Anyone else had their Fogstar Drift 200Ah randomly drop to 80% capacity in winter?

Picked up my Fogstar Drift 200Ah LiFePO4 back in spring and it's been brilliant all summer — easily hitting the full 200Ah and keeping my motorhome fridge, lights, and a small inverter happy for days. Then the temperatures dropped below about 8°C and suddenly my Victron SmartShunt is showing a usable capacity closer to 160Ah. No alarms, no BMS cutoffs, just quietly sulking like it's had a row with the cold.

I know LiFePO4 loses capacity in the cold — that's not news — but 20% feels steep for barely-below-10°C temps. Charging's been fine via a Victron IP22 30A on absorption/float profiles. The battery isn't in a sealed bay; it's under the rear bed with some ventilation, so ambient temps in there are probably following outside pretty closely overnight.

Wondering if anyone else with a Drift specifically has seen this, or if I should be looking at insulating the battery compartment before I blame the cells. Is a 20% drop at 8–10°C within normal LiFePO4 tolerance, or is Fogstar's BMS being overly cautious?

@SolarRob classic cold temperature derating — LiFePO4 chemistry genuinely does shed usable capacity as the mercury drops. My Fogstar 200Ah in the van behaves similarly once the battery itself dips below around 10°C.

Worth distinguishing between actual capacity loss and your BMS being cautious about low-temp charging protection. If you're plugged into shore power overnight in the cold, the BMS may well be throttling the charge acceptance rather than the discharge side.

I wrapped mine in 10mm thermal foam between the battery and the van floor — made a noticeable difference. Nothing fancy, just Kingspan offcuts from a skip.

The capacity will largely come back come March, so don't panic about permanent degradation just yet.

@ValleyOffGrid is spot on, and worth adding that the Fogstar Drift specifically uses cells that start noticeable derating somewhere around 10°C and below. At 0°C you can realistically expect only 70-80% of rated capacity, so what @SolarRob is seeing sounds pretty normal rather than a fault.

A few things worth trying — if your motorhome has any heating running overnight, positioning the battery somewhere it benefits from residual warmth makes a real difference. Also worth checking your BMS discharge cutoff voltage; some folk tighten that up in winter to protect the cells, which further reduces usable capacity.

The good news is it'll bounce straight back come spring. Nothing wrong with your battery!

Same thing hit my setup in the shepherd's hut last winter. Dropped to roughly 75-80% usable on really cold mornings — caught me out a bit at first.

Worth knowing it's not permanent damage though. Once temps climb back up during the day you'll see capacity recover.

Practical fix that helped me: I insulated the battery compartment properly and added a small heat mat on a thermostat. Keeps the cells above ~5°C overnight and the derating becomes much less dramatic. Victron BMV-712 makes it easy to track what's actually happening in real time too — I'd be guessing otherwise.

@SolarRob the Fogstar Drift does handle the cold better than some cheaper cells I've used previously, so you're not in bad shape. Just needs a bit of babying through the first proper winter.

Living this exact reality on the narrowboat right now. The bilge where my Fogstar sits dropped to about 4°C last week and I watched my usable capacity shrink noticeably — what hurt more though was the charging side. LiFePO4 really doesn't want to accept charge below 5°C, so my Victron MPPT was essentially sitting there twiddling its thumbs on frosty mornings waiting for the battery to warm up before bulk charging kicked in properly.

If you haven't already, worth digging into your BMS low-temperature charge cutoff settings. Some Drift units are conservative there. I've also started running a small self-regulating heat mat underneath mine — draws minimal power but keeps the cells above that critical threshold overnight. Made a genuine difference once I sorted it.

@FormerMechanic15 the shepherd's hut scenario sounds brutal — any insulation around the battery itself?

Something worth flagging that I don't think's been mentioned yet — if you're regularly discharging at higher currents in the cold (running the inverter especially), the voltage sag gets noticeably worse too, which can trigger low voltage cutoffs earlier than you'd expect. Your BMS might be cutting out before you've actually hit the real capacity floor.

Worth checking whether your Drift's BMS logs show cutoff reasons if you can access them. A few people on here have found they're losing usable capacity to premature BVC trips rather than the cells themselves being the limiting factor.

@MarineOllie a narrowboat bilge at 4°C is rough — even a cheap foam wrap around the battery makes a surprising difference to that thermal floor overnight. Doesn't need to be fancy.

Bit of a tangent but worth asking — has anyone actually checked whether their Victron BMV (or whatever you're using for monitoring) is accounting for temperature in its SOC readings?

I spent a bewildered fortnight last winter convinced my shepherd's hut battery was dying, only to discover my shunt was reporting capacity without any temperature compensation factored in. The actual usable capacity was fine for the conditions — my expectations just hadn't caught up with basic physics.

So before anyone panics about the Fogstar specifically — are you comparing 80% of rated capacity, or 80% of what you were getting in August? Because those are very different problems with very different solutions. 🙂