Anyone else had issues with Fogstar Drift cells drifting badly after a few charge cycles?

Just finished building a 48V 200Ah LiFePO4 bank using Fogstar Drift 280Ah prismatic cells (8S configuration). Topped balance took ages but eventually got all cells sitting within 2-3mV at 3.65V. Looked perfect. Connected the whole lot to my Victron Multiplus-II 48/5000 and started running it through proper charge/discharge cycles.

By cycle four, I've got one cell sitting noticeably high — hitting 3.68V while the others are still around 3.45V. The JK BMS is catching it and shutting down well before I'm getting anywhere near usable capacity. Tried a second top balance overnight and it settled back down, but by cycle six it's drifting again. Same cell every time, which makes me think it's either a weak cell or something odd with capacity matching out of the factory.

Has anyone else seen this with the Drift range specifically, or is this just par for the course with budget prismatic cells? Wondering whether Fogstar will entertain a warranty claim if I can prove it's genuinely underperforming, or whether I'm just stuck capacity-testing each cell individually and hoping for the best.

@Mark1978 been through exactly this with my boat bank — 8S of 280Ah cells, same story.

What sorted it for me was being more brutal with the top balance. I left mine on the bench connected in parallel for 48 hours at 3.65V through a proper bench PSU, not just a charger. The cells were drawing almost nothing by the end, genuinely settled.

The other thing worth checking: cell compression. Prismatic cells need even clamping pressure or internal resistance varies and drift looks worse than it actually is. I built a simple plywood jig with threaded rod — made a noticeable difference.

Also, what BMS are you running? Some cheaper units have charge balancing that's too aggressive early on and actually creates apparent drift. A Daly will behave very differently to a JK or Victron Smart BMS in those first few cycles.

Hey @Mark1978, sounds frustrating but pretty normal behaviour with new prismatic cells to be honest. The thing people don't always mention is that top balancing at 3.65V only tells half the story — it's the mid-range where the real differences show up once you start cycling properly.

What I'd suggest is don't keep chasing the top balance repeatedly. Let the pack cycle through several charge/discharge sessions and check what's happening around the 50-60% SOC mark. If cells are still drifting badly after 10-15 cycles then it's worth investigating further.

Also worth double-checking your BMS cell wire connections are all solid — a slightly loose sense wire can make a perfectly balanced cell look miles out. What BMS are you running with it?

Hey @Mark1978, worth checking whether your BMS is actually balancing passively during charge or only kicking in near top of charge. A lot of cheaper BMS units have a balancing start threshold that's too close to the charge cutoff voltage, so it barely gets chance to work before the charger stops. Try dropping your charge current right down for the absorption phase — gives the balancer more time to do its job. Also, 2-3mV at top balance is decent but those cells will often settle into their "real" capacity after 10-20 proper cycles, so don't panic just yet. What BMS are you running?

Hey @Mark1978, one thing worth mentioning that nobody's touched on yet — have you checked your cell compression? Prismatic LiFePO4 cells really do benefit from being held under moderate, even pressure. Without it, the internal layers can expand unevenly during cycling and that alone can cause cells to behave inconsistently and drift more than they should.

Also, how many cycles are you actually into it? I found my 280Ah prismatics were still "finding their feet" around cycles 10-20 before settling down. The capacity ratings from Fogstar are tested at fairly low C rates too, so if you're pushing them harder than that the weaker cells will show up more quickly.

Don't panic just yet — give it a few more cycles and see if the drift stabilises. What BMS are you running?

@Mark1978 — 2-3mV after top balance is practically perfect, your cells haven't "drifted badly," they've just remembered they're new and need about 20-30 proper cycles before they settle down and stop being dramatic about it.

@Mark1978 just to add something practical — after your next full charge cycle, leave the pack resting for 24 hours before checking cell voltages again. Surface charge can mask what's actually happening and make cells look like they're drifting when they're not. If you're still seeing significant spread after that resting period, that's when I'd start investigating more seriously. Also, what capacity are you actually drawing down to? If you're regularly cycling below 20% SOC on brand new cells, that can exaggerate voltage spread during the settling-in phase before the cells have properly conditioned themselves. Give it 10-15 full cycles and things should tighten up considerably in my experience.

@Mark1978 worth adding — how confident are you in your BMS settings? A lot of people set their cell overvoltage protection right at 3.65V, which means the balancer is fighting against cutoff during top balancing. If your BMS is tripping early on any cells, you'll see what looks like drift but is actually just inconsistent charge termination. Try logging individual cell voltages throughout a full charge cycle rather than just checking at the top — that'll tell you a lot more about where the divergence is actually happening.

Good point from @CamperTel there. Also worth checking your charge termination voltage on the charger itself — if it's cutting off cleanly at 3.65V per cell rather than continuing to push current, that makes a big difference to how settled the cells are post-charge.

One thing I'd add: are you bottom balancing or relying solely on that top balance? With Fogstar Drift cells specifically I've found giving them a couple of full cycles before judging the drift behaviour is worthwhile — they do tend to settle down noticeably once they've had a bit of use. Don't panic just yet!