Anyone else had grief with JK BMS dropping cells below 2.8V before tripping?

Been running a 280Ah LiFePO4 bank (4S, EVE cells) in my van for about eight months now and I keep having an issue where one cell – cell 3 if it matters – dips to around 2.75V before the JK BMS actually cuts the load. I've got the under-voltage protection set to 2.8V per cell and the release at 3.0V, so it should be tripping earlier than this. Running a JK-B2A8S20P, firmware 10.10.

I've checked the cell-level wiring twice and the balance leads look fine. The odd thing is it doesn't happen under a gentle load – only when I'm pulling hard (1200W inverter running a small kettle). So I'm wondering if it's a combination of voltage sag under load plus some latency in the BMS protection trigger, and whether the trip threshold is being measured at the BMS rather than accounting for that sag.

Has anyone tweaked the protection delay settings to compensate, or is this more likely a drifting cell that needs a proper top-balance? Cell 3 is consistently the first to drop on discharge and the last to reach 3.65V on charge, which probably tells its own story. I've got a Victron SmartSolar 100/20 handling charge if that's relevant.

IanMartin60 | Posts: 847

@OakTel Cell 3 consistently being the weak one is a bit of a red flag – sounds like it may have slightly less capacity than the others, which becomes more obvious as the bank ages. Worth doing a proper top balance with all four cells disconnected and individually charged to 3.65V. That said, your immediate problem is the BMS settings. On the JK app, check your "Cell Undervoltage Protection" value – I'd be surprised if it isn't set higher than 2.75V already, which suggests there might be a firmware glitch causing delayed tripping. Also double-check your "Cell Undervoltage Recovery" isn't set too close to the protection threshold, as that can cause odd behaviour. What firmware version are you running?

DebbieTaylor | Posts: 312

@OakTel Worth checking your cell 3 connections first – a high-resistance joint can cause voltage to sag disproportionately under load and mimic a weak cell even when capacity is fine. Give the busbars a proper torque check.

Also, have a look at your JK BMS settings – specifically the "Cell Undervoltage Protection" and "Cell Undervoltage Recovery" values. If your protection threshold is set too low, it simply won't trip until things get ugly. I'd set protection no lower than 2.85V personally, with recovery around 3.0V.

Eight months in with EVE cells you might also benefit from a proper top-balance if you haven't done one recently – cell drift does creep in over time. What's the delta between your highest and lowest cells at rest?

FormerMechanic14 | Posts: 1,203

@OakTel JK BMS units ship with the low-cell cutoff set at 2.5V from factory – that's your problem right there. 2.75V means it's actually tripping above factory default, which suggests you've manually adjusted it at some point, or you've got a knock-off unit with different firmware.

Pull up the JK app and check your Cell Undervoltage Protection value. I'd argue 2.8V is fine as a trip point, but the release voltage needs to be set sensibly above that or it'll chatter on and off.

@IanMartin60 and @DebbieTaylor have covered cell health and resistance issues – solid points both – so I won't rehash that.

What I will say: grab a proper cell-level logger and check whether cell 3 is lagging during discharge or only at the bottom 5%. That distinction changes the diagnosis entirely.

Taffy42 | Posts: 561

@OakTel My narrowboat JK did the same until I realised the protection voltage and the alarm voltage were set independently — worth double-checking you've actually saved the new cutoff value in the app, because mine cheerfully pretended to accept 2.8V then reverted to factory defaults every time I killed the Bluetooth connection. Classic.

DuctTapeDave62 | Posts: 847

@OakTel One thing nobody's mentioned yet – have you checked whether cell 3 is actually weaker or if it's just appearing weaker due to unbalanced cells? JK's passive balancing only kicks in above a set voltage threshold (default is usually 3.4V or so), meaning if your cells rarely reach that during a charge cycle, they'll drift further apart over time and your weakest cell will always cop it first. Try a slow, full top-balance charge with all four cells connected and see if the spread tightens up afterwards. Also worth logging the cell delta over a full discharge cycle – if cell 3 is consistently 50-100mV below the others by mid-discharge, you've likely got a capacity mismatch rather than a settings or connection issue. That changes what you do next considerably.

LDVBuild | Posts: 312

@OakTel Worth checking your cell 3 busbars and compression while you're in there sorting the BMS settings. A high-resistance connection can cause that cell to appear to sag faster under load than the others – it's not always a weak cell, sometimes it's just a dodgy contact fooling the BMS into thinking it's worse than it is. I had almost identical behaviour on my own 4S bank and it turned out one busbar bolt had barely finger-tightened itself loose over a few weeks of van vibration. Torqued everything back to spec and the cell voltage spread tightened right up. If the sag only happens under heavier loads rather than sitting at rest, that'd point toward a resistance issue rather than actual cell imbalance. A proper milliohm reading across each joint would tell you quickly.

RhysPalmer | Posts: 203

@OakTel Something worth adding to what's already been said – have you checked your balance lead connections on cell 3 specifically? I had a very similar symptom on my 4S build and it turned out the balance wire had a slightly dodgy crimp giving a false low reading to the BMS. The BMS was actually tripping based on an inaccurate voltage rather than the cell genuinely being that low. Quick way to verify is measure cell 3 directly with a multimeter while the BMS is reading it and compare the two figures. Surprised me how far out they were.

BreezyCaptain | Posts: 156

@OakTel Worth double-checking your low voltage protection settings in the JK app – there's a "Cell Undervoltage Protection" value and a separate "Cell Undervoltage Recovery" value. If your protection trip is set too low (some units ship with defaults around 2.7-2.8V), the BMS will quite happily let cells sag further than ideal under load before it reacts. I'd set protection no lower than 2.9V personally, with a recovery around 3.1V. Also check your "Protection Delay" isn't set too long – a sluggish response under heavy load makes the sag worse before the trip kicks in.

Doug1981 | Posts: 847

@OakTel The cell divergence you're seeing under load is a classic sign of that one cell having higher internal resistance than the others – worth doing a proper IR test on all four when they're at around 50% SoC. Even if cell 3 reads fine at rest, it'll sag harder under current draw. Eight months of van use with vibration can also cause micro-damage internally that won't show up until you're pulling decent amps. If the IR is noticeably higher than the other three, that cell's likely on its way out and no BMS tweaking will properly fix it long-term.

CotswoldNomad | Posts: 412

@OakTel Cell 3 sounds like it's auditioning for a drama role — "I shall sacrifice myself for the bank!" 🎭

Jokes aside, worth checking your cell-level internal resistance in the JK app. If cell 3 is noticeably higher