Anyone else had their JK BMS trip on high cell deviation during bulk charge — or is it just me?

Finally got my 280Ah EVE LiFePO4 pack wired up properly in the van — 4S, so 12V nominal — and it's been running nicely for a couple of weeks. But yesterday I had the solar hammering in (proper rare Scottish sunshine, about 420W through the Victron MPPT) and the JK BMS-24S cut the pack out with a "cell voltage difference" alarm. Delta was showing around 85mV at the time, which triggered the 80mV protection I'd set.

Thing is, these are brand new EVE 280Ah Grade A cells and I've only done about 8 cycles on them. I've read they can be a bit unbalanced early on and need time to settle, but I wasn't expecting it to be this dramatic. The JK's active balancer was running flat out but clearly couldn't keep up during the bulk surge. Top-of-charge voltages were ranging from about 3.42V on the laggard cell up to 3.54V on the highest — so not a dangerous difference, just enough to pop the protection threshold.

Has anyone dealt with this on a fresh pack? I'm wondering whether to bump the deviation protection up to 120mV temporarily while the cells top-balance, or whether I should just do a proper bench top-balance with a bench PSU before relying on the BMS balancer alone. Also curious what balancing current the JK is actually delivering in practice — mine's the 2A active balancer version but I'm not sure I'm seeing that in real life.

@DODNerd — classic top-of-charge drama, that. Had exactly this with my shepherd's hut build last spring. Four EVE 280Ah cells, JK BMS, Victron MPPT pushing hard on a sunny April morning.

What sorted it for me was backing the absorption voltage down slightly — I was running 14.6V and dropped to 14.4V. The cells just weren't balanced tightly enough yet to handle that aggressive top end without one racing ahead.

Also worth checking your cell internal resistance readings in the JK app. One of mine was sitting noticeably higher than the others — turned out a busbar connection was only finger-tight. Proper torque on those M6 bolts made a significant difference overnight.

Give it ten or fifteen full cycles before you panic. New cells often settle their balance naturally once they've been through a few proper charge-discharge runs.

@DODNerd classic JK moment — mine did exactly the same until I bumped the cell deviation threshold from the default 300mV down to... wait, no, up... honestly just poke around in the JK app until it stops being dramatic about it 😂

Real fix though: your EVE cells probably need a proper top-balance before the BMS stops throwing a wobble every time the Victron MPPT actually does its job in actual British sunshine (both of you were equally surprised, I'd wager).

Stick a bench charger on each cell individually, get them all sitting pretty at 3.65V, then reassemble — the deviation alarm should calm right down once they're matched properly.

Seen this exact scenario on my boat build — sunny day, charger pushing hard, and the BMS throws a wobble right when you least expect it.

What sorted it for me was slowing the charge rate down during bulk. I was running a Victron SmartSolar and dropping the absorption current limit made a noticeable difference — the cells got more time to stay in step with each other rather than racing ahead unevenly.

Worth also checking your cell interconnect torque. Sounds boring, but I had one slightly loose busbar causing one cell to read artificially high under load, which absolutely triggered that deviation alarm repeatedly until I caught it.

@DODNerd — before you chase BMS settings down a rabbit hole, grab a multimeter and check each cell terminal voltage directly during charge. Sometimes the fix is embarrassingly physical rather than digital.

@DODNerd worth checking whether your cells were properly top-balanced before you built the pack. If they weren't sitting at matching voltages before assembly, you'll chase this problem forever — the deviation just gets worse as you approach full. I had similar grief with my emergency backup setup before I pulled everything apart and did a proper top-balance at 3.65V per cell using a bench PSU. Took a weekend but sorted it completely. Also — what's your charge current set to? Pushing EVE 280Ah cells too hard accelerates the imbalance near the top. Dropping the bulk current a bit can buy your passive balancer enough time to keep up.

@DODNerd one thing nobody's mentioned yet — have a look at your bus bar connections and cell terminal torque. I had almost identical behaviour on my 304Ah pack and it turned out one terminal was slightly undertorqued, maybe 2Nm down on where it should've been. Perfectly fine under normal loads but when the solar was really piling amps in, that marginal resistance was enough to skew the readings on that cell and trigger the deviation alarm. Worth going round with a torque wrench before you start tweaking BMS parameters — no point chasing a software fix if it's actually a mechanical issue. EVE's spec is 4Nm on those M6 terminals if I recall correctly, though double-check your cell datasheet.

@DODNerd had exactly this on my static van build — JK BMS, same EVE 280Ah cells, 4S. What I eventually traced it down to was the BMS protection thresholds being too tight out of the box. The default high-cell-deviation trigger on mine was set to something like 30mV, which sounds reasonable until you're pushing 40A+ through cells that haven't fully settled yet.

Worth logging your cell voltages during bulk using the JK app — the Bluetooth monitoring is actually decent. If you're seeing one cell consistently pulling ahead rather than random shuffling, that points more toward @MoorCamper's top-balance point. If it's different cells each time, I'd lean toward either threshold config or what @HazelChild mentioned.

Temporarily widening the deviation threshold to 60-80mV whilst you investigate isn't dangerous — just removes the false trips so you can gather proper data first.

Great shout from @HazelChild on the connections — I'd add that it's worth checking the actual contact surface area on those bus bars too, not just torque. Thin or poorly fitted bars create resistance that only shows up under real load.

Also worth looking at your charge current relative to cell capacity. If your solar controller is pushing hard and the JK is set to a tight cell deviation threshold (default is often quite low), you'll trip it even with a healthy pack. Try bumping the threshold up slightly in the JK app whilst you investigate — just as a diagnostic step, not a permanent fix.

@DODNerd worth checking whether your JK is set to balance during charge only or always on — mine was defaulting to charge-only and barely touched the cells until they were already nearly full, by which point the deviation had already triggered the protection. Switched it to always-on balancing and the drama disappeared overnight. The passive balancing on these JK units is pretty feeble anyway (75mA if you're lucky), so the earlier it starts chipping away, the better.

@DODNerd this is a really common one with EVE cells during that first big solar input. One thing nobody's mentioned yet — have you checked your cell deviation trigger threshold in the JK app? Factory default is often set quite tight, sometimes as low as 30mV. If your cells aren't fully top-balanced yet, that'll trip almost every time bulk charge kicks in hard. Try nudging it up temporarily to 60-80mV while you run a few proper top-balance cycles. Once the cells settle in and track each other properly, you can tighten it back down. Took my pack about three or four good cycles before deviation calmed right down.