Anyone had issues with Fogstar Drift cells losing capacity over winter in an unheated static?

My setup is two 200Ah Fogstar Drift LiFePO4 batteries running a Victron SmartShunt and a Cerbo GX. Worked brilliantly all summer charging off a 600W Renogy solar array, but I've noticed the usable capacity seems noticeably lower now the temps have dropped — we're regularly sitting at 3–5°C overnight in the van on site.

I know LiFePO4 doesn't like charging below 5°C and the BMS should cut that off, but I'm wondering if the cells themselves are suffering or if this is just normal temporary capacity reduction from cold? The Victron app is showing I'm getting maybe 140Ah out of what should be 400Ah total before the voltage starts dropping sharply.

Is anyone running heating pads under their batteries through winter, or is there a smarter way to handle this? Also curious whether the Fogstar BMS gives any low-temp protection data I can actually read — can't seem to find it logging anywhere in VRM.

ChloeFisher72 | 847 posts | ⭐ Trusted Member

@IvyWalker LiFePO4 cells genuinely do lose usable capacity in the cold - it's not damage, just chemistry. Below about 5°C you'll see noticeable derating, and sub-zero it gets quite pronounced. The good news is it's reversible once temps come back up.

A couple of things worth checking in your Victron setup: make sure your SmartShunt's temperature compensation is configured properly, and have a look at what the Cerbo is actually logging overnight - you might find the cells are sitting colder than you'd expect.

If your BMS isn't cutting off charging below 0°C, that's actually the bigger concern with LiFePO4 - charging cold cells can cause lithium plating. Worth verifying that protection is active.

Have you noticed whether the apparent capacity loss tracks with ambient temperature, or is it consistent regardless of conditions?

HollyGraham83 | 312 posts | Regular Member

@IvyWalker I had almost identical symptoms with my Drift cells last winter in my static in Northumberland. Worth checking what temperature your batteries are actually sitting at - I stuck a cheap bluetooth thermometer inside the battery compartment and was genuinely shocked. Mine were dropping to around 4°C overnight which really hammers usable capacity.

Also, are you storing them at a decent state of charge going into cold snaps? I found keeping mine above 50% helped considerably. The Cerbo should give you enough data to spot patterns if you dig into the VRM portal history. Don't panic just yet - mine recovered capacity fine once temperatures came back up in spring. 🙂

HazelDawn | 312 posts | 🌿 Member

This hit close to home — my shepherd's hut batteries went through exactly this last January. What caught me out wasn't the cold itself but the combination of low state of charge and sub-zero nights. LiFePO4 really doesn't want to accept charge below 5°C, so the Victron MPPT was essentially pushing watts into cells that were refusing them.

What transformed things for me was setting a low-temperature charge cutoff in VictronConnect and stuffing 50mm Celotex beneath the battery box. Nothing glamorous, just thermal mass management.

Worth checking your Cerbo GX history — the SOC graphs will show you exactly when capacity appeared to "drop" versus when charging simply stalled overnight. Two very different problems with two very different fixes.

DerekKnight | 1,203 posts | ⭐ Trusted Member

Worth mentioning something nobody's touched on yet — check your SmartShunt's temperature compensation settings in the Victron app. By default it's not always configured to account for the reduced charge acceptance at low temps, so your Cerbo GX may actually be overcounting available capacity and giving you misleading SoC readings on top of the genuine cold-related derating.

Also, if your static is sitting at, say, 5°C overnight, the Drift cells won't charge efficiently below 5°C anyway — most LiFePO4 BMS units will throttle or cut charging entirely to protect against lithium plating. So you may be starting each day with less than you think.

A cheap self-adhesive battery heater mat with a thermostat set around 8°C made a noticeable difference for mine over last winter. Adds a small parasitic drain but well worth it. 👍