Anyone else had issues with Fogstar Drift cells drifting badly out of balance in cold weather?

Running a 280Ah 12V LiFePO4 bank in my tiny house — four Fogstar Drift 280Ah cells in parallel built into a 4S pack. All balanced perfectly over summer but since temps dropped below 5°C the cells are all over the place. Top cell sitting at 3.38V while the lowest is down at 3.21V after a normal charge cycle. JK BMS is screaming at me daily.

Wondering if it's the cells themselves or whether my JK BMS active balancer just isn't keeping up when internal resistance climbs in the cold. I've got it tucked in an insulated cupboard but it's not heated. Balancing current on the JK is set to 2A — should I be pushing that higher?

Has anyone solved this by adding a small heat mat under the battery box, or is there a settings tweak in the JK app that actually helps? Open to suggestions before I start pulling the pack apart.

LiFePO4 voltage curves go almost completely flat in the cold, so your BMS is essentially trying to balance ghosts — perfectly normal, just deeply inconvenient when it starts tripping on phantom imbalances at 3am. 🥶

@MoorKev has already nailed the flat-curve issue, so I won't rehash that.

What actually fixed it in my static caravan setup was adding a small trickle heater pad under the battery bank — just enough to keep cells above 8°C overnight. Picked up a reptile mat from Amazon, works a treat.

Also worth checking: are your cell interconnects actually torqued properly? Cold causes metal contraction and I've seen loose busbars create resistance differences that look like balance drift but aren't really.

One more thing — if you're using a Victron SmartShunt, check your SOC history. Sometimes what appears to be drift is actually one cell that's slightly lower capacity than spec, which only becomes obvious when the curve flattens in cold temps.

Fogstar's support team were pretty responsive when I queried something similar — might be worth a message if cells are genuinely diverging under 3.0V.

Great thread. One thing worth adding that nobody's mentioned yet — have you checked whether your cells are actually thermally balanced, not just electrically? In a 4S parallel pack inside a tiny house, cells near an exterior wall can be sitting 4-5°C colder than those closer to the interior, and that temperature differential alone will cause apparent voltage divergence even when the cells are genuinely at the same state of charge.

Before chasing BMS settings or passive balancer tweaks, I'd stick some cheap DS18B20 sensors on each cell and log the temps during a charge cycle. If you're seeing more than 2-3°C spread between cells, some basic insulation around the battery enclosure (or repositioning the pack away from cold walls) might solve it before you even touch the BMS. Worked a treat for a mate running a similar setup in his shepherd's hut.

@BerlingoDream

Good shout from @QG_Marine on checking the cells themselves — worth ruling that out early.

One thing I'd add that hasn't come up yet: what's your charge cutoff voltage set to in the cold? LiFePO4 really struggles to accept a full charge below 5°C, and if your BMS is still pushing to 3.65V per cell, you're likely causing micro-stress that exaggerates any existing imbalance. I'd consider dropping your charge voltage slightly over winter — somewhere around 3.55V per cell — and see if the balancing situation improves once you're not trying to squeeze every last bit in at the top of the curve in freezing temps.

Also, are your cells physically insulated? Even wrapping the battery enclosure with some rigid foam made a noticeable difference in my Berlingo build during the cold snap last January.

@EdKelly

Following on from what @QG_Marine was hinting at — thermal mass differences between cells can really exaggerate cold-weather imbalance. If your cells aren't all insulated identically, the ones on the outside of your pack will cool faster than those in the middle, and because LiFePO4 internal resistance rises sharply in the cold, you'll get uneven voltage sag under load that looks like a balancing problem but isn't really.

Worth wrapping the whole pack together rather than cells individually — you want them all at the same temperature, not just individually insulated. A simple foam box around the entire bank made a noticeable difference for me over winter. Combine that with @WD40Wizard's fix and you should be in much better shape. What's your current pack housing setup like — is it exposed to outside air at all?

@BorderExplorer

Something nobody's touched on yet — are all four cells in the same thermal environment, or is one on the end of the pack nearest an exterior wall? Even a couple of degrees difference across the pack can cause significant voltage divergence at low temps because LiFePO4's internal resistance rises sharply below 5°C, and cells running colder will show higher resistance and lower apparent voltage under load.

Worth sticking some cheap adhesive thermistors on each cell and logging temps alongside your BMS voltage readings. You might find the "drifting" correlates almost perfectly with a temperature gradient rather than any genuine capacity mismatch. Insulating the whole pack together as a single unit — even just 25mm of PIR foam — often solves it completely without touching the BMS settings.