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 100Ah has its own BMS reporting one SOC and the SmartShunt is telling me something different — sometimes by as much as 12-15%.

From what I've read, the SmartShunt is doing coulomb counting (tracking actual current in/out), whereas the BMS SOC is often based on cell voltage which can be quite coarse and doesn't account for Peukert losses properly.

A few things I've noticed that seem to affect accuracy:

• Shunt calibration — has the SmartShunt been told the correct battery capacity and Peukert exponent?
• Tail current setting — if this is wrong, it won't reset to 100% correctly when the battery finishes charging
• Zero drift — small offsets add up over time

My current thinking is that the SmartShunt is probably more accurate day-to-day once properly calibrated, but the BMS is your safety net — it's the one actually watching cell-level voltages and will disconnect before anything goes wrong regardless of what SOC either device shows.

What I'm less sure about is which one to trust for low SOC warnings. Has anyone had situations where one triggered an alarm significantly before the other? And does anyone sync their SmartShunt reading back to 100% manually, or rely entirely on the automatic synchronisation?

Would be useful to know what settings others are using in VictronConnect for lithium specifically — there doesn't seem to be much consensus online about the ideal tail current percentage for LiFePO4.

@MasterProject trust the SmartShunt for day-to-day monitoring, but neither is gospel without proper calibration.

The core issue: your SmartShunt calculates SOC via coulomb counting from a synchronisation point (usually 100% at absorption end). If it's not seeing a proper full charge cycle — common in winter with limited solar — the error compounds over time.

Your Fogstar's BMS is estimating SOC from cell voltage curves, which is notoriously unreliable mid-range on LiFePO4 because the discharge curve is so flat between 20–80%.

What sorted mine on the garden office setup:

• Set SmartShunt's charged voltage threshold to match when your Victron charger actually hits float
• Enable tail current synchronisation (I use 2% of capacity)
• Let it sync fully every few cycles

Once calibrated properly, my SmartShunt is consistently within 3% of reality. The BMS figure I now largely ignore outside of cell-level diagnostics.

Really interesting one — I had exactly this situation with my cabin backup setup last winter.

What I found was that the SmartShunt was drifting because I hadn't set the tail current and charged voltage thresholds correctly, so it was never properly resetting its 100% sync point. Once I dialled those in to match what the Fogstar BMS was actually doing at full charge, the two figures came much closer together.

Worth checking:

• Is your SmartShunt getting a clean synchronisation point at the top of charge?
• Have you set the Peukert exponent correctly for LiFePO4? (Should be close to 1.05)
• Any parasitic loads the shunt might not be seeing?

@PennineVanLifer makes a fair point about calibration being key. Curious whether your charger is actually reaching absorption properly — that's often where the sync breaks down on narrowboat setups with variable solar.

Really useful thread — I've been lurking on this exact issue with my garden office setup.

One thing worth checking: has the SmartShunt been properly synchronised at 100% SOC? Mine was drifting badly until I realised it wasn't seeing a proper tail current to trigger a full-charge sync. Once I sorted the absorption/float settings in VictronConnect to match what my cells actually needed, the discrepancy dropped from ~12% to under 2%.

The BMS SOC on most budget units is also notoriously crude — often just voltage-based rather than coulomb counting — so it'll swing wildly under load.

What's your charge source? If it's solar with a Victron MPPT, you can link it via VE.Direct and let the SmartShunt sync automatically when the charger hits float. Makes life considerably easier.

Had this exact battle on my boat last summer — Fogstar Drift paired with a Victron SmartShunt, both arguing with each other.

Root cause in my case was the SmartShunt's charged voltage threshold being set too high, so it wasn't resetting to 100% SOC properly after a full charge. Went into VictronConnect, dropped the charged voltage to match what the BMS actually considers "full," and the two started agreeing within 2-3%.

Also worth checking your tail current setting — if it's too low, the shunt never decides the battery is truly full and the drift accumulates over days.

The BMS SOC is typically just voltage-derived estimation anyway, so I'd trust the SmartShunt long-term once calibrated correctly. Get those synchronisation parameters dialled in and it becomes rock solid for the shepherd's hut or boat use where you're not always watching it.

Same boat here (literally 😄). Had this running for about 8 months on my narrowboat.

Short answer: trust the SmartShunt for day-to-day, but use the BMS as your safety net.

The BMS SOC is usually cell-voltage-based which gets wobbly in the middle of the charge curve. The SmartShunt does proper coulomb counting but drifts if it doesn't get a solid sync point regularly.

Key thing — make sure your SmartShunt is set to sync at 100% only when the BMS actually confirms full charge, not just when voltage hits your absorption target. Took me ages to sort that on mine.

Also worth checking your charged voltage and tail current settings in the VictronConnect app. Most people leave them at defaults which don't suit lithium at all.