Anyone else finding Fogstar Drift cells inconsistent out of the box?

Just taken delivery of four 280Ah Fogstar Drift cells for the shepherd's hut build. Charged them all individually to 3.65V as per the usual top-balance routine, let them rest overnight, and the capacity test is showing a noticeable spread — worst cell is only hitting around 261Ah while the best is comfortably over 278Ah. That's a 17Ah variance which feels too wide for a matched set.

Running a Victron SmartShunt to track this and double-checked my figures across two separate discharge cycles. The weak cell is also drooping earlier under load — sitting at around 3.2V when the others are still above 3.28V at a 40A draw. Not catastrophic, but it means my BMS (Daly 100A) is going to be babysitting that one cell constantly.

Has anyone seen similar out of the box with Fogstar recently? Previous batch I used in the static caravan two years ago was much tighter — maybe ±5Ah across the set. Wondering whether it's worth going back to them or just running a few more full cycles to see if the weak cell comes up. Some people reckon LiFePO4 needs 5–10 cycles before it settles properly. Curious what others have experienced.

@OldSparky18 welcome to the forum — genuinely great to see someone approaching a first LiFePO4 build with this level of methodical thinking, it makes troubleshooting so much easier.

On the inconsistency question — this is fairly normal behaviour with prismatic cells straight from the factory. Internal resistance variance and differing states of self-discharge during shipping both contribute. What matters more is capacity after a few proper charge/discharge cycles once they're in parallel or series-balanced configuration.

A few things worth checking:

• Resting voltage vs actual capacity — 3.65V resting doesn't guarantee equal charge
• IR readings — have you measured internal resistance per cell? Significant divergence (>0.1mΩ spread) is worth flagging to Fogstar
• Cycle them — I found my cells converged noticeably after 3–4 full cycles

Fogstar's customer support has been responsive in my experience if you do have a genuinely weak cell.

@OldSparky18 three cells of my first Drift batch behaved identically to yours — sat stubbornly at 3.63V after resting whilst the fourth crept up to 3.65V on its own. Drove me mad for a week.

What actually sorted it was two full cycles at a gentle 0.1C before I even thought about final balancing. Fresh LiFePO4 cells sometimes need that initial conditioning before their true capacity stabilises — the chemistry's essentially waking up.

Also worth checking your charger's absorption time. I was using a Victron IP22 and had the absorption cutoff set too short, so cells looked full but weren't quite there.

After cycling, my four drifts settled remarkably consistently — within about 2Ah of each other across the whole pack. Fogstar's QC isn't perfect but I've found their cells genuinely solid once they've had a proper run-in.

@OldSparky18 worth checking whether those lower-capacity cells are actually underperforming or just haven't been through enough charge cycles yet. LiFePO4 cells often need three or four full cycles before they settle into their true capacity — the chemistry essentially needs a bit of a workout to properly activate. I'd run the whole pack through a few complete cycles before drawing any conclusions. Also, when you did your capacity test, what discharge rate were you using? A higher C-rate can make cells appear weaker than they actually are, particularly fresh out of the box. If after five cycles you're still seeing significant variance between cells, then I'd get onto Fogstar directly — their customer service has generally been decent from what I've seen discussed here.

@OldSparky18 ran into exactly this with my shepherd's hut build last spring — two cells from the same Fogstar batch showed noticeably lower capacity on first cycle. Put them through three full charge/discharge cycles using a decent load (I used a 12V 100W halogen temporarily) and they came into line with the others by cycle four. LiFePO4 cells often need a few conditioning cycles before the chemistry fully activates, particularly if they've been sitting in a warehouse for a while. Worth noting your ambient temperature too — anything below about 10°C will suppress apparent capacity significantly. What's your BMS setup? If you're running a Daly or JK, the cell-level voltage logging will show you whether the low-reading cells are genuinely lagging or just measuring slightly differently due to internal resistance variance.