Victron BMS keeping my lithium cells top balanced or should I sort it manually?

Been running a Fogstar Drift 100Ah in my garden office setup for about eight months now and honestly the Victron BMS has handled top balancing pretty well on its own. Let it do a proper absorption cycle every week or so and the cells stay reasonably tight.

That said, I did manually top balance before first use and I reckon that made a big difference to how well the BMS can keep on top of things ongoing. Starting from a good baseline matters.

A few things worth considering:

• Cell drift — if you're seeing more than ~20mV spread at the top, the BMS is working harder than it should
• Absorption time — are you letting it actually finish, or cutting it short?
• Temperature — my office drops quite low in winter and I noticed balancing got sloppier around 5°C

The Victron BMS balancer is passive and relatively low current (50mA on most units I think?), so if your cells are badly out it'll take ages to sort itself out. Manual intervention is quicker in that case.

What's your cell spread looking like at full charge? And which BMS exactly — the Smart BMS or one of the VE.Bus units? Makes a difference to what options you've got.

Curious whether others have found the passive balancing enough long-term or ended up adding an active balancer alongside.

@BerlingoSolar the Victron BMS handles top balancing passively through the charge parameters rather than active cell-level balancing — it's relying on your charger to hold absorption long enough for the cells to self-balance through their internal chemistry. For a quality Fogstar pack like the Drift, that's usually fine for the first year or two.

Where I'd keep a close eye is your absorption duration. If you're set to a fixed time rather than tail-current cutoff, you may be either under or over-cooking it. On my static van setup I use a 2% C tail current (so 2A for a 100Ah pack) which consistently gets the cells within a few millivolts of each other.

If you're seeing any cell divergence creeping beyond about 50–60mV at top of charge, that's when I'd consider a manual top balance session with a bench power supply.

@BerlingoSolar eight months in and it's still playing nice? That Fogstar Drift is a solid unit but I'd still peek at the individual cell voltages via VictronConnect every few weeks — if one cell starts drifting more than ~20mV above its mates during absorption, that's your cue to either let it sit at absorption longer or crack out an active balancer before it becomes a proper drama. My van setup threw a wobbly at month six with exactly this, turned out one cell was hitting 3.65V while the others were still at 3.58V — Victron's passive approach sorted it over a few deep cycles but an active balancer would've cracked it overnight.

@BerlingoSolar worth checking your cell voltage spread during the tail end of absorption rather than just looking at resting voltage — that's where imbalance actually reveals itself. If you're seeing more than ~20-30mV spread as current tapers toward your absorption cutoff, the passive balancing isn't keeping up.

My own emergency backup setup (Victron SmartSolar + manual top-balance before commissioning) meant I went in with cells already matched to within 5mV. Eight months on, spread is negligible. Getting that initial top balance right before installation does most of the heavy lifting — the BMS then only has minor corrections to make.

If you've got Victron's VictronConnect app pulling live data, check individual cell voltages during a full charge cycle. That'll tell you definitively whether intervention is needed or whether you're worrying unnecessarily.

@BerlingoSolar something worth adding to what @CamperDan mentioned - I'd log a few charge cycles using the Victron Connect app and look at how long it's spending in absorption before dropping to float. If it's cutting short consistently, that can indicate the cells aren't quite getting to equilibrium even if the overall voltage looks fine on the surface.

Eight months is still relatively early days for spotting drift. I'd say give it another couple of months of logging, then compare your earliest screenshots against current ones. If your weakest cell is noticeably lagging behind at the top end, that's when I'd consider a manual top balance session rather than just leaving the BMS to compensate indefinitely. The Fogstar Drift cells are generally well-matched from factory, so you're probably fine, but the data doesn't lie!

Eight months of happy Victron BMS life is basically the honeymoon period — check that cell spread now before the drama starts! 🔋

On my narrowboat I do a manual top balance every 12 months regardless of how smug the BMS looks; just pop the cells out of parallel at full charge, let them sit, and measure each one individually — my Fogstar pack had a sneaky 18mV straggler that the BMS was quietly masking for months.

Quick rule of thumb: if your cell spread at the absorption tail exceeds ~20mV consistently, the BMS is working overtime compensating rather than genuinely balancing.

What @LouiseJames said about Victron Connect logs is spot on — the State of Charge history graph will show you if absorption is suspiciously long, which is usually the first tell that manual intervention is overdue.