Anyone else had grief with JK BMS dropping cells under load? Possible balancing issue

I've been running a 280Ah 16-cell LiFePO4 pack in my shed setup for about eight months now — four 16S packs wired in parallel feeding a Victron Multiplus 2000. Generally it's been brilliant, but lately I've noticed one cell group (cell 4, if the JK app is to be believed) dropping to around 2.9V under any decent load, say 800W+, while the others sit comfortably above 3.1V. As soon as the load drops off, it bounces straight back up within a few seconds.

The JK BMS is the 200A active balancer model — firmware is up to date as far as I can tell (v11.26). Balancing kicks in above 3.4V per cell, which is fine for top balancing, but I'm wondering if that threshold is too high to catch a lagging cell during discharge. I've tried lowering the balance start voltage in the app but honestly I'm not confident I've got the settings right.

My gut says it's either a duff cell that needs replacing, or possibly a dodgy connection on that particular bus bar — I torqued everything to 4Nm when I built it but maybe I should recheck. Has anyone seen this kind of voltage sag on a single cell and managed to diagnose whether it's a cell issue versus a high-resistance connection? Keen to hear how you went about it before I start pulling the pack apart.

Hey @GlenDixon, classic JK symptom that. Before assuming it's a balancing issue, I'd check your cell internal resistance readings in the JK app — if one or two cells are creeping up compared to the others, that's often what causes the voltage sag under load rather than pure balance drift.

Also worth checking your busbar connections with a proper torque wrench. Loose connections can masquerade as weak cells when you're pulling any decent current.

What's your balance start voltage set to? I had mine too low initially and the balancer was barely doing anything meaningful. Bumping it to 3.40V made a noticeable difference over a few charge cycles.

Eight months is also around the time you'd expect any manufacturing variation between cells to start becoming more visible. Might be worth doing a top balance if you haven't already.