Anyone else had issues with Fogstar Drift cells balancing slowly at low SOC?

Running a 280Ah 12V LiFePO4 bank in my cabin setup — four Fogstar Drift 280Ah cells in a 4S arrangement with a Daly Smart BMS (100A). Generally really happy with the setup but I've noticed the cells take ages to balance when the bank drops below around 20% SOC. Talking 48+ hours of slow top-up solar before they're back in sync.

At higher SOC (above 50%) the passive balancing keeps up fine. It's specifically that low-voltage range where one cell seems to lag — typically cell 3, sitting around 10-15mV below the others at 3.28V while the rest are at 3.29-3.30V. Not a massive spread but it bugs me and I wonder if it's masking a capacity issue in that cell.

Has anyone switched from passive to active balancing on a similar build? I've seen the Heltec active balancers mentioned a fair bit here — wondering if one of those alongside (or replacing) the Daly would sort it. Or is a 15mV spread genuinely nothing to worry about and I'm just being paranoid about a fairly new bank?

@WattAndrea — interesting one, I'm watching this thread closely as I've got Fogstar Drift cells going into my garden office build soon.

Quick question though: what's your balancing current set to on the Daly? I've read that the passive balancing on those BMS units is quite low (around 35–40mA?), which apparently really struggles to pull cells together when they're sitting at the bottom of the SOC curve where the voltage differences are tiny but meaningful.

Have you considered enabling active balancing or swapping to something like a JK BMS which handles low-SOC balancing noticeably better?

Also — are the cells diverging consistently on the same cell each time, or is it rotating? If it's always the same cell that lags, that might point to a slightly weaker cell rather than a BMS limitation.

Had this exact thing with my narrowboat bank — Daly Smart BMS passive balancing is notoriously weak, only kicks in above a certain voltage threshold so at low SOC it's basically doing nothing.

Two options worth trying:

1. Active balancer — chuck a cheap active balancer on alongside the Daly. Picked up a JK one for about £15, made a noticeable difference
2. Top balance the cells properly if you haven't already — a lot of people skip this and it haunts them forever

The Fogstar Drift cells themselves are solid in my experience, doubt it's the cells. More likely your Daly just isn't up to the job on its own.

@HollyGazer worth doing a proper top balance before you even commission yours, saves the headache later.

@WattAndrea this rings a bell from when I was commissioning my boat bank a couple of years back. Spent three frustrated weekends wondering why my cells kept drifting apart around 10-20% SOC.

Turned out my cells just needed a proper top-balance before trusting the BMS to keep things tidy. I pulled them all individually up to 3.65V with a bench power supply, let them sit, and the Daly suddenly had far less work to do.

Also worth checking your cell interconnect torque — a slightly loose busbar creates enough resistance to cause phantom imbalance readings that look like genuine cell drift.

If patience runs out, the JK BMS active balancers are worth the upgrade cost for a permanent fix. Made a noticeable difference on my setup when winter capacity margins got tight and every percent of SOC mattered.

@WattAndrea worth checking whether your Daly is even attempting to balance below your configured start voltage — mine defaulted to 3.40V trigger which essentially meant it sat idle for the majority of the usable LiFePO4 plateau.

What transformed my 280Ah cabin bank was switching to an active balancer running alongside the Daly rather than replacing it. I'm using a cheap 2A active unit from AliExpress — nothing fancy — and the cells now track within 8–10mV even at the bottom 20% SOC where passive balancing is basically useless due to the flat discharge curve.

The Drift cells themselves are solid in my experience; this is almost certainly a balancing strategy issue rather than anything wrong with the cells. If budget allows, a JK BMS with its built-in active balancer would solve this cleanly in one unit.

Been through this myself with a similar setup. The Fogstar Drift cells are actually pretty well matched from the factory in my experience, so if you're seeing drift at low SOC it's almost certainly the Daly struggling rather than the cells themselves.

One thing worth trying — do a few slow, gentle top balancing cycles first to get everything singing from the same hymn sheet, then let the Daly maintain from there. The passive balancing really only needs to handle minor drift once you've done that groundwork properly.

Also, what's your absorption voltage set to? I found tweaking that made a noticeable difference to how evenly my cells finished. If you're running too short an absorption phase the weaker cell just never quite catches up. @OldSailor86 will probably back me up on this — it's a patience game during initial commissioning really.

@WattAndrea I had almost identical behaviour with my Drift cells last winter. What sorted it for me was doing a proper top-balance before first use — individually charging each cell to 3.65V with a bench power supply and holding them there until current dropped right off. Even "matched" factory cells can drift a bit in transit or storage, and that initial imbalance tends to show up most noticeably at the bottom of the SOC range where the voltage curve is so flat. If you haven't done a full top-balance already, it's well worth the effort. Also, are you regularly reaching a full absorb charge? Cells that rarely see a proper full charge tend to drift further over time. @Thommo40 is right that the Drifts are decent quality out of the box, but they still benefit from that initial conditioning.

@WattAndrea one thing worth trying that hasn't been mentioned yet — give each cell a slow, gentle top-balance charge individually using a bench power supply or even a decent iCharger set to 3.65V with a low current limit (around 2-5A). I did this with my Drift cells when I first built my bank and the passive balancing on the Daly had a much easier job afterwards. Also double-check your balance start voltage threshold in the Daly app — the default is sometimes set surprisingly high, meaning it barely activates during normal cycling. What sort of voltage spread are you actually seeing between cells at low SOC?

@TrevorLamb beat me to the individual top-balance suggestion, but I'll add — once you've done that, chuck a Victron SmartSolar on passive balance duty for a week and let it cook slowly; my narrowboat Drift cells went from 18mV spread to under 3mV doing exactly that, basically zero effort involved. The Daly Smart BMS active balancer is about as powerful as a disappointed sigh, so don't rely on it alone to sort things out at low SOC.

@TrevorLamb and @DevonNomad have covered the top-balance and Smart Shunt angles well. One thing I'd add from my own tiny house build — the Daly Smart BMS passive balancing current is notoriously feeble, typically only 30–68mA depending on firmware. At low SOC where cell voltages are compressed in that flat LiFePO4 curve, that's simply not enough to resolve meaningful divergence in a reasonable timeframe.

Worth checking whether your Daly app shows balancing actually triggering — mine would show the balance icon active but barely shifting delta-V over hours. If that's what you're seeing, you might consider a dedicated active balancer (the Heltec or QUCC units are decent and cheap) running alongside the Daly.