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

Had exactly this on the narrowboat last winter — SmartShunt reading 73% while the Daly BMS was showing 58%. Drove me mad for weeks.

Short answer: neither is perfectly accurate, but for different reasons.

The SmartShunt uses coulomb counting, which means it tracks every amp going in and out. Sounds great, but it drifts over time if your "full" sync point isn't calibrated properly. If the shunt hasn't seen a proper tail current recently, it'll wander off.

The BMS is typically estimating SOC from cell voltages, which is notoriously unreliable mid-range on LiFePO4 — the voltage curve is almost completely flat between about 20–80%, so small voltage differences mean huge SOC swings in the reading.

What I actually do now:

• Trust the SmartShunt for day-to-day monitoring (it's more granular)
• Use the BMS as a protection layer, not an SOC gauge
• Make sure the SmartShunt syncs to 100% properly — mine only does it when charge current drops below 1A, which I had to set manually in VictronConnect

The key thing is getting your sync settings right in VictronConnect. Charged voltage threshold, tail current, detection time — worth spending an hour on those properly.

Anyone running a more sophisticated BMS like a JK or JKBMS finding better agreement with their SmartShunt? Curious whether the smarter BMS units are any more trustworthy for SOC.

@BoatEwan had exactly this circus on my narrowboat last year with a Victron SmartShunt vs a JK BMS — maddening doesn't cover it.

The crux nobody mentions: both devices are guessing, but via completely different methods. The SmartShunt is coulomb-counting from a reference point (your last "full" sync), so any drift in your shunt calibration or a dodgy sync threshold compounds over days. The BMS is typically estimating from cell voltages, which is notoriously nonlinear across the flat middle section of LiFePO4's discharge curve — basically useless between roughly 20-80% SOC.

My solution: trust the SmartShunt for day-to-day figures, but force a genuine full charge periodically to resync it properly. Set your absorption properly in Victron Connect so it actually completes — half-hearted charges are the root cause of most SmartShunt drift I've seen.

Really interesting thread this. Worth adding that the SmartShunt's accuracy lives or dies by how well you've configured the charged voltage threshold and tail current settings — if those are even slightly off, the coulomb counting never gets a proper reset and the drift compounds over time.

@BoatEwan on a narrowboat you're probably doing lots of partial cycles which makes this worse. I'd suggest setting a proper absorption endpoint that your system reliably hits, so the SmartShunt gets a clean sync regularly.

The BMS SOC is often just a rough voltage-based estimate anyway, especially with LFP's flat discharge curve — it's genuinely terrible for that chemistry.

Personally I lean toward trusting the SmartShunt once it's properly calibrated, but treat both as advisory rather than gospel until you've nailed the settings.

The dirty secret nobody mentions: both devices are integrating errors over time, they're just making different errors — Daly's cell-level readings drift under load, SmartShunt's coulomb counting punishes you for a dodgy charged voltage threshold or a tail current set too high.

My motorhome setup runs Fogstar Drift cells with a JK BMS and a SmartShunt in parallel — after six months I trust neither in isolation, but when they agree within ~5% I sleep fine.

Key thing @RhysThompson95 is dancing around: your SmartShunt is only as honest as your last full synchronisation point. If it's not hitting that 100% sync regularly, errors stack like unpaid speeding fines.

Force a proper full charge, let the tail current drop to whatever you've configured, then compare both readings — that's your baseline sanity check.

@FormerCop nailed it really. Both drift, just differently.

What sorted mine on the motorhome was forcing a proper full charge sync more regularly — like actually letting the Victron reach absorption and float properly rather than cutting it short. SmartShunt resets its 100% reference point and suddenly the two readings stop being so embarrassing.

Also worth checking your Peukert exponent setting in the SmartShunt. Default is usually 1.25 but that's very much a generic figure. Get it wrong and your SOC reading will be garbage under any kind of load.

BMS voltage-based SOC is basically useless under load anyway — voltage sags all over the place. Coulomb counting (what the SmartShunt does) is theoretically better but needs those sync events to stay honest.

Personally I use the SmartShunt as my day-to-day reference and treat the BMS as a safety net, not a gauge.

@FormerCop makes a fair point about drift, but worth adding — the SmartShunt will never know what the BMS is quietly doing behind the scenes. If your Daly is doing any cell balancing or has tripped a protection and reconnected, the shunt's coulomb counting just carries on regardless, completely oblivious.

On my cabin setup I ended up treating the SmartShunt as the "fuel gauge" for daily use and the BMS as the safety net. Different jobs really.

One thing that helped me was nailing the Peukert exponent and charge efficiency factor in the Victron app — most people leave these at default and wonder why the numbers drift apart over a few cycles. Worth a tweak if you've got Fogstar or similar LiFePO4 cells.