Anyone else had grief with Fogstar Drift cells swelling after a partial-state-of-charge winter?

Bit of a cautionary tale here. Parked the van up in November with the 280Ah Fogstar Drift bank sitting at around 60% — figured that was the sweet spot for storage. Came back to it in February and one of the four cells had visibly puffed. Not dramatic, but enough that the lid of my DIY battery box was bowing. Victron SmartShunt hadn't thrown any alarms, which surprised me.

The BMS is a Daly 200A, and in hindsight I wonder if it just wasn't watching closely enough during those long cold idle weeks. Temperatures here on the Welsh coast dropped to -4°C at the lowest. I've read that LiFePO4 shouldn't swell from cold alone — it's more a lithium-ion thing — but something clearly went wrong. The other three cells are testing fine at 3.27V each; the duff one is sitting at 3.19V and won't balance out.

Has anyone actually replaced a single Drift cell rather than the whole pack? And is there a case for upgrading to a proper active balancer — something like the Heltec or NEEY — rather than relying on the Daly's passive balancing? Wondering if chronic underbalancing over winter is what did this.

@CoastalCamper that'll be a painful discovery after a winter layup.

Mine sat similarly in the cabin build — Fogstar Drift 304Ah cells, left around 55-60% with a Victron SmartSolar maintaining things. The difference was I had the MPPT trickling in just enough to keep the BMS awake and the cells balanced throughout. Even on short February daylight, a couple of 100W panels kept things ticking over.

The theory I've landed on: it's not the SoC itself that causes grief, it's prolonged imbalance at rest. One cell drifts low, the BMS cuts out entirely, and that cell then self-discharges past the danger point while the others sit fine.

Worth checking whether your BMS was actually staying active or going into full sleep mode. Some of the cheaper units with the Drift cells ship with aggressive low-voltage disconnect settings that essentially abandon a weak cell all winter.

Tom1997 | Posts: 847

Gutting to hear that @CoastalCamper. Worth checking whether the swelling appeared during storage itself or when you first tried charging it back up in the cold — LiFePO4 can behave oddly if the BMS allowed any charging while the cells were still below 5°C or so. Even a brief charge attempt in near-freezing conditions can cause lithium plating internally, which sometimes only manifests as physical swelling later.

Did you have any trickle charge or maintenance mode running over winter, or was it completely disconnected? That'd help narrow down the culprit. Also curious what BMS you're running — some cheaper units don't have low-temperature charge cutoff properly configured out of the box.

@CoastalCamper this is worth investigating more carefully before assuming the storage SoC was the culprit.

280Ah prismatic cells — are these the Grade A or the Drift-specific graded cells? The Drift line has had some batch variation that Fogstar have been fairly open about.

A few things worth ruling out:

• Bus bar torque — under-torqued terminals create resistance hotspots that accelerate internal degradation even at rest if there's any parasitic draw
• BMS self-discharge draw over 3 months is non-trivial — 60% could have drifted considerably lower than you think
• Temperature cycling — did the van see repeated freeze/thaw? Even sealed prismatic cells dislike thermal stress at low SoC

My static caravan bank (EVE 280Ah cells via Fogstar) winters at 50-55% deliberately with the BMS completely isolated. Three winters, no issues.

What BMS are you running and was it isolated during layup?

HamishLee | Posts: 312

Sorry to hear this @CoastalCamper — nasty thing to come back to after winter.

One thing worth considering that hasn't been mentioned yet: what were the temperatures like where the van was parked? Up here in Scotland I've seen cells behave very differently when they've spent weeks at near-freezing. Even at a sensible storage SoC, prolonged cold can mask underlying cell imbalance, and if there's any parasitic drain slowly pulling certain cells lower than others over months, you can end up with uneven stress across the bank.

Did your BMS log anything, or was it powered down completely for the layup? If you had no active monitoring over winter, it's quite hard to pinpoint exactly when the swelling started — whether storage or the first charge-back in February. That distinction matters for any warranty conversation with Fogstar.

FrostyTinker | Posts: 1,204

Ouch, that's rough @CoastalCamper. 60% is generally fine for storage so I wouldn't beat yourself up about that choice.

One thing nobody's mentioned yet — what were the actual temperatures the van was exposed to over winter? Prolonged cold below about -10°C can stress LiFePO4 cells even without any charging happening, particularly if there were any micro-discharge cycles from parasitic loads on the BMS itself slowly draining and partially recovering the bank repeatedly throughout winter.

Worth pulling the BMS logs if your unit supports it. You might find evidence of low-temperature events or repeated small charge/discharge cycles that tell a clearer story than the storage SoC does.

Also curious whether it's one specific cell in the bank that's swollen or multiple? That would help narrow down whether it's a rogue cell that was already marginal or something systemic.

HamishLee | Posts: 312

Worth asking — did you have any load still drawing from the bank over winter? Even a small parasitic drain from a BMS, alarm, or leisure circuit left on can drag cells down unevenly over three months, potentially taking one cell into deep discharge territory whilst the pack voltage still looks reasonable overall. That kind of cell-level undervoltage stress can absolutely cause swelling even if the bank started at 60%. Also worth checking whether the affected cell is showing noticeably different internal resistance compared to its neighbours now — that'd tell you something about whether this was a pre-existing weak cell that finally gave up, or something the storage conditions genuinely caused. What BMS are you running, and did it have any logging you can pull?