Anyone else had their Fogstar Drift cells drop capacity over winter?

Picked up four 280Ah LiFePO4 cells from Fogstar back in spring and built a 24V 280Ah battery for my garden office. Been running it through a Victron SmartSolar MPPT 100/30 with a JK BMS — all seemed solid through summer.

Come November the usable capacity feels noticeably lower. I'm only pulling maybe 200–220Ah before the BMS cuts off, whereas I was comfortably hitting 260Ah+ in warmer months. Temps in the office have been dropping to around 5–8°C overnight. I've got low-temp cutoff set at 5°C on the JK so it shouldn't be charging below that, but discharge is still allowed.

Is this just normal LiFePO4 behaviour at lower temps reducing usable capacity, or should I be looking at whether something's gone wrong with the cells themselves? Worth doing a proper top-balance again, or is that not going to help if it's purely temperature-related? Curious what others have seen with Fogstar cells specifically over their first winter.

My shepherd's hut cells did the same thing last January — turned out I'd let them sit at 40% SOC in -4°C for a fortnight, which LiFePO4 absolutely despises for capacity retention. Worth checking your JK BMS logs for any low-temp charging events, because if the MPPT was attempting top-ups on frosty mornings without a temp sensor fitted, you may have caused some minor anode lithium plating. Not catastrophic, but cumulative. A few full charge/discharge cycles through spring sometimes coaxes a bit of capacity back — think of it as waking the cells up from their seasonal sulk. 🏕️

BazLewis72 | 847 posts | ⚡ Solar Addict

@MelKing83 Worth checking your JK BMS logs for any low-temperature cutoffs over winter — the JK will throttle or disconnect charging if cells drop below a certain threshold, which can skew your apparent capacity figures. LiFePO4 genuinely does lose usable capacity in the cold anyway (not permanent, just temporary), but if the BMS was cutting in repeatedly you might have also missed full top-balance opportunities for weeks at a time.

Before assuming the cells have degraded, I'd do a proper capacity test now the temperatures have come back up — fully charge, then discharge at a controlled rate and log the Ah out. Fogstar's Drift cells have a solid reputation so I'd be surprised if there's genuine degradation this early. What temperatures was your garden office hitting overnight?

ValleyAmy | 412 posts | 🔋 Off-Grid Enthusiast

@MelKing83 As @BazLewis72 suggests the BMS logs are a good starting point, but also worth bearing in mind that LiFePO4 cells genuinely do lose usable capacity in the cold — it's not necessarily permanent damage. At 5°C you might only be accessing 80-85% of rated capacity, and if temps dropped near freezing in that garden office overnight, the chemistry just doesn't perform the same.

Have you insulated the battery enclosure at all? Even wrapping it in some Rockwool or foam board makes a surprising difference. Some folk on here run a small self-regulating heat mat on a thermostat during winter months.

Try running a capacity test now temperatures are picking back up and see if you're back to normal figures before worrying too much about cell degradation. Fogstar's cells have generally had a decent reputation on here. 👍

My static caravan cells threw a similar wobble last February — turns out LiFePO4 just has a proper sulk below 5°C and the capacity figures look alarming until everything warms up again. Give it a proper charge cycle once temperatures recover and you'll likely find your 280Ah has come back to life like it never left. 🥶

Liam1990 | 1,203 posts | ⚡ Solar Addict

Worth adding that even if the cells aren't hitting the BMS low-temp cutoff threshold, internal resistance climbs significantly between 0–10°C with LiFePO4 chemistry — you'll see usable capacity drop 15–20% purely from that, which can look like genuine capacity loss on the Victron dashboard. In my motorhome setup I log State of Health via the SmartSolar's VRM portal alongside ambient temperature, and the correlation is really obvious through a British winter.

@MelKing83 — before assuming the Fogstar cells themselves are degraded, I'd pull a proper capacity test at a controlled temperature above 15°C. Discharge at 0.2C through a known load, measure actual Ah out. If you're hitting 260Ah+ you've almost certainly got a thermal performance issue rather than genuine cell degradation. Fogstar's Drift cells have been pretty solid in my experience; don't write them off prematurely.