Anyone else had issues with a JK BMS dropping cells randomly during high discharge?

Been running a 280Ah LiFePO4 pack in my van for about eight months now — four 3.2V Eve cells in series with a JK BMS (the 200A version). Generally been brilliant, but lately I'm getting a weird fault where one cell apparently drops to 2.8V under load and trips the BMS, even though when I check with my multimeter straight after it's sitting perfectly fine at 3.28V like the others.

It's happening most often when I'm pulling around 120–130A through a 3000W inverter — kettle, microwave, that sort of thing. The BMS app shows cell 3 as the culprit every single time. I've re-torqued all the busbars (0.4Nm as spec'd), checked the sense wire connections, and I can't see anything obviously loose. Cells are about 18 months old and came from a well-regarded group buy through a fellow on another forum.

Wondering if it's a weak cell developing internal resistance, a dodgy sense wire somewhere in the harness, or just a BMS firmware glitch — the unit is on v11.25 and I haven't updated it yet. Has anyone seen this pattern before, or got a reliable way to test which it actually is without pulling the whole pack apart?

Doug1981 | 847 posts

@Mike1997 Had almost identical symptoms on my setup last autumn. Worth checking your cell interconnect busbars first — sounds obvious but a slightly loose bolt can cause enough resistance to fool the BMS into thinking a cell is dropping under load. I torqued mine down thinking they were fine, then actually used a proper torque wrench and found two were significantly under spec.

Also worth pulling the BMS app logs if you haven't already — the JK records cell voltage history which should tell you which cell is flagging and whether it's consistent. If it's always the same cell, you might just have a weak cell rather than a BMS fault.

What's your typical discharge rate when it happens? Mine only showed up above about 80A continuous.

LiFePO4_King | 1,203 posts

@Mike1997 One thing worth ruling out before anything else — have you checked the JK app for your cell voltage differentials under load? The 200A JK has a default cell undervoltage protection set quite conservatively from factory, and if one cell is drifting even slightly out of balance during heavy discharge, it'll trip the BMS before the others catch up.

Also worth having a look at your busbar connections with a proper torque wrench. I had a similar head-scratcher that turned out to be a single M6 bolt that had worked itself loose — barely noticeable by hand but causing enough resistance to create a voltage sag on that cell under load.

What's your typical discharge rate when it drops out? Makes a difference diagnosing whether it's a balancing issue or something more mechanical.

GazJones | 312 posts

@Mike1997 Had a similar head-scratcher on my 4S Eve pack last year. One thing neither @Doug1981 nor @LiFePO4_King have touched on yet — check the actual BMS firmware version in the app. There was a known bug in some earlier JK firmware builds that caused spurious cell undervoltage trips during high current draws, even when the cell was absolutely fine. JK quietly pushed an update that sorted it for most people. Also worth double-checking your protection voltage settings — sometimes they get knocked slightly during an app reconnect. What's your low voltage cutoff currently set to?

Debbie1999 | 156 posts

@Mike1997 Something nobody's mentioned yet — have you checked your balance wire connections at the BMS itself? I had almost identical random cell drops on my JK last spring and spent ages chasing it through the app before realising one of the balance lead JST connectors had a slightly loose pin. Under high discharge the pack vibrates enough that it was causing intermittent contact. Pulled the connector, reseated every pin individually and the problem vanished completely. Worth doing before you go any deeper into settings or firmware. In a van environment especially, vibration does nasty things to connections over time that you'd never see just doing a visual check. Take five minutes with a multimeter checking resistance on each balance lead too whilst you're at it.

Tommo | 847 posts

Worth adding something nobody's touched on yet — the JK 200A has a known quirk where the over-current protection trips temporarily under sharp inductive load spikes, even when your average draw is well within spec. In my Transit build I was seeing exactly this with a compressor fridge kicking in alongside an inverter. The BMS read the surge as a fault condition and dropped out briefly.

Fix that worked for me: add a small capacitor bank across the output terminals (I used a couple of supercaps salvaged from an old UPS). Smooths the inrush considerably.

Also worth checking — what cable cross-section are you running between cells and BMS? Undersized inter-cell links create enough resistance to generate a voltage differential that the JK misreads as a weak cell. I'd want minimum 35mm² for 200A continuous.

What does the JK app show for cell delta voltage at the moment of dropout?

SIE_Electric | 203 posts

@Mike1997 Eight months in and it's already having a mid-life crisis — classic LiFePO4 behaviour. 🔋

Jokes aside, one thing nobody's flagged yet: have you checked your cell interconnect busbars for micro-arcing? Sounds exotic but loose bu

MountainBarry | 412 posts

Had almost identical grief with my cabin pack last winter — turned out to be the cell-level sense wire crimps. Looked fine visually but under load the resistance spiked enough to trick the BMS into thinking a cell was dropping. Pulled every connector, re-crimped with proper ferrules, and the phantom faults vanished completely.

@Debbie1999 is on the right track with balance wires, but I'd specifically focus on the crimp quality rather than just continuity. A multimeter won't always catch a dodgy crimp that only misbehaves under 150A+ draw. Grab some decent ferrule crimpers — transformed the reliability of my whole setup.

MistyRigger | 412 posts

@Mike1997 One thing worth checking that hasn't been mentioned — have a look at your negative busbar connections rather than just the cell terminals. I had almost identical symptoms on my 230Ah pack and spent weeks chasing ghost faults before finding a slightly loose negative busbar bolt that was causing momentary voltage drop under load. The BMS was reading it as a cell dropping out. Give everything a proper torque check with a calibrated wrench, not just finger-tight. Copper busbars can also develop oxidation underneath that isn't visible on the surface. Worth pulling them off and giving the contact faces a clean.