Anyone else finding their Fogstar Drift 200Ah barely hitting 180Ah usable in winter?

Mine's sitting in an uninsulated timber frame lean-to off the side of the tiny house — temps have been dropping to around 4°C out there overnight and the Victron BMV-712 is showing I'm getting roughly 178Ah before the low voltage alarm kicks in.

Fogstar's spec sheet says usable capacity drops at low temps, which, fair enough, basic chemistry innit. But 18Ah of missing capacity is starting to sting when you're trying to run a 12V compressor fridge through the night alongside a couple of LED circuits.

Has anyone actually bothered wrapping their battery in insulation or chucking a small heat mat on a thermostat to keep it above 10°C? Wondering if the power consumed keeping it warm would even offset the capacity gains — or if I'm just going to grump through until April like last year.

@ROW_OffGrid yeah that tracks — LiFePO4 starts losing usable capacity noticeably once you're below about 10°C, and at 4°C you're realistically looking at 85-90% of rated capacity anyway. My cabin setup sees similar overnight temps and I just accept it as winter tax on the system.

Honestly the bigger concern at those temps isn't the usable capacity dip, it's whether your BMS is cutting off charging if the cells themselves are cold — most Fogstar Drift units will refuse to accept charge below 0°C which is the right call, but worth checking your morning charge profile on the BMV-712 to confirm it's actually pulling current when the sun comes up rather than sitting idle waiting for cells to warm.

A bit of insulation board around the battery enclosure makes a surprising difference.

Welcome to the forum @ROW_OffGrid! 🎉

Honestly 180Ah from a 200Ah LiFePO4 at 4°C is pretty decent — don't be too disheartened. I've seen my boat bank drop way more dramatically than that in a cold snap.

Practical fix that made a real difference for me: a small bit of closed-cell foam insulation wrapped around the battery helps retain the heat generated during charge/discharge cycles. Dead cheap from B&Q.

Also worth checking if your Victron BMV-712 has the temperature sensor connected — if you're not already compensating for temp in your readings, your actual usable capacity might be slightly better than it looks on screen.

Spring's not far off and those numbers will creep back up naturally. In the meantime, insulate what you can! 🔋

@ROW_OffGrid worth considering some basic insulation around the battery itself if moving it indoors isn't an option — even wrapping it in rigid foam board (leaving ventilation gaps) can make a surprising difference to the ambient temp it's sitting in. A small self-regulating heat mat on a thermostat set to kick in around 5°C is another option some folks on here swear by, keeps the cells just warm enough without any real energy cost to speak of.

As @WattAndrea and @JimWilson have covered the capacity side well, I'd just add — keep an eye on your charge rates too. Cold LiFePO4 doesn't just discharge less, it also accepts charge more slowly, so your morning solar recovery might be lagging more than you'd expect. The Fogstar BMS should protect against charging below 0°C but sluggish acceptance starts well before that.

@ROW_OffGrid I had almost the exact same situation last winter with my shepherd's hut build — Fogstar Drift sitting in a poorly ventilated under-bench compartment, temperatures tanking overnight, and the BMV-712 telling a rather sorry tale by morning.

What actually shifted things for me was wrapping the battery in a self-regulating heat tape (the kind plumbers use for pipe frost protection) set to kick in around 5°C, then boxing it in with 50mm Celotex. Draws minimal power, keeps the cells just warm enough that you're not losing that chunk of capacity before you've even started your day.

Worth checking your charge settings too — Victron's low-temp charge cutoff should be configured properly if it isn't already. Charging a cold LiFePO4 does it no favours long-term.

@ROW_OffGrid — just to add some numbers to what others have said: LiFePO4 capacity drops noticeably below about 10°C, and at 4°C you're realistically looking at 85-90% of rated capacity, so 180Ah is pretty much textbook behaviour rather than a fault.

One thing worth checking on your BMV-712 — make sure your Peukert exponent and charge efficiency factor are dialled in correctly for LiFePO4 (Peukert around 1.05, efficiency ~99%). Incorrect settings can make the readings look worse than reality.

Also worth knowing: the Fogstar Drift has a low-temperature charging cutoff, so if your panels are trying to top it up on a cold morning before the battery warms up, the BMS may be quietly refusing charge. Might explain any unexpected SOC behaviour you're seeing. 🔋

@ROW_OffGrid 180Ah from a 200Ah at 4°C is honestly not too bad — you're probably losing another chunk to the BMV's state of charge calibration as well, so the real-world chemistry loss might be slightly less dramatic than it looks on the display. Worth doing a full discharge/charge cycle once things warm up a bit to let the BMV recalibrate properly.

One thing nobody's mentioned yet — check whether your Drift has low-temperature charge protection active. Most do, and if overnight temps are creeping near 0°C you may find the BMS cutting charge entirely at some point, which would compound the issue. The discharge side handles cold better than charging does with LiFePO4.