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

Picked up 4x 280Ah LiFePO4 cells from Fogstar last spring, built a 12V 280Ah bank with a Daly BMS. Summer was brilliant — barely touched 90% most days with the solar keeping up.

Come November though, I noticed the usable capacity dropping noticeably. Resting voltage looks fine, BMS isn't throwing any faults, but I'm only getting maybe 220-230Ah out before the low voltage cutoff kicks in. That's a solid 50Ah down on what I was seeing in August.

Cells are in a wooden enclosure in my garage — unheated, probably sitting around 4-6°C overnight lately. I know LiFePO4 doesn't love the cold but I didn't expect this much of a hit. Running a Victron SmartShunt so the numbers should be reasonably accurate.

Anyone seen similar? Is this just temperature derating and it'll come back in spring, or should I be looking at something else — cell imbalance, dodgy BMS calibration? Worth doing a full top balance again?

@OllieThompson73 LiFePO4 doesn't actually lose capacity in cold — what you're seeing is voltage sag under load making the BMS cut out early, or your charger backing off because the absorption voltage looks "reached" sooner in cold temps.

A few things to check:

• What temperature are your cells sitting at? Below ~5°C and you'll see noticeable capacity reduction in real-world use
• Daly BMS — decent budget option but its low-temp charging cutoff can be aggressive
• Check your Victron (or whatever charger you're using) absorption voltage — might need bumping slightly in winter

My boat bank did exactly this first winter. Cells were fine — just cold. Once I insulated the battery box and added a small heat mat on a thermostat, sorted it completely.

Run a proper capacity test at controlled temp before assuming the cells are degraded.

RussMartin replied:

@OllieThompson73 Worth adding to what @RayWatson81 is getting at — it's not just voltage sag fooling your BMS, cold temperatures genuinely increase internal resistance in LiFePO4, so you'll see more sag under the same load than you would in summer. Your actual stored capacity is largely fine, but you're effectively getting less usable capacity because the BMS cuts out earlier to protect the cells.

Try insulating the battery box if it's in an unheated space — even a simple foam enclosure makes a noticeable difference. Some folk run a small heating mat on a thermostat inside the enclosure, set to kick in around 5°C. Your Daly should be fine once the cells are a bit warmer. What temperatures are you actually seeing in your battery space overnight?

@OllieThompson73 worth checking whether your Daly BMS is cutting out early due to low temp protection triggering. Mine did exactly this over last winter in my tiny house setup — the BMS was reading the cell temps as too low and throttling discharge before the cells were actually depleted. What are your overnight lows where the battery bank is housed? If it's an uninsulated space dropping below 5°C that could be your culprit rather than any actual capacity loss. A simple temperature log overnight would tell you a lot.

Good points from everyone above. One thing I'd add — have you checked what your actual resting voltage is first thing in the morning before any loads kick in? LiFePO4 has that notoriously flat discharge curve, so even a small genuine capacity reduction from the cold can look dramatic on a state-of-charge readout because you're already sitting in that flat middle section.

Also worth logging your actual amp-hour consumption with a decent shunt meter like a Victron BMV if you haven't already. Gives you proper data rather than guessing from voltage alone. I'd bet once spring arrives and temps climb back above 10°C you'll find things look a lot healthier again. @MV_Marine's point about the BMS cutting early is definitely worth ruling out first though — that'd be my starting point before worrying about the cells themselves.

LazySparky replied:

@OllieThompson73 One thing nobody's mentioned yet — what's your charge voltage set to? Some Daly units ship with conservative defaults that were already marginal in summer but become more noticeable once your panels are producing less and the cells are sat lower for longer. Worth logging your actual absorption voltage and comparing it against Fogstar's recommended spec for those cells. Also, are you running any load overnight? Sometimes what looks like capacity loss is actually a parasitic drain you've not noticed before — small inverter standby, a router, fridge cycling. These things add up considerably over a long winter night. Rule out the boring stuff first before assuming the cells themselves are the issue. They're decent cells — unlikely to have degraded significantly in less than a year.

JackieEdwards75 replied:

@OllieThompson73 Worth considering the temperature coefficient on LiFePO4 as well — you genuinely do lose usable capacity in the cold, it's not just the BMS playing up. My 280Ah cells behave noticeably differently below about 5°C, maybe 10-15% less usable capacity even when everything else is spot on. If your battery bank is in an uninsulated space like a shed or garage, that'll compound things considerably over winter. A simple solution is wrapping the cells in some foam insulation — not airtight obviously, just enough to hold a bit of warmth. Pair that with what @LazySparky said about charge voltage and you might find things improve noticeably. What temperatures are you actually seeing in the battery location?