Victron BMS keeping my lithium cells at different SOC — anyone else had this?

Been having a similar head-scratcher on my boat recently. Running a 4-cell 280Ah LiFePO4 bank (Fogstar cells) with a Victron Smart BMS, and I noticed two of my cells sitting noticeably higher than the others when checking through the Victron Connect app — we're talking maybe 50-60mV difference at rest.

From what I've read, is this a balancing issue or could it point to a genuinely weaker cell? I'm not entirely sure how to tell the difference, which is partly why I'm posting.

A few questions for anyone who's been through this:

• Does the Victron BMS do passive balancing only, and is that sufficient for a bank that's seen a couple of years of use?
• Would it be worth running a full top-balance on the cells outside the BMS to reset things?
• Could this be causing my usable capacity to be lower than it should be, because the BMS is cutting off based on that one high cell?

That last point feels like it might explain why my fridge seems to be pulling the bank down faster than expected lately — though that could also just be a dodgy connection somewhere, I suppose.

Anyone running Victron BMS with older LiFePO4 cells had similar drift? Curious whether this sorted itself out over time or whether it needed active intervention. The shepherd's hut I'm building will have a similar Victron setup, so keen to understand this properly before I replicate any mistakes! 😄

@CornishBoater oh mate, I had exactly this with my static caravan setup last summer. Four Fogstar cells, Victron BMS, same story — two cells racing ahead while the others lagged behind like they couldn't be bothered.

Turned out my cells just needed a proper top-balance before the BMS took over. I'd assumed they came balanced from the supplier. They did not.

What I did:

• Disconnected everything
• Charged each cell individually to 3.65v with a bench PSU
• Let them sit overnight, topped up stragglers
• Then reconnected the BMS

After that the Victron balanced beautifully during the absorption tail. Takes a few charge cycles to fully settle mind you — don't panic if it's still slightly off after the first one.

What does your cell voltage spread actually look like at top of charge? That'd tell us a lot.

@CornishBoater this is something I've been watching closely on my narrowboat setup. Have you checked whether your absorption voltage is set correctly in the Victron app? I found my cells were drifting apart because the charger was cutting off before the BMS had chance to properly top-balance them.

Also worth asking — how old are the cells? Mine started showing similar behaviour after about 18 months of heavy cycling. Sometimes it's just a sign that one cell has slightly higher internal resistance and needs a manual top-balance session with a bench power supply to bring everything back in line.

What does your cell voltage spread look like at rest vs under load? That difference can tell you a lot about whether it's a balancing issue or something more concerning with a specific cell.

@CornishBoater worth checking your cell interconnect torque first — loose busbar connections cause resistance imbalance which shows up exactly like this. Had it on my shepherd's hut build with EVE cells, two cells looked perpetually behind until I went round with a torque wrench and found one joint barely finger-tight.

Also — are the "high" cells always the same ones, or does it rotate? If it's always the same cells sitting high, I'd suspect those specific cells have slightly lower capacity than rated. Fogstar's QC is generally decent but cell matching isn't perfect batch to batch.

The Victron BMS passive balancing is pretty weak at anything other than top-of-charge anyway. It won't rescue a genuine capacity mismatch mid-cycle.

@CornishBoater worth separating two distinct problems here before chasing your tail. Cell imbalance at top of charge is usually a capacity mismatch — one cell hits 3.65V early and the BMS backs off before the others are full. Cell imbalance mid-SOC points more toward internal resistance differences.

On my narrowboat bank (280Ah EVE cells, Victron SmartShunt + Cerbo), I found running a proper top-balance cycle with a bench power supply at 3.65V per cell individually — holding until current drops below ~100mA — fixed persistent drift completely. Takes hours but worth doing once properly.

@ExPostie is right about busbar torque too. I'd check that first as it costs nothing. Fogstar cells specifically seem to ship slightly out of balance in my experience, so don't assume the BMS is faulty before ruling out the basics.

What's the actual voltage spread you're seeing, and at what SOC?

@CornishBoater had almost identical with my Fogstar cells last summer. Worth checking whether the imbalance is consistent or shifts around between cycles — if it's always the same two cells sitting high, that points more toward capacity variation between cells rather than a BMS or connection issue. Fogstar cells are generally decent but there's always some spread from the factory.

One thing nobody's mentioned yet — have you done a proper top balance on the cells individually before connecting the BMS? If they went in slightly mismatched from the start, the BMS will struggle to compensate long-term. A decent bench charger on each cell individually can make a surprising difference and often sorts it completely. Only takes an afternoon.

What voltage differential are we actually talking here? A few millivolts at top of charge is fairly normal.