Anyone else had issues balancing a secondhand LiFePO4 pack after sitting idle?

Picked up a 200Ah 24V LiFePO4 bank off eBay last month — eight 3.2V 280Ah prismatic cells in 8S configuration with a Daly 100A BMS. Bloke said it had been sat in his garage for about 18 months doing nothing. Paid £180 for the lot which seemed decent enough.

Problem is, after a couple of charge cycles I'm getting massive voltage spread across the cells. Top of charge three of them are sitting at 3.65V whilst two others are barely hitting 3.41V. The Daly is throwing an overvoltage fault and cutting out before the pack is anywhere near full. I've tried doing a slow top-balance at 3.65V per cell with a bench power supply on each one individually, which brought things closer, but after one full cycle it drifts apart again.

I'm wondering if those two lagging cells have actually lost capacity, or whether it's just a calibration/balance issue that'll sort itself out over a few more cycles. Has anyone used an active balancer alongside a Daly on a second hand pack like this? Seen some cheap JK Active Balancers on AliExpress for around £35 and wondering if that'd be worth trying before I write off the weak cells entirely.

@Gibbo39 been exactly there with a batch of EVE cells I rescued last year — sat idle does horrible things to cell balance, they drift all over the shop.

What worked for me was top balancing manually before trusting the Daly to sort it. Charge each cell individually to 3.65V with a bench power supply, then assemble and let the BMS do a full cycle or two. Takes patience but the cells usually wake up properly.

Worth checking your weakest cell voltage under load too — idle drift can mask a genuinely degraded cell that'll drag the whole pack down once you start pulling real amps.

Daly BMS units aren't the quickest balancers either, the passive balancing current is tiny. If the drift persists after a few cycles, a dedicated active balancer board makes a noticeable difference.

Great topic @Gibbo39. One thing worth trying before you give up on any stubborn cells — do a very slow top balance at 3.65V per cell using a bench power supply, keeping current down to around 1-2A. Cells that have sat idle for an extended period often just need a gentle wake-up rather than the aggressive charge a BMS might throw at them initially. I'd also recommend temporarily bypassing the Daly and balancing each cell individually so you can see exactly which ones are lagging. The Daly's passive balancer is notoriously slow at around 60mA, so if your cells are significantly out, it'll take an age to sort itself out naturally. Once you've got them within about 20mV of each other, the BMS should cope fine from there.

My Fogstar cells threw a similar wobbly after a winter of neglect — turns out one cell was so far out it was practically sending a postcard from a different voltage altogether.

@Gibbo39 worth connecting each cell individually to a bench power supply and logging the resting voltage after a 24-hour soak — not immediately after charging. Cells that have been idle that long often mask self-discharge issues until you let them settle properly.

On the Daly BMS specifically: the balancing current is notoriously weak (often only 30–40mA passive). If your cell spread is more than ~100mV, the Daly simply won't close that gap in any reasonable timeframe under normal cycling. Consider bypassing it temporarily and doing a proper top-balance externally — I used a bench supply set to 3.65V per cell with a 1Ω resistor in series to trickle them level, took about 48 hours on my 280Ah EVE cells.

Also check internal resistance with a proper meter — anything above 0.5mΩ difference between cells after balancing usually indicates a genuinely degraded cell worth investigating further.

@Gibbo39 I've been down this exact rabbit hole with a secondhand pack. One thing nobody's mentioned yet — before you do anything else, check the internal resistance of each cell individually if you've got a decent capacity tester or even a basic YR1035. Cells that have been sat idle for 18 months can develop quite different internal resistance profiles, and a cell that looks balanced by voltage alone can still be a duffer that'll drag the whole pack down under load. I picked up a supposedly matched 280Ah set last year and two cells had IR nearly three times the others — voltages looked fine on the surface though. Swapping or isolating the high-resistance cells made a massive difference before I even started proper balancing. Worth ruling out before you invest too much time on the BMS side of things.