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

Had this exact headache last year with my cabin setup. Running a Fogstar Drift 200Ah and the SmartShunt was consistently reading about 8% higher than the BMS.

Short answer — trust the BMS for lithium. It's reading cell-level voltages directly and knows what's actually going on inside the pack. The SmartShunt is doing coulomb counting which drifts over time, especially if your synchronisation points aren't dialled in properly.

That said, the SmartShunt is still worth having because:

• Better historical data and Victron Connect graphs
• Catches issues the BMS won't flag on the DC system side
• Plays nicer with MPPT/Multiplus for system integration

The fix that sorted mine was setting the charged voltage and tail current correctly in the SmartShunt settings. Most people leave these at defaults which are wrong for LiFePO4. I dropped charged voltage to 27.2V (24V system) and tail current to around 4%. Took a few full cycles but they came within 2% of each other after that.

Also worth checking — is your BMS actively balancing? If top balancing is off, both readings will be all over the place anyway.

What BMS are you running? Some of the cheaper units have dodgy SOC algorithms themselves, so sometimes neither is that reliable 😅

Anyone else got thoughts on this? Curious whether people are using the BMS figure to reset the SmartShunt automatically or doing it manually.

@Chopper makes a fair point about the BMS, but I'd add a small caveat from my own experience on the boat.

When I first commissioned my setup — Victron SmartShunt paired with a 280Ah LiFePO4 — the SmartShunt was wildly off until I dialled in the Peukert exponent and charge efficiency factor properly in VictronConnect. Once calibrated correctly and given a few full cycles to synchronise, the two readings converged to within 2-3%.

The BMS is your safety net, absolutely. But a well-configured SmartShunt gives you far better trending data — useful for spotting a cell drifting before the BMS screams at you.

Key thing: make sure your SmartShunt is set to sync at 100% only when the BMS confirms a genuine full charge. Default settings often cause it to reset prematurely.

The SmartShunt is basically that friend who insists they know a shortcut but the BMS is the one who actually lives there — calibrate your shunt properly after a full charge cycle and the gap usually shrinks to something you can live with. 🔋

From my shepherd's hut setup, I give the Fogstar BMS the final word on protection cutoffs, but I do keep an eye on the SmartShunt trends for spotting capacity fade over time — two tools, two jobs, neither is wrong exactly.

Key thing people miss: if your SmartShunt hasn't been tail current configured correctly for lithium, it'll never catch a proper 100% reset and the drift compounds week on week.

Really interesting thread — I've been wrestling with exactly this on my narrowboat setup!

Quick question for @Chopper and @WattAMess25 — does the gap between the two readings get worse over time, or does it stay fairly consistent at that 8%?

On mine I noticed the SmartShunt drifts further off the longer it goes without hitting a proper full charge to reset the sync point. Once I sorted the absorption voltage settings in Victron's app it tightened up considerably.

Also wondering — are either of you running the SmartShunt alongside a Victron MPPT? Apparently when they're all talking over VE.Smart networking the coulomb counting gets a bit more reliable. Not sure if that changes the BMS vs SmartShunt argument fundamentally, but it seemed to help in my case.

Something worth adding from my narrowboat experience — the SmartShunt's accuracy lives or dies by how well you've configured it.

I spent three frustrating weeks last winter convinced my Victron unit was faulty. Turned out my charged voltage threshold and tail current settings were completely wrong for my Fogstar cells. Once I dialled those in properly through the VictronConnect app, the gap between SmartShunt and BMS closed to under 2%.

The BMS wins in a genuine emergency — it will cut before damage occurs. But a properly calibrated SmartShunt gives you far better day-to-day visibility because it tracks coulombs continuously rather than relying on voltage curves.

My rule aboard: BMS is the bouncer, SmartShunt is the fuel gauge. You need both doing their jobs properly. If they're disagreeing significantly, the settings are almost always where I'd look first.

@CopperRoamer narrowboat installs are particularly tricky because you've got variable loads from inverters, bilge pumps, and engine alternators all happening simultaneously — the SmartShunt's coulomb counting can drift quite badly if the charged voltage threshold isn't set precisely right.

The key thing nobody's mentioned yet: your SmartShunt resets its 100% SOC reference point when it sees a tail current (typically 4% of capacity) at your charge voltage. If your absorption voltage is even 0.1V off from what the BMS considers "full," they'll never agree on what 100% looks like.

On my narrowboat I found setting the SmartShunt's charged voltage to match exactly what the Fogstar's BMS reports via Victron VE.Bus data completely resolved the discrepancy. Check your Peukert exponent too — default 1.25 is wrong for most lithium; I use 1.05.