Anyone else had issues with Fogstar Drift cells swelling after a harsh winter?

Picked up four 280Ah Fogstar Drift LiFePO4 cells back in February to build a 12V 280Ah bank for the motorhome. Followed the usual top-balance routine, stuck them in a basic DIY plywood box under the bed, and all was well through spring and summer — pulling around 100–120Ah a day no bother.

Come December though, parked up in Dorset with overnight temps dropping to -3°C or -4°C, I noticed one of the middle cells had developed a slight but definite bulge on the side face. The Daly BMS isn't throwing any fault codes and voltage sits at 3.28V at rest, same as the others. Capacity feels marginally down but nothing dramatic yet — maybe 5% off on a full cycle test.

Wondering if this is thermal expansion from the cold rather than genuine cell damage, or whether I've got a dud on my hands. The cells weren't being charged below 0°C — I've got a Victron SmartSolar with the lithium low-temp cutoff enabled — so charge protection should've been doing its job. Has anyone sent a Drift cell back to Fogstar under warranty mid-winter? Curious how they handled it.

Hey @DorsetDweller, sorry to hear you're having bother with those! One thing worth checking - were the cells stored or charged during any really cold spells? LiFePO4 cells absolutely hate being charged below about 5°C, and even though they discharge fine in the cold, charging them when temperatures drop near freezing can cause lithium plating internally which sometimes leads to swelling over time. The plywood box won't have offered much thermal protection against ground-level cold either.

Worth contacting Fogstar directly - they've generally had decent customer service from what I've seen mentioned here before. Take plenty of photos first though and document everything.

What BMS are you running? Some cheaper units don't have low-temperature charge cutoff protection built in, which could be a contributing factor if the van was sitting unused on charge during cold nights.

Haven't had swelling personally with my Drift cells, but @DorsetDweller one thing that jumps out is the plywood enclosure - did you have any compression on the cells over winter? LiFePO4 cells really do benefit from gentle clamping pressure (around 10-12 PSI is commonly recommended), and without it they can expand more freely during charge cycles, especially if they've been sitting partially discharged in the cold. What were your low temperature cut-off settings on the BMS? If the motorhome got below 5°C and the BMS wasn't properly configured to prevent charging below that threshold, you could have caused some internal stress even without realising it. Worth contacting Fogstar directly too - they've generally been decent with warranty queries in my experience. What does the actual swelling look like, is it uniform across all four cells or just one or two?

Really sorry to hear this @DorsetDweller. One thing nobody's mentioned yet - what BMS are you running? LiFePO4 cells shouldn't swell under normal conditions, but if your BMS has been allowing even occasional over-charge or under-discharge events during those cold snaps, that cumulative stress can absolutely cause physical deformation over time. Worth pulling the BMS logs if it supports that, or at least checking your charge voltage setpoints - some motorhome split-charge setups default to lead-acid profiles which are way too high for lithium. Also, have you been in touch with Fogstar directly? They've generally had decent customer service from what I've seen on here, and if the cells are relatively new they may well want to know about it. Swelling on cells that young would be unusual and worth documenting with photos before doing anything else.