Victron SmartShunt showing different SOC to my BMS — which one do I trust?

Had exactly this going on with my boat setup last winter — Victron SmartShunt sitting at 78% whilst the BMS was reading 65%. Drove me mad for weeks.

From what I've worked out, the SmartShunt is doing coulomb counting (tracking amps in/out), so it's only as accurate as its calibration point. If it hasn't had a proper full charge to reset/sync recently, the drift compounds over time. The BMS on the other hand is typically reading cell voltages and estimating from there — which has its own limitations given how flat the LiFePO4 discharge curve is mid-range.

A few things worth checking:

• Has your SmartShunt been configured with the correct battery capacity and Peukert exponent?
• When did it last hit 100% and hold there long enough to synchronise?
• What BMS are you running? Some of the cheaper units have pretty optimistic SOC algorithms

Personally I've come to trust the SmartShunt more for day-to-day trends, but I keep an eye on the BMS for anything cell-level. If my Fogstar cells are showing any imbalance, that overrides everything else.

Curious what others do — do you treat one as the "master" reading, or run them both as a cross-check? And has anyone actually dialled in their SmartShunt well enough to fully trust it long-term?

@SparkyGrafter — the SmartShunt is integrating current over time (coulomb counting), so it's only as good as its calibration and your charged voltage / tail current settings; if those are even slightly off, it drifts like a dinghy with no anchor.

The BMS figure is typically voltage-based with a lookup table — dead accurate at the extremes of charge but hopeless in the mushy middle of LFP's flat discharge curve.

Brutal truth: neither is gospel, but a properly configured SmartShunt wins long-term if it's allowed to sync at 100% regularly (full charge, low tail current, hold time all dialled in via the Victron app).

Check your charged voltage isn't set below your actual absorption voltage — classic gotcha that stops it ever resetting to 100%.

Good question to dig into. With my static caravan setup I've found the synchronisation point is everything — if your SmartShunt hasn't seen a proper full charge recently, its coulomb counting drifts badly.

A few things worth checking:

• Is your "charged voltage" threshold set correctly in VictronConnect? If it's too high, the shunt never resets to 100%
• What's your tail current setting? Too low and it won't recognise the absorption end-point
• Does your BMS use a different chemistry curve to calculate SOC? Some cheaper BMS units are notoriously optimistic

Personally I trust the SmartShunt long-term but only once it's properly calibrated. The BMS voltage-based reading is more of a snapshot.

Worth connecting to VRM portal if you haven't — you can see exactly when (or if) your shunt last synchronised to 100%.

Reply by Davo:

@SparkyGrafter I had almost identical grief with my narrowboat setup. One thing worth checking that nobody's mentioned yet — your SmartShunt's charged voltage threshold and tail current settings. If these aren't set correctly for LiFePO4, the shunt never gets a proper synchronisation point and the drift compounds over time. Mine was set to defaults that were frankly designed for lead-acid.

Also worth looking at your Peukert exponent — set it to 1.05 for lithium rather than the higher values suited to lead-acid. Makes a surprising difference to accuracy under high discharge loads.

For what it's worth, I'd trust the BMS for cell-level protection decisions, but a properly calibrated SmartShunt often gives better overall capacity tracking once you've sorted the sync settings. The two should complement each other rather than fight.

Really useful thread this. One thing I'd add — have you checked your Peukert exponent settings in the SmartShunt? Mine was way off for my Fogstar Drift LiFePO4 cells and it was causing the coulomb counting to drift badly under high loads.

Also worth considering: LiFePO4 has that notoriously flat voltage curve, which can confuse voltage-based SOC estimates on some BMS units. So it might actually be the BMS that's wrong here rather than the SmartShunt.

@BlownFuse makes a good point about synchronisation — I'd also double-check what your "charged voltage" threshold is set to in VictronConnect. If it's triggering a sync too early, your SmartShunt resets to 100% prematurely and everything drifts from there.