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

Had this exact head-scratcher on my boat last summer. After a lot of faffing about I came to the conclusion that neither is completely right, but the SmartShunt is usually closer — provided you've got your settings dialled in properly.

The BMS SOC on most units (especially cheaper ones) is notoriously optimistic. It's often based on voltage alone or a fairly crude coulomb counter that never gets a proper calibration reset unless you hit 100% charge regularly. If you're partial-cycling like most of us do, it drifts badly over time.

The SmartShunt's coulomb counting is more precise, but it lives and dies by:

• Correct battery capacity entered (Ah)
• Peukert exponent set properly for your chemistry
• Charged voltage and tail current thresholds — these are what trigger the 100% sync point

If that sync never fires, the shunt just accumulates error indefinitely.

What I'd suggest:

1. Double-check your "charged voltage" is actually being reached during absorption
2. Set tail current to roughly 1-2% of capacity
3. Let it do a proper full charge and watch whether the shunt resets to 100%

Once mine was set up correctly the SmartShunt was rock solid. The BMS percentage became something I basically ignore unless it's throwing actual alarms.

What BMS are you running? Some of the Daly units are particularly bad for this. JK BMS tends to be more honest in my experience.

@DaiYoung56 good point about the SmartShunt generally being more reliable, but worth adding — it's only as good as its calibration. If you haven't set your battery capacity, Peukert exponent, and charge efficiency factor correctly in the VictronConnect app, it'll drift badly over time.

My setup in the van runs a Fogstar Drift 200Ah with its built-in BMS, and the SmartShunt sits about 2-3% different most of the time. I've learned to trust the SmartShunt for trending (is it going up or down, how fast) and the BMS for protection (cell-level stuff the SmartShunt simply can't see).

The real fix is making sure you sync the SmartShunt at 100% SOC properly — it needs a genuine absorption completion, not just hitting bulk voltage.

@FenlandVanLifer makes a fair point about calibration. The bit nobody mentions enough is tail current — if your SmartShunt's charged detection threshold isn't set correctly for your specific battery, it'll never properly sync its 100% point and the drift compounds over time.

On my garden office setup with a Fogstar Drift 100Ah, I had a persistent 8% discrepancy until I actually read the battery spec sheet and matched the tail current setting properly in VictronConnect. Took ten minutes, sorted it completely.

The BMS SOC is often just a rough voltage-based estimate anyway — fine as a sanity check, not something to navigate by. Trust the SmartShunt, but only after you've configured it properly for your chemistry and capacity.

@HamishMitchell hit on something I learned the hard way on my narrowboat — tail current is genuinely the silent killer of SmartShunt accuracy.

But there's another layer worth mentioning: synchronisation points. The SmartShunt only resets to 100% when it sees the battery hit absorption and the current drops below that tail threshold. If your solar ever just barely tips into absorption without properly finishing the cycle, the shunt never syncs, and the drift compounds over days.

I went weeks thinking my 200Ah Fogstar lithium was at 60% when it was closer to 80%. Turned out my Victron MPPT absorption time was set too short for the install.

Check your VRM history — if you're not seeing regular "synchronised to 100%" events logged, that's your culprit before you start blaming the BMS.

@PeteJames @HamishMitchell both nailed the tail current issue. One thing I'd add from my garden office setup — charged voltage threshold trips people up just as much. If your SmartShunt thinks "fully charged" is 14.2V but your charger only ever hits 14.0V under load, it never synchronises and the drift compounds over weeks.

Also worth checking your Peukert exponent in the Victron app. Default is 1.25 which suits flooded lead acid — if you're on lithium (Fogstar, CATL cells etc.) drop it to 1.05 or the shunt will underestimate remaining capacity under high loads.

BMS SOC is usually just coulomb counting with no synchronisation logic, so it drifts worse long-term. SmartShunt wins, but only once it's properly configured.