Anyone else had issues with a JK BMS cutting out under load with LiFePO4 cells?

Been running a 280Ah LiFePO4 bank (4 x 70Ah EVE cells in series) in my campervan for about six months now, paired with a JK BMS — the 200A model. Everything seemed spot on until last week when I started getting random cutouts under heavier load. We're talking the BMS tripping when I fire up my Webasto diesel heater and the 12V compressor fridge at the same time, pulling maybe 30–40A total. Nowhere near the rated limit, so I'm a bit stumped.

I've checked the main cable connections and they look solid — 35mm² cable throughout, lugged and crimped properly. Temps are fine, cells are balanced to within 5mV at rest. The JK app is showing an overcurrent protection trigger but the settings are at 120A with a 300ms delay, so it really shouldn't be firing at those draw levels. I've also got a Victron SmartShunt in the circuit and that's logging peaks of around 38A max, which confirms it's not a genuine overcurrent event.

Has anyone seen the JK BMS behave oddly with inductive loads — motors, pumps, that sort of thing? I'm wondering if the compressor startup spike is somehow confusing the BMS even though the sustained draw is low. Could it be a firmware thing? Mine's on v11.25. Happy to try a firmware update if that's been known to fix similar behaviour, just a bit nervous about bricking it mid-trip.

@NickMason yeah mine did exactly this — scared me half to death the first time, thought I'd somehow managed to brick £400 worth of cells by looking at them wrong.

Turned out my balance wires weren't quite seated properly in the JST connector. One slightly dodgy connection was feeding the BMS garbage voltage readings and it was throwing a wobbly under

@NickMason First thing I'd check is your cell-level voltages under load — if one cell is sagging significantly more than the others, the JK will trigger an undervoltage cutout even if the overall pack voltage looks fine. Classic sign of a weak cell or a bad connection at one of the busbars.

Also worth pulling the JK app and checking your cell undervoltage protection setting. Factory default is sometimes quite conservative (2.8V or even higher on some firmware versions).

Had similar grief with my EVE 280Ah bank — turned out a busbar bolt had worked slightly loose. Torque them down to spec, clean the contact surfaces, and recheck.

What's your discharge cutoff set to in the BMS config, and are you seeing any significant cell delta during the load event? That'll tell us a lot.

@NickMason this is almost certainly a low-voltage cutout on a single weak cell rather than a BMS fault per se. The JK is actually doing its job correctly.

What I'd specifically check: connect via the JK BMS app (Bluetooth) and watch individual cell voltages in real time whilst applying load. You're looking for one cell dropping away from the others — if it hits the under-voltage protection threshold (default is usually 2.8V on the JK) before the others, the BMS will disconnect.

On my EVE 280Ah bank I had one cell with noticeably higher internal resistance — visible immediately under a 100A+ draw. Solved it with a proper top-balance cycle using a bench power supply across each cell individually before reassembly.

Also worth checking your cell interconnect torque — loose busbars cause exactly this kind of voltage sag on one cell.

GrumpyHermit | 847 posts | Joined 3 years ago

@NickMason worth checking your cell interconnect torque as well — loose busbars cause resistance which causes voltage drop under load which triggers exactly what @DownsCamper and @BatteryDaz are describing, but on a cell that's actually perfectly healthy. Had this exact situation with my own EVE 280Ah bank and nearly sent a perfectly good cell back before I thought to check the connections properly with a torque wrench. EVE recommend 4Nm on those terminals, most people under-tighten them. Chuck a thermal camera or even just your hand near the busbars under load — any warmth and that's your culprit. Dead simple fix if so.

Also, which firmware are you running on the JK? Some older versions had slightly aggressive OVP/UVP defaults that benefit from updating.

@NickMason had almost identical with my boat bank — JK 200A, EVE 280Ah cells. Turned out to be the balance wires. One had a dodgy crimp at the JST connector end, causing the BMS to read a phantom low-voltage on that cell and trip the discharge protection.

Worth pulling the balance lead connector off and inspecting every crimp individually. Also check the JK app — under Cell Diagnosis it'll log whether your cutout was OVP, UVP, or OCP. That narrows it down fast without guesswork.

@GrumpyHermit is right about torque too — EVE cells are notorious for needing proper compression or you'll get resistance spikes under load that look like cell sag.

If it keeps happening after all that, the JK 200A has a known firmware bug on earlier batches — worth checking you're on the latest.

@NickMason I had this exact scenario in my shepherd's hut build last year — 4 x EVE 280Ah cells with the JK 100A. What hadn't been mentioned yet: check your BMS protection settings in the JK app. The default overcurrent trip point is often set quite conservatively from the factory, and if you're running an inverter with any meaningful startup surge (kettle, power tools, etc.) it'll nuisance-trip before the cells are even stressed.

Connect via Bluetooth, go into Protection Parameters and look at your Discharge Overcurrent 1 and Overcurrent 2 values. Mine were set at 80A and 120A straight out the box on a 200A unit — seemed daft. Bumping them to something sensible (and adjusting the delay timing) sorted the random cutouts completely.

Worth eliminating before you start suspecting the cells themselves.

JennyWilson | 312 posts | Joined 2 years ago

@NickMason one thing nobody's mentioned yet — have you checked your BMS temperature sensor placement? Mine was doing something similar and it turned out the sensor had shifted away from the cells and was reading ambient temperature instead of actual cell temp. On cold mornings the BMS was convinced the cells were too cold to deliver current and was throttling output aggressively. Repositioning it properly with some thermal paste and kapton tape sorted it completely. Worth ruling out before you go pulling everything apart! What temperatures have you been seeing in the van lately?