Victron 200Ah lithium keeping cells out of balance after full charge — BMS fault or something else?

Had this exact thing on my garden office setup last summer. Turned out the top balancing wasn't finishing properly because the absorb stage was cutting out too early — Victron MPPT was set to a slightly low absorption voltage (think it was 14.1V when it wanted 14.4V for proper top balance).

Worth checking:

• Absorption voltage — should be 14.2–14.6V depending on your specific cells
• Absorption time — if it's too short the BMS never gets chance to bleed off the high cells
• Tail current setting — if charge terminates before current drops low enough, same problem

Also, is this the built-in Victron lithium battery or a third-party with a Victron-compatible BMS? Makes a difference. The proper Victron Smart Lithium uses DVCC over VE.Bus/VE.Direct and handles a lot of this automatically — but only if DVCC is actually enabled in VenusOS/CCGX etc.

If it's a budget pack (no shame, I've got Fogstar in the van), the BMS might just be a bit rubbish at passive balancing. They mostly only balance at the top anyway and some are painfully slow — like 50mA slow.

What's the cell delta showing at full charge? If it's under 50mV I honestly wouldn't lose sleep. Over 100mV consistently and something's actually wrong.

Anyone else seen this after a period of partial state-of-charge cycling? Curious whether it's more common when the pack hasn't seen a proper full charge in a while.

@PeakVanLifer — yes, and the sneaky culprit is often the tail current setting on the MPPT rather than absorb time itself. If tail current is too high (say 4-5% rather than 1-2% of capacity), the charger declares "job done" before the cells have had chance to properly top-balance.

Worth checking in VictronConnect:

• Tail current: drop to 1% of battery capacity
• Absorb voltage: confirm you're hitting 28.8V (for 24V system) or 57.6V for 48V — not some conservative figure the installer left in
• Repeated absorption: enable it if using a Victron SmartSolar

My Fogstar 200Ah cells were doing exactly this last winter. Dropped tail current to 1% and the imbalance sorted itself within three charge cycles. BMS was blameless throughout — poor thing was getting the blame unfairly!

@TracyAllen is absolutely right about tail current, and it's worth being specific about why it matters for top balancing.

When the MPPT exits absorb early due to tail current being met, the cells that were slightly high never get the opportunity to bleed back into equilibrium via the BMS's passive balancing circuit — which only activates meaningfully at the top of the charge curve, typically above 3.4V/cell.

On my Fogstar 200Ah setup I found the default 1A tail current was far too aggressive. Bumped it to 4A (roughly 2% of capacity, which is the commonly cited figure for LiFePO4) and the pack came into balance within a couple of cycles.

Check your VictronConnect app under the MPPT's expert settings — the tail current threshold is buried there. Also verify your absorb voltage is hitting 14.2V minimum, not drifting low due to temperature compensation overcorrecting.

@TracyAllen and @MasterCamper have nailed the tail current angle, so I'll add the next diagnostic step: once you've sorted the tail current, actually watch your cell voltages during that final absorb phase using something like VictronConnect or Venus OS. What you're looking for is whether the BMS is quietly throttling charge current before the cells get a proper chance to top-balance — this is classic behaviour with cheaper integrated BMS units that have tight overvoltage protection per-cell.

On my cabin setup with Fogstar Drift cells, I had to nudge the absorb voltage up fractionally (3.55V/cell rather than 3.50V) to give the balancer enough headroom to actually do its job. The BMS needs that voltage differential to drive balancing current. Without it, you're just parking at a plateau and wondering why cells drift further apart every cycle.

@PeakVanLifer once you've worked through what the others have suggested, it's also worth having a look at your BMS balance settings directly if you can access them. Some BMS units have a balance start voltage threshold — if it's set too low relative to your actual absorption voltage, balancing kicks in too early and never gets the chance to properly equalise the top end of the cells. I've seen this on a couple of Victron setups where the BMS was third-party. Worth checking whether balancing is configured to only activate above say 3.45V per cell, so it's actually working during that final absorb phase rather than wasting capacity earlier in the charge cycle.