Anyone else had issues with Fogstar Drift cells losing capacity after a cold winter?

Picked up a 200Ah Fogstar Drift lithium last spring for the shepherd's hut build — been running it through a Victron SmartSolar and a Cerbo GX, all set up properly with the right charge profile. Worked brilliantly through summer and autumn, sitting at around 195Ah usable without any issues.

Come January/February though, with overnight temps regularly hitting -5°C or lower out here, I noticed the BMS cutting out earlier than expected. Now that we're into warmer weather again, it's only recovering to about 178Ah — so I've lost roughly 17Ah of capacity and it doesn't seem to be coming back.

I've checked the Victron data logs and there's no sign of it ever being charged below 0°C (the BMS should have protected against that anyway), so I'm not sure if it's genuine cell degradation or something else going on. Has anyone else seen this with the Drift range specifically, or with LiFePO4 in general after a harsh winter? Worth contacting Fogstar directly or is this within normal tolerance for the first year?

@TerryScott72 had similar concerns going into last winter with mine in the shepherds hut setup. What temps are you actually seeing inside the battery compartment? LiFePO4 drops apparent capacity noticeably below about 5°C even if the cells are fine — it's not necessarily permanent loss.

Worth checking your Cerbo logs for lowest recorded temps. Mine showed the battery hitting 2°C on a few nights and the capacity looked terrible until it warmed up again.

If you're not already, a bit of insulation around the battery box makes a proper difference. I used some rigid PIR board last autumn — cheap fix before assuming the cells are duff.

@TerryScott72 had similar concerns running Drift cells on the narrowboat over last winter — we hit a few nights down near -5°C moored up in Cheshire and I was watching the SOC like a hawk.

What I found: the cells themselves were fine, but apparent capacity loss was mostly the BMS throttling charge acceptance in the cold, not actual degradation. Once temps came back up into double figures the figures normalised.

Worth checking:

• Is your Cerbo logging historical SOC? Compare a full charge cycle now vs last autumn
• What's your low-temp charge cutoff set to on the SmartSolar?

LiFePO4 chemistry genuinely doesn't like charging below 5°C — if the BMS was doing its job it may have been rejecting charge on cold mornings, which looks like capacity loss but isn't permanent damage.

Give it a proper calibration cycle now it's warmer and see where you land.

Hey @TerryScott72 — worth checking whether your Cerbo is logging historical data so you can actually compare capacity figures pre and post-winter rather than going on feel alone. The VRM portal is brilliant for this if you haven't dug into it yet.

One thing I'd flag: LiFePO4 cells don't actually lose capacity from cold exposure itself, but if they were charged whilst below about 5°C that can cause lithium plating internally which does cause permanent degradation. Did you have any low-temperature charge protection set up, or was the SmartSolar just cracking on regardless through those cold snaps?

A proper capacity test — full charge, controlled discharge to cutoff, measure the actual amp-hours — will tell you definitively whether you've got a real problem or just the cells behaving oddly in the cold. What's your BMS showing for state of health?

Really useful thread this. One thing I'd add that nobody's mentioned yet — LiFePO4 cells don't just lose capacity in the cold, they also raise their internal resistance, which can make your BMS trip out under load and give a false impression of capacity loss. Worth doing a proper capacity test in controlled conditions (mild day, steady discharge rate) before concluding the cells have actually degraded. If you're seeing the same figures as last spring once temperatures are back up, the cells are probably fine. @TerryScott72 what's your actual resting voltage looking like at the moment? That can tell you quite a bit about the state of things before you go pulling the whole system apart.

Great thread. Just to build on what @WillRoss is getting at — the internal resistance increase in the cold is worth paying attention to because it can make your BMS trip on voltage sag under load even when the cells themselves are fine. I had this exact thing happen with a DIY pack last February. Looked like capacity loss but was actually the BMS cutting out momentarily during high draw. Worth checking your BMS logs if it records that data, and also running a proper capacity test now the temperatures have come back up. If you're genuinely losing capacity rather than experiencing cold-related performance dips, that's a different conversation — but I'd rule out the resistance/sag issue first before worrying about permanent degradation. Fogstar's support have been fairly decent in my experience if you do need to escalate it.

Good point from @SimonPalmer on internal resistance — worth adding that in a motorhome context I've seen this cause the BMS to trip under load even when the cell voltage looks fine at rest. The battery appears healthy sitting there, but the moment you pull 50A+ through a cold pack the voltage sag hits the low-voltage cutoff and the BMS disconnects.

What I do now is keep a small 240V immersion heater blanket wrapped around the battery bank when hooked up on site over winter — keeps the cells above 5°C and the difference in usable capacity is night and day. Obviously not viable off-grid, but for a shepherd's hut with mains access it's worth considering.

Also check whether your Fogstar BMS has a low-temp charge protection threshold — most modern ones do, and that could explain the reduced accepted charge.

@RustyRoamer raises a valid point about BMS cutouts in motorhome installs, but for a static setup like @TerryScott72's shepherd's hut, the bigger risk is actually charging into a cold pack rather than discharge cutout. LiFePO4 really doesn't like accepting charge below around 5°C — lithium plating on the anode is a genuine degradation mechanism, not just a theoretical concern. If the Victron SmartSolar doesn't have a temperature sensor fitted to the battery, that's the first thing I'd check. The Cerbo GX can pull battery temp data if the BMS reports it over VE.Can or BLE — worth verifying that charge inhibit is actually functioning rather than assuming it is.

Something worth adding to what @Keith1977 and others have touched on — have you checked your State of Charge readings against actual resting voltage after a full charge cycle? Sometimes what looks like capacity loss is actually the Cerbo's SOC drifting out of calibration over winter, especially if the battery sat partially discharged in the cold for any length of time. Worth doing a proper full charge followed by a controlled discharge test to get a realistic capacity figure before assuming the cells themselves have degraded. The Drift cells are generally decent quality — might be a measurement issue rather than genuine loss.