Victron BMV says 100% but cells are miles apart — BMS issue?

Had this exact saga on my static last summer — Victron BMV was sat there grinning at me showing 100% while my Fogstar cells were doing completely different things entirely.

Turned out my BMS wasn't actually balancing properly at top of charge, it was just claiming it was. Classic case of the BMV trusting the voltage tail-off and calling it done, when in reality a couple of cells were lagging behind like they'd had one too many.

Few things worth checking:

• Cell drift — chuck a proper cell-level monitor on there (I use a Heltec between the BMS and the battery terminals) and see what the spread actually looks like at alleged 100%
• BMS passive vs active balancing — passive balancers at low current are basically just warming the air, especially if your cells have drifted more than ~50mV apart
• Victron sync settings — your BMV might be syncing to 100% based on voltage alone rather than tail current, worth revisiting those parameters in VictronConnect

On my boat I ended up doing a manual top-balance on the cells individually before reconnecting — took an afternoon but sorted it right out.

What BMS are you running? Some of the cheaper units are basically just protection boards pretending to be smart. If it's a JK or similar you can usually pull the individual cell voltages via Bluetooth and catch it red-handed.

Anyone else had grief with BMV sync parameters causing this kind of phantom 100%? Feels like it comes up every few months on here.

@DazHenderson77 classic case of the BMV measuring pack voltage while the BMS is quietly having a meltdown trying to balance cells that are already miles apart — your Fogstar cells aren't the villain here, it's almost certainly a passive balancer that only kicks in at top-of-charge and can't keep up if one cell is drifting badly.

Check your BMS balance current rating — anything under 100mA on a large pack is basically useless, like trying to drain a bath with a teaspoon. Worth logging individual cell voltages via the BMS app (if it has one) to find your rogue cell, then do a manual top-balance with a bench power supply before trusting any state-of-charge reading again.

Active balancing BMS upgrade transformed my van setup — never looked back.

@DazHenderson77 yeah the BMV doesn't care what individual cells are doing, it just sees pack voltage and does its maths — could be showing 100% while one cell is tanking and another is railing high.

Get yourself into the BMS app and look at individual cell voltages. If you're seeing more than about 50-100mV spread at top of charge you've got a balancing problem, not really a BMV problem.

Could be:

• passive balancing just too slow/weak
• a genuinely weak cell
• dodgy cell connections

My Fogstar setup had similar early on — turned out one of my busbars wasn't torqued properly so one cell was reading artificially high resistance. Sorted that and the spread tightened right up.

BMV is only ever as good as what you tell it about your battery.

@DazHenderson77 the key thing nobody's mentioned yet is why the cells drift apart in the first place — usually it's either mismatched capacity between cells from the outset, or differential self-discharge rates. Your BMV will happily declare 100% the moment pack voltage hits your absorption setpoint, completely blind to whether one rogue cell has already hit 3.65V whilst another is still sitting at 3.45V.

Worth checking whether your BMS is actually passive balancing (bleeds energy from high cells) or whether it's doing anything at all — some cheaper units have balancing that only activates above a threshold voltage.

On my narrowboat I run a Daly BMS alongside the BMV and cross-reference cell deltas regularly. If your delta is routinely >50mV at top-of-charge, those cells need a proper top-balance cycle before anything else will fix it properly.

@DevonDweller raises the critical point — cell drift origin matters enormously for diagnosis.

Worth adding: if your BMS is passive balancing only (most budget units are), it can only dissipate energy from high cells as heat. It cannot pull a low cell up. So if you've got a genuinely weak cell with higher internal resistance, passive balancing will never fix the underlying problem — it'll just mask the symptom temporarily.

Run a proper capacity test on each cell individually. I did this with my Fogstar 280Ah cells using a bench load — found one cell was only delivering ~240Ah whilst the others hit 278Ah+. That's your culprit right there.

Also check your absorption voltage is actually reaching cells uniformly — poor busbar connections create voltage drop that the BMV never sees but your weak cell absolutely feels.

@DevonDweller makes a fair point about root cause, but I'd add — don't overlook the shunt placement either. Had this exact headache on my shepherd's hut build. BMV shunt was wired before the BMS, so it was never seeing the balancing current at all. Completely blind to what was happening at cell level.

Also worth checking whether your BMS is actually passive balancing or just claiming to. Plenty of cheap units will log a balancing event but the balance current is so feeble (think 30–50mA) it can't keep up with even moderate drift. If you're running Fogstar cells with any decent capacity behind them, that's essentially useless.

A proper active balancer wired in alongside — NEEY do a decent one — made more difference than fiddling with BMS settings ever did on mine.