Anyone else had issues with Fogstar Drift cells drifting badly during balancing?

Picked up four 100Ah Fogstar Drift LiFePO4 cells back in April to build a 12V 200Ah battery (2P2S). Been running them with a Daly 100A BMS and a Victron SmartSolar 100/30 on a 400W roof array.

Everything looked fine for the first couple of months but lately I'm noticing one cell pair consistently hits 3.65V and triggers the BMS high-voltage cutoff while the other pair is still sitting around 3.48V. That's a 170mV gap which seems way off. Top-balancing them manually with a bench power supply brought them back into line, but within a few weeks the drift creeps back in.

Is this a passive balancing limitation with the Daly, or could one of the cells actually be losing capacity? Has anyone swapped to an active balancer — something like the Heltec or a Daly Smart with active balance — and actually seen a measurable improvement, or is it mostly marketing?

@AlanPalmer57 classic symptom of cells that haven't been properly top-balanced before assembly. With a 2P2S configuration the Daly's passive balancing current is tiny — typically 30–50mA — which is nowhere near enough to correct significant capacity divergence between your parallel pairs.

Before anything else, I'd disconnect the pack, split it back to individual cells and charge each one independently to 3.65V with a bench power supply or decent lab charger, holding at that voltage until current drops below ~100mA. Repeat the cycle a couple of times.

I did exactly this with my Fogstar cells going into my garden office build — took about a weekend but eliminated the drift entirely. The Daly is fine for protection duties but don't rely on it to do the balancing heavy lifting. Worth checking cell internal resistance while you're at it; anything above ~0.3mΩ divergence between cells is worth flagging to Fogstar directly.

Really interested in this thread as I'm planning a similar build for emergency backup. Quick question for @AlanPalmer57 — are you seeing the drift mainly at the top end of charge (above 3.45V per cell) or is it spreading across the whole SOC range?

Reason I ask is I've read that Fogstar Drift cells can sometimes have quite different internal resistance figures between batches, which would cause drift throughout the cycle rather than just at the top. Worth checking your BMS cell voltage logs if the Daly gives you that data.

Also — have you tried slowing the charge current right down temporarily? Sometimes forcing a slow absorption phase lets a partially balanced pack settle before you do a proper top-balance. Curious what numbers you're actually seeing between the worst and best cells.