Anyone else had issues with Fogstar Drift cells going out of balance after a few months?

Fitted a 200Ah 12V LiFePO4 last spring using four Fogstar Drift 50Ah cells with a Daly 100A BMS. First few months were spot on, all cells sitting within a few millivolts of each other. Now cell 3 keeps drifting high during charge — hitting 3.65V while the others are still around 3.45V, causing the BMS to cut out early.

Tried a few manual top-balance sessions using my bench PSU but it keeps drifting back within a week or two. Running a Victron SmartSolar 100/30 so charge profile seems solid, nothing obviously wrong there. Van's used most weekends for camping trips, so the pack gets cycled fairly regularly.

Wondering if cell 3 is genuinely degrading or if the Daly BMS passive balancing just can't keep up. Anyone swapped out a single cell in a pack like this, or is it better to just upgrade to an active balancer at this point?

@Brummie86 classic symptom of a cell that's drifted slightly lower in capacity — probably a minor internal resistance increase rather than anything catastrophic. Your Daly BMS won't actively balance until cells hit the top-of-charge knee (around 3.45V+), so if you're not regularly reaching full charge, imbalances quietly accumulate over months.

Few things worth trying:

• Do a full absorption charge to 14.6V and let the BMS passive balancing run for a good hour or two at the top
• Check your cell interconnect torque — loose busbars cause resistance differences that mimic weak cells
• Log individual cell voltages under load — a weak cell will sag disproportionately

The Daly passive balancer is fairly feeble (around 30-50mA). If balance issues persist, an external active balancer like the JK active balancer is worth the £20-odd investment.

@Brummie86 worth checking whether your Daly is actually balancing or just monitoring. The passive balancing on most Daly units only activates above ~3.4V per cell and bleeds a measly 30-50mA — practically useless if cell 3 has drifted more than a few percent in capacity from the others.

What I'd suggest: manually top-balance the four cells individually with a bench power supply before reconnecting — hold each at 3.65V until current drops below ~100mA. This resets your baseline properly.

Also log your charge voltage. If your charger is hitting 14.6V and the Daly's balance function still can't keep up, that points to @KentBoater's capacity divergence diagnosis rather than a BMS fault.

I had similar creep on my 280Ah build after about eight months — top-balance sorted it completely and cells have stayed within 8mV ever since.

@Brummie86 Had almost the identical setup last year on my shed build. One thing worth trying before you conclude it's a duff cell — give the whole pack a proper top balance. Disconnect everything, charge each cell individually to 3.65V with a bench power supply or decent single-cell charger, then reassemble. Sometimes cells drift apart simply because they were never perfectly balanced at the start, and it compounds over time.

Also worth double-checking your cell 3 bus bar connections are properly torqued. A slightly loose connection creates resistance which makes that cell appear weaker than it actually is. I chased a "bad cell" for weeks before finding a terminal that just needed another quarter turn.

If after a proper top balance it still keeps lagging, then I'd lean towards what @KentBoater is suggesting about reduced capacity.

@Brummie86 had something similar with my garden office build. Cell 3 going high at top of charge is usually the one that's slightly higher capacity rather than the weak link — it charges faster so hits the BMS high voltage cutoff first.

Worth doing a full top balance outside the pack. Parallel all four cells at 3.65V with a decent bench PSU, let the current drop right off. Did this with mine and they've stayed within 10mV ever since.

Also — what's your charge voltage set to? If you're pushing 14.6V with a Daly doing minimal balancing, you're giving it very little time to catch up before cutoff. Drop to 14.4V and see if it helps first before pulling the pack apart.

@Brummie86 on my narrowboat build I had cell drift creep in around the 6-month mark too — turned out my bus bars weren't torqued evenly and one connection had slightly higher resistance, causing uneven charge acceptance over time.

Worth pulling the bus bars off cell 3 specifically and checking for any discolouration or oxidation on the terminals. Even a tiny bit of resistance there will show up as cell imbalance under load/charge.

Also worth noting — Daly passive balancing only kicks in above ~3.4V and bleeds off maybe 30-60mA. If your charge current is anything decent that balancer is basically fighting a losing battle. A small active balancer (I added a cheap JK one) alongside the Daly made a noticeable difference within a few cycles.