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

Picked up four 280Ah Fogstar Drift LiFePO4 cells back in March to build a 12V 280Ah bank for the T6. Balanced them carefully before first use, all sitting within 2mV at rest. Ran them through the Daly 100A BMS and everything looked tidy for the first couple of months.

Now around month four, I'm seeing one cell consistently lagging behind — hits 3.45V while the other three are already nudging 3.55V under charge. Under load it's the first to sag too, dropping to around 3.1V when the others are still above 3.2V. Top-balanced again last week with a bench PSU, got them all back to within 5mV, but within a few cycles the same cell is drifting again. Capacity hasn't collapsed noticeably yet but it's clearly the weak link.

Wondering if this is a duff cell or just normal variation shaking out over real-world cycles. The Daly is doing its job catching the high-voltage cutoff but I'm half-tempted to swap to an active balancer — looking at the Heltec 2A unit. Has anyone actually found active balancing fixes persistent drift like this, or is a cell that keeps lagging just a cell that needs replacing?

Had this exact thing with my van build. Cells looked mint out of the box but by month three one was lagging noticeably under load.

Turned out my Daly wasn't balancing aggressively enough — switched to an active balancer running alongside it and the drift basically stopped. The Daly passive balancing just can't keep up once you're putting proper cycles through them.

Worth checking your charge absorption time too. If your Victron (or whatever you're running) isn't holding absorption long enough, the cells never get a proper top balance and drift compounds over time.

@T6Solar what voltage are you hitting at the top? If you're not reaching 3.45V per cell consistently the balancer has nothing to work with.

@T6Solar and @Brummie86 — worth checking whether your Daly BMS is actually balancing whilst under load or only at top-of-charge. The passive balancing on most Daly units is woefully weak (30–50mA typically), which means if one cell has even slightly higher self-discharge than the others, it'll fall progressively further behind each cycle.

I had a similar drift issue in my Sprinter build after about four months. Switched from the Daly to a Victron Smart BMS paired with a dedicated active balancer module and the cells pulled back into alignment within roughly 20 charge cycles.

Also worth logging individual cell voltages under heavy load — my "drifting" cell actually had elevated internal resistance rather than genuine capacity loss, which is a different problem entirely.

What charger profile are you running? Absorption voltage and duration can mask drift or accelerate it depending on settings.

@PaulaFisher91 makes a solid point about the Daly's balancing window. Worth adding: passive balancing dissipates energy as heat, so if your cells are drifting more than ~10mV at rest after a full charge cycle, the BMS passive balancer simply can't keep pace with the underlying capacity variance between cells.

What I'd do first is a proper top-balance — disconnect from the BMS entirely, charge each cell individually to 3.65V with a bench PSU, let them sit 24 hours, measure again. If they're still diverging significantly after that, you've likely got a genuinely weak cell rather than a balancing issue.

On my narrowboat I run Victron's active balancer alongside a Daly for exactly this reason — the Daly protects, the active balancer does the actual work. Costs another £25–30 but eliminates this headache almost entirely.

Went through almost identical heartache with my static caravan setup last winter. Three months in, one cell in my 200Ah bank started lagging noticeably during heavy morning loads — kettle, electric blanket, the usual suspects.

What actually sorted it for me was switching to a Victron SmartShunt alongside the BMS. Suddenly I could see what was happening in granular detail rather than guessing. Turned out my charging profile was slightly off — absorption voltage just a touch too high, which was stressing that one cell disproportionately.

@StuCampbell's point about passive balancing rings very true. The energy dissipated as heat during balancing isn't trivial, particularly in an enclosed space.

Worth reviewing your charging parameters before assuming the cells themselves are faulty — in my experience the cells are often innocent.

@T6Solar had similar grief with a 4-cell pack last summer. One thing nobody's mentioned yet — have you checked the internal resistance of each cell individually? A cell that's drifting under load but looks fine at rest often has elevated IR compared to its neighbours, even if it passed initial checks. Grab a decent charger with IR measurement or a dedicated meter and compare all four. If one's crept up significantly it'll sag harder under draw and trigger your BMS cutoff early, which looks like capacity drift but is actually a weak cell struggling. Fogstar's warranty support has been decent in my experience if you can demonstrate the fault with data — log your cell voltages under a known load and present that alongside the IR readings.

Really useful thread this. @12VNerd makes me curious what you were about to say about inter-cell connections — hoping you finish that thought because loose bus bars or undertorqued terminal connections can cause a cell to appear to drift when really you're just seeing resistance-induced voltage drop under load. Worth giving every connection a proper torque check with a calibrated wrench, not just hand-tight.

@T6Solar what's your charge rate looking like? If you're pushing anything above 0.2C into a pack where one cell is slightly weaker internally, it'll consistently fall behind the others and the Daly won't catch it fast enough at its default balancing threshold. Dropping charge current temporarily can sometimes help you work out whether it's a genuine capacity issue or a balancing lag problem before you go replacing cells unnecessarily.