available at lower temps, but if yours cut entirely, check whether it was actually attempting a charge cycle from solar or a charger at the point it tripped.

For a shepherd's hut application, a few practical options:

• Self-heating cells (Fogstar do a heated variant)
• A small insulated battery enclosure — surprising how much difference it makes
• Ensure charging only resumes once ambient recovers

In my static caravan setup I keep the battery compartment slightly above ambient using a basic thermostatically-controlled heat mat. Not elegant, but it's seen three winters without incident.

Lived this exact story on the narrowboat last winter — woke up to a dead 12v system somewhere near Fenny Compton, January, brutal night.

What saved me going forward was fitting a small self-regulating heat mat directly against the battery casing, wired to a thermostat that kicks in around 5°C. Pulls barely anything overnight and keeps the cells just warm enough that the BMS stays happy.

Worth noting: the discharge cutoff is usually lower than the charge cutoff on most Fogstar units — so you might still be able to run loads even when the battery refuses to accept charge. Useful to understand which protection actually triggered yours @JA_Solar, as the response is different for each.

A cheap temp sensor logging overnight through something like a Victron Cerbo would tell you exactly what happened and when.

Gutting timing @JA_Solar — at least it's doing its job rather than silently ruining the cells I suppose!

Had a similar scare on my narrowboat last February. My fix was a cheap reptile heat mat cable-tied to the battery and wired through a simple thermostat module (the STC-1000 type, about a fiver on Amazon). Kicks on around 5°C and keeps everything above the cutoff threshold overnight.

Worth checking whether your Fogstar BMS has adjustable low-temp parameters too — some can be tweaked via a Bluetooth app. Did yours come with any monitoring app at all?

For the shepherd's hut specifically, even just boxing the battery in with some rigid foam insulation makes a surprising difference — thermal mass does a lot of the work for you when temps only dip briefly.

Really feel for you @JA_Solar — been there myself with a setup in a Welsh hillside cabin a couple of winters back. Worth noting that even if charging protection cuts in below 5°C, you can often still discharge from the battery at those temps, so if your whole system went dark it might be worth checking whether the load disconnect triggered separately, or whether you've got a wiring issue compounding things. Also, a small self-regulating heat mat wrapped around the battery and wired to a cheap thermostat controller can sort this completely — mine kicks in at 5°C and barely draws anything. Costs me pennies over a winter. @HollyGaz is right that the BMS protecting itself is the right outcome, but definitely worth a longer-term fix before next winter rather than relying on it every cold snap!

@JA_Solar been through this exact misery in my own cabin setup. The Drift's low-temp cutoff kicks in around 5°C for charging — perfectly sensible behaviour from the BMS but absolutely maddening at 2am.

Practical fix that sorted me out: a small self-regulating heat mat wrapped around the battery, wired to a thermostat controller. Mine triggers at 8°C so the cells never get anywhere near cutoff temp. Costs pence to run and the difference is night and day.

Longer term, insulating your battery compartment properly makes a massive difference — closed-cell foam does wonders. If your hut has any thermal mass at all, the battery temp will lag behind ambient by several hours anyway.

Worth checking your Victron (if you're running one) for historical battery temp data — might tell you exactly when and how cold things got.

@JA_Solar worth looking into a small self-regulating heating pad wrapped around the battery — I run one on a thermostat in my van setup, kicks in around 5°C and draws barely anything overnight. Keeps the cells happy without babysitting it.

Also if you're on Victron kit, you can set up a low-temp charge disconnect via the BMS assistant so at least you're not fighting the BMS — it just pauses charging until temps recover rather than cutting everything.

3°C inside the hut is pretty marginal — might be worth insulating the battery compartment better before next winter hits. Even a simple foam-lined box makes a surprising difference.

Narrowboat winters sorted this out for me sharpish — stuffed mine in an insulated box with a cheap reptile heat mat on a thermostat, because apparently my lithium and my bearded dragon have the same temperature requirements.

Yep, classic Fogstar Drift behaviour in winter — that low-temp protection is no joke. Had the same happen in my setup last year before I sorted it.

What worked for me was moving the battery inside the thermal envelope of the building rather than against an external wall. Made a bigger difference than any heating pad honestly.

Also worth checking your Victron BMV or whatever you're monitoring with — you can set low-temp alerts so you're not waking up to a dead system, at least get some warning before it cuts.

@LB_Camper and @Stu1991 have the heating solutions covered well already.

Been through exactly this in my shepherd's hut two winters running. Worth noting that the Victron SmartShunt will log your battery temperature history if you've got it paired — useful for seeing how quickly temps are dropping overnight and whether a heating solution is actually working.

One thing nobody's mentioned: check your Fogstar's BMS cutoff threshold in the documentation. Some units trip at 5°C, not 0°C, which catches people out. If yours is cutting at 3°C internal hut temp, the battery itself may be reading colder — thermal mass on a concrete or timber floor pulls heat away faster than you'd expect.