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

Had exactly this headache last winter with my garden office setup. Fogstar Drift 200Ah with its built-in BMS showing 74% whilst the SmartShunt was reading 58%. Drove me absolutely mental for weeks.

After a lot of digging, the SmartShunt was the culprit in my case — the configured battery capacity was slightly off, and the charged voltage threshold wasn't set correctly so it wasn't resetting to 100% properly at the top. Once I sorted those parameters in the VictronConnect app it started tracking much better.

That said, I don't think it's always the SmartShunt's fault. The BMS SOC is typically voltage-based (which is notoriously unreliable mid-curve on LiFePO4) whereas the SmartShunt does proper coulomb counting. In theory the SmartShunt should be more accurate — but only if:

• Your Peukert exponent is set correctly
• The charge efficiency factor is right
• It's getting clean synchronisation points at 100%

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

1. Does your BMS actually do coulomb counting, or is it purely voltage-based SOC?
2. How often does your SmartShunt get a proper full-charge sync?
3. Are you running the SmartShunt as a standalone unit or integrated into a wider Victron system via Cerbo?

Still not 100% confident mine is right even now. There's a part of me that wonders if both are wrong in different directions and the truth is somewhere in the middle — which is a deeply unsatisfying conclusion after spending money on decent kit.

What are others doing — trusting one over the other, or averaging them out?

@DodgyMechanic had this exact same drama on my boat last year 😅

Honestly? Trust the SmartShunt for day-to-day use, but the BMS for protection limits.

The SmartShunt is doing proper coulomb counting — it's actually measuring amps in and out. The BMS SOC is often just voltage-based estimation which is notoriously dodgy, especially under load.

Key thing though — make sure your SmartShunt is properly configured:

• Correct battery capacity set
• Charged voltage threshold matches your actual absorption finish
• Tail current % is right (I use 4% on mine)

If the SmartShunt never hits a proper "sync point" it'll drift over time. Mine was miles off until I sorted that.

Also worth checking if Peukert exponent is set correctly for lithium — default is often still set for lead acid 🤦

@DodgyMechanic I've wrestled with this on both my boat and shepherd's hut builds — worth checking whether your SmartShunt is properly synchronising.

A few questions that might help diagnose it:

• What have you set as your charged voltage threshold in the VictronConnect app?
• Is your tail current configured correctly for a LiFePO4 chemistry?
• How often is the shunt actually hitting a 100% sync point?

The BMS SOC is often just a rough voltage-based estimate, which can drift badly under load. The SmartShunt's coulomb counting is generally more accurate if it's set up correctly for your specific battery.

On my Fogstar setup I had to tweak the charged detection voltage down slightly — the defaults felt like they were calibrated more for lead-acid than lithium.

What firmware version is your SmartShunt running?

@DodgyMechanic the root cause nobody's mentioned yet: the BMS is measuring voltage and interpolating SOC from a lookup table, whilst the SmartShunt is doing actual coulomb counting. Voltage-based SOC is notoriously wobbly under load — LiFePO4's flat discharge curve makes it even worse.

That said, @BoatLouise is right that the SmartShunt drifts over time without a proper synchronisation event (hitting absorption voltage to reset to 100%). If your solar isn't regularly getting the battery to a genuine full charge, the shunt just accumulates errors.

My Victron setup on the cabin EV charging circuit had a 23% discrepancy until I sorted the charged voltage and tail current parameters in VictronConnect. Those defaults are often wrong for LiFePO4.

Set the SmartShunt correctly → trust it daily. Use the BMS reading as a sanity check for anything really alarming.

@DailySolar is correct on the voltage interpolation point, but there's another factor worth adding: Peukert's exponent.

The SmartShunt compensates for it, most BMS units don't. Under heavy discharge loads your BMS is essentially lying to you because it's not accounting for the fact that you're pulling capacity out faster than the nominal rate allows for.

Also check your SmartShunt's charged voltage threshold and tail current settings — if it's not synchronising at 100% correctly, the coulomb counting drifts over time and compounds the error. Mine was set to Victron defaults which didn't match my Fogstar's actual charge profile. Once I corrected those two parameters the discrepancy dropped from ~15% to under 3%.

Set it up properly and the SmartShunt wins every time. The BMS SOC readout is largely decorative on most units.