EG4 12v 400AH battery cell overvoltage issue

Had something similar crop up on my van build last summer — one cell kept throwing overvoltage faults even though the pack overall was sitting at a reasonable state of charge.

Turned out my Victron MPPT wasn't quite dialled in properly and was pushing absorption voltage slightly too high, which was stressing that one cell more than the others. A few tweaks to the charge profile sorted it, but it did make me wonder whether the cells were already slightly mismatched out of the box.

A couple of questions for anyone who's dealt with this:

• Did you top-balance the cells before first use? I skipped this step initially (impatient, as always) and I reckon it contributed to my issues
• What's your absorption voltage set to? I've seen people running EG4 cells at different upper limits and I'm curious what's working
• Is the BMS logging the fault history? Worth pulling the data if you can — helps narrow down whether it's a recurring cell or random

Also wondering if this is a known weak point with EG4 specifically or just the usual lithium lottery. My Fogstar cells on the garden office build have been rock solid by comparison, though that's a different chemistry and form factor entirely.

Has anyone contacted EG4 support directly about this? Curious whether they're helpful or just point you at the manual. Would be good to know before deciding whether to push for a replacement cell.

@ExJoiner19 curious what your cell voltage spread looked like before the fault triggered — was it a gradual drift or did one cell just shoot up suddenly?

I've got a Fogstar Drift 200Ah in my static caravan setup and I've been keeping an eye on individual cell voltages through the Victron app. Mine shows a fairly consistent spread but I've heard these LiFePO4 packs can mask imbalance issues until you hit the top of charge where it all becomes obvious.

Did you end up doing a manual top balance, or did the BMS sort itself out? Also wondering whether your charger absorption voltage was set too high — I've seen people running 14.6v on setups that probably shouldn't go above 14.4v.

@ExJoiner19 classic passive balancing limitation tbh. If your charger was pushing any meaningful current when that fault triggered, the weak cell just couldn't keep up.

Worth checking whether the BMS is actually cutting at cell level or pack level — some cheaper units are a bit vague about that in the logs.

On the EG4 specifically, I'd be looking hard at the internal resistance on that outlier cell. Grab a proper meter reading under load, not just resting voltage. Resting it'll look fine, under load it'll tell a different story.

Had a similar drift on a Fogstar pack in my static — one cell was consistently 20–30mV above the rest during absorption. Ended up being a dodgy cell from factory. Warranty process was painless enough.

@ExJoiner19 worth checking whether your Victron MPPT absorption voltage is set too close to the BMS overvoltage threshold — classic rookie trap. If absorption is 14.4V and your BMS trips at 14.6V, any mild cell imbalance can push a weak cell over the edge before balancing has had a chance to do its job.

What I did on my motorhome build was drop absorption to 14.2V and extend the duration slightly. Gives the passive balancer time to bleed down the high cells before you're dancing on that threshold.

Also — check your cabling resistance. Uneven resistance between cells and the BMS sense wires gives you phantom voltage readings. Caught me out completely last October with my Fogstar cells.

@TelHall sudden drift rather than gradual usually points to a dodgy sense wire or connection rather than a genuinely weak cell, in my experience.

@LiFePO4Nerd raises something I've been wondering about for my garden office setup — how much of a margin do people typically leave between the MPPT absorption ceiling and the BMS cutoff threshold?

On my Fogstar 12v 200Ah I've got absorption at 14.2v and the BMS trips at 14.6v, so roughly 0.4v headroom. Is that considered sufficient, or should I be pushing that gap wider?

Also, @ExJoiner19 — did you identify whether the rogue cell was a pre-existing weak cell from the factory, or something that developed over time? Wondering whether periodic individual cell voltage logging would catch this kind of drift early before it actually triggers a fault.