Victron BMS keeps throwing a low cell alarm but voltages look fine on the app

Had exactly this with my Fogstar Drift 200Ah last winter. Turned out one cell was marginally out of balance — looked totally fine on the main dashboard but when I drilled into the individual cell voltages in VictronConnect it was sitting about 40mV lower than the others under load. Wouldn't flag it at rest but the moment I put the inverter under any real demand it'd dip and trigger the alarm.

Few things worth checking:

• Cell delta under load — not just at rest. That's the key one
• Firmware on the BMS itself — Victron pushed an update a while back that changed some threshold sensitivities
• Loose cell interconnects. Sounds daft but a slightly loose busbar can cause a false voltage drop reading

Also worth looking at your temperature sensor placement if you're running one. Mine was giving slightly skewed readings because it was sat too close to one end of the pack.

If the cell delta is genuinely small (under 20mV at rest) and it's only flagging under load, it might just need a proper top balance. I did mine with a bench power supply at 3.65V per cell and it sorted it completely.

What capacity pack are you running and how old is it? That'd help narrow it down. Others on here will have seen this too — @BatteryBodger and a few of the LiFePO4 crowd seem to know these Victron quirks inside out.

Classic case of the app showing you the average and letting you believe everything's rosy — like checking your bank balance but forgetting about the direct debits.

@RetiredEngineer86 nailed it, but worth adding: cold temperatures make cell imbalance dramatically worse, so if this started last winter that's almost certainly your culprit rather than a dodgy cell per se. My narrowboat setup does the same every November until the battery bank warms up a bit.

Also worth checking your balance current in the Victron BMS settings — if it's set too low it'll struggle to keep up, especially on a 200Ah pack. Bump it up and give it a few full charge cycles to sort itself out before assuming the Fogstar's gone duff.

@RetiredEngineer86 nailed the diagnosis, but worth adding — that marginal cell will likely drift further out of balance over time, especially once temperatures drop. Cold genuinely hammers internal resistance asymmetry between cells.

What I'd do: enable the detailed cell voltage history logging in VictronConnect and watch it over a full charge/discharge cycle. The rogue cell usually reveals itself dramatically at the top or bottom of the curve — fine through the middle, then suddenly sprints ahead or lags behind everyone else like that one colleague who disappears five minutes before the meeting ends.

Also check your absorption voltage setting. If it's slightly low, the BMS never gets a proper balancing window. Bumping it 0.05V sometimes cures what looks like a hardware problem entirely. Fogstar Drifts are decent cells but they do appreciate a proper top-balance periodically.

@TracyAllen is right about the drift — cold temps absolutely accelerate it too, which is why winter caught you out.

Worth checking your cell deviation threshold in VictronConnect; default's often set looser than it should be for LiFePO4. I had mine alarming on a 0.08V spread that looked "fine" until I actually watched it under load — that's when a weak cell shows its true colours.

Run a proper top-balance if you haven't already; chuck it on a decent CC/CV charger cell by cell if your BMS won't do it. Fogstar cells are generally decent quality but even matched cells drift after a season.

Also — and I cannot stress this enough — the Victron app dashboard is lying to you by omission. Always dig into the raw cell data. Always.

@RetiredEngineer86 glad you got to the bottom of it! One thing worth adding that nobody's mentioned yet — have you tried running a manual balance cycle with the cells at a higher state of charge? The Victron BMS balances passively and only kicks in properly above a certain voltage threshold (usually around 3.4V per cell). If you're regularly stopping short of a full charge, the balancer never gets a proper chance to do its job. Try leaving it on a slow top-up charge for a few hours once it hits absorption — sometimes that's all it needs to bring a drifting cell back into line. Worked a treat for me with a similar issue on my Battle Born setup. Worth ruling out before you start worrying about cell replacement anyway! 🙂

@CamperRachel don't leave us on a cliffhanger like that 😂 — but yeah, beyond what everyone's already covered, I'd also check your Victron Cerbo (if you've got one) is actually logging the cell-level data rather than just pack-level, because mine was cheerfully averaging everything out like a dishonest school report until I dug into the DVCC settings. Garden office setup taught me that lesson the hard way — Fogstar cells looked immaculate on the dashboard while one quietly threw a wobbler all winter.