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

My static van has had this exact soap opera running for two years — BMS says 87%, SmartShunt reckons 71%, and I'm just stood there like a referee at a biscuit fight.

Short answer: trust the SmartShunt for day-to-day SOC, but don't completely ignore your BMS.

Here's the thing — most cheap BMS units (looking at you, every unbranded unit on eBay) calculate SOC based on voltage curves alone, which is about as accurate as guessing the weather by sniffing the air. The Victron SmartShunt is doing proper coulomb counting, tracking every amp in and out, which is fundamentally more reliable if you've set it up correctly.

The "if" is doing a lot of heavy lifting there, mind you.

Key things to check on your SmartShunt setup:

• Charged voltage set correctly for your chemistry (LiFePO4 ≠ lead acid, obviously)
• Tail current dialled in properly
• Peukert exponent — leave it at 1.05 for lithium

Also, when did you last let the bank hit 100% for a full sync? The SmartShunt needs that reference point or it drifts like my van does when the wind picks up.

What BMS are you running? Some of the better ones (JK, Daly with active balancer) are actually decent — others are basically decorative. Worth knowing before deciding which one's telling porkies.

@LutonBuild the SmartShunt wins if you've configured it properly — and that's the critical caveat.

The BMS SOC is typically just voltage-based lookup, which is notoriously unreliable mid-charge on LiFePO4 because the discharge curve is so flat.

The SmartShunt does coulomb counting, which is genuinely accurate provided:

• Charged current threshold is set correctly (I use 1% of capacity)
• Tail current is dialled in for your battery chemistry
• You've entered accurate capacity — Fogstar Drift cells drift slightly from rated spec, worth checking
• It's synchronising properly at full charge (watching for those full-charge resets in VictronConnect history)

Two years of drift suggests it's possibly never syncing correctly at 100%. Check your absorption voltage is actually hitting the BMS high-voltage cutoff threshold — if it falls short, the shunt never resets.

@LutonBuild "referee at a biscuit fight" — I'm stealing that phrase immediately, possibly for the name of my narrowboat.

From my own setup: the SmartShunt is only as clever as the bloke who configured it. Wrong battery capacity entered? Wrong Peukert exponent? Never properly sync'd at 100%? It'll drift

@LutonBuild the real villain here is synchronisation — if your SmartShunt hasn't seen a proper 100% full charge to reset itself, it just drifts off like a confused uncle at a motorway services.

Mine was 16% out on the Fogstar 280Ah bank until I sorted the charged voltage and tail current settings in VictronConnect — took twenty minutes and now both agree like old mates at a pub.

BMS is essentially just doing cell maths; SmartShunt is counting coulombs — apples and slightly-better-informed apples.

@LutonBuild ran into this exact thing with my van build. Worth checking your charged voltage threshold in the SmartShunt settings — mine was set to default 13.2V which was triggering a false sync way too early on my Fogstar Drift cells. Bumped it up to 14.1V and the two readings slowly converged over a few cycles.

Also worth noting: if your BMS is doing any cell balancing, it might be reporting a slightly optimistic figure based on the highest cell rather than pack average. Seen that catch people out.

The SmartShunt will drift over time regardless — it needs that clean sync point @HiluxBuild mentioned to self-correct. Without it you're just accumulating error.