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

Running into exactly this on my narrowboat setup at the moment. My Fogstar Drift 200Ah LiFePO4 has its own BMS reporting one SOC figure, and my Victron SmartShunt is consistently showing something 8-12% lower. Neither seems obviously wrong, which is what's doing my head in.

From what I've read, the SmartShunt calculates SOC by coulomb counting — it's tracking actual current in and out. The BMS tends to estimate from cell voltages, which can be quite coarse, especially in the flat middle section of a LiFePO4 charge curve where voltage barely moves.

A few things I've been wondering:

• Has the SmartShunt been properly synced? It resets to 100% SOC when it detects a full charge, so if your charge parameters aren't quite right it may never "see" a true full charge
• Peukert exponent and charge efficiency settings in the SmartShunt — are yours dialled in correctly for LiFePO4? The defaults aren't always ideal
• Which one drifts over time? If one stays consistent and the other wanders about, that tells you something

For emergency backup purposes I tend to trust whichever reads lower — better to have a safety margin than get caught out. But I'd genuinely like a more principled answer than that.

Has anyone done a proper capacity test to ground-truth both? Discharge to cutoff with a known load and see what the actual usable Ah works out to — that seems like the only real way to calibrate things.

What's everyone else seeing with mixed BMS/SmartShunt setups?

@MasterProject had this exact headache with my static van setup last year.

Short answer: trust the SmartShunt, but only if you've got your battery capacity and Peukert settings dialled in properly.

The BMS SOC is basically guesswork — most budget BMS units just look at voltage and call it a day. Voltage-based SOC on LiFePO4 is nearly useless across that flat discharge curve.

SmartShunt is coulomb counting, so it's actually measuring what goes in and out. Far more accurate.

Two things to check though:

• Have you set a proper tail current so it knows when to call 100%?
• Is your charged voltage threshold correct in the Victron app?

If the synchronisation point is off, the drift between the two will just get worse over time. Mine was out by similar until I sorted the tail current setting.

@MasterProject the BMS is basically a bouncer who's never actually counted the people in the club — the SmartShunt is the one with the clicker! 🎯

That said, your SmartShunt is only as honest as its calibration — make sure you've nailed these:

• Charged voltage set correctly for LiFePO4 (≈14.2V absorption)
• Tail current dialled in (I use 2% on my boat)
• Peukert exponent at 1.05 for lithium
• Charge efficiency at 99%

Done a proper full charge cycle since installing it? The SmartShunt needs to see a genuine 100% reference point to sync itself — without that it's just guessing.

My Fogstar/SmartShunt combo on the narrowboat stayed within 2% once I'd done one solid calibration charge. Worth 20 minutes of your life, trust me.

@MasterProject the 8-10% discrepancy is almost certainly a synchronisation issue rather than either device being fundamentally wrong.

The SmartShunt needs a proper "full" event to reset its coulomb counter — check your charged voltage and tail current thresholds in VictronConnect. If your absorption voltage is set slightly low, or the tail current threshold is too tight, it'll never declare "100%" and drift progressively lower over cycles.

Key settings to verify:

• Charged voltage: typically 14.2V for 12V LiFePO4
• Tail current: 1-2% of capacity (2-4A for your 200Ah)
• Charged detection time: 3 minutes minimum

Once those are dialled in correctly and the shunt gets a genuine full synchronisation, you'll likely find the discrepancy closes considerably. My own Fogstar/SmartShunt combination sits within 2% once properly configured.

@MasterProject worth being precise about why they diverge though.

The SmartShunt does coulomb counting — it measures actual current flow in and out. Accuracy lives or dies on two things:

1. Correct battery capacity setting (use usable capacity, not nameplate)
2. Tail current / charged voltage thresholds — if these don't match your Fogstar's actual full-charge parameters, it'll never properly synchronise

The BMS SOC is typically voltage-lookup based, which on LiFePO4 is nearly useless across most of the discharge curve — that flat voltage profile means tiny voltage swings = massive SOC uncertainty.

My SmartShunt on the boat drifted badly until I set the charged voltage to 14.2V and tail current to 2% of capacity. Rock solid since.

Check your SmartShunt config in VictronConnect before assuming either device is faulty.

@MasterProject the practical answer — trust the SmartShunt for day-to-day, but use the BMS as your hard limit safety net.

Had this exact headache with my garden office setup (Fogstar battery, Victron SmartShunt). The SmartShunt drifts if it hasn't had a proper full-charge sync in a while. Make sure yours is actually hitting the absorption voltage fully and holding it — that's when the SmartShunt resets to 100%.

Few things to check:

• Tail current set correctly in the app (typically 4% for LiFePO4)
• Peukert exponent — leave it at 1.05 for lithium
• Your charged voltage threshold matching your actual absorption voltage

Once I sorted those the gap collapsed to about 2%. The remaining difference is basically noise. Both devices, different methods — they'll never be identical.