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

Mine disagreed by nearly 15% for weeks until I realised my SmartShunt had the wrong battery capacity entered — classic "garbage in, garbage out" situation right there.

Honestly though, even with correct settings, I'd lean toward the BMS for actual cell-level truth and the SmartShunt for trend-watching — they're doing slightly different jobs when you think about it.

A few things worth checking if yours is way off:

• Charged voltage threshold — is the SmartShunt actually recognising a full charge and resetting to 100%?
• Tail current setting — this catches a lot of people out, especially on LiFePO4
• Peukert exponent — set it to 1.05 for lithium, not the lead-acid default
• Battery capacity — sounds obvious but double-check your Fogstar/whatever cells are entered correctly in VictronConnect

My shepherd's hut setup ran on dodgy SOC readings for an embarrassing amount of time before I sat down with the Victron docs properly — the SmartShunt is brilliant kit but it does need a good configuration session to earn its keep.

Has anyone found their BMS SOC drifting badly after a partial cycle? Mine does occasionally and I'm never quite sure whether to trust it or just do a full top-balance and start fresh — curious what others do.

@BatteryPaula the BMS knows what's actually happening inside each cell, while the SmartShunt is just doing maths on current flow and quietly lying to you every time you forget to hit a proper 100% reset point — so I'd trust the BMS like a suspicious narrowboat neighbour who actually watched you come in.

@BatteryPaula good catch on the capacity setting — catches loads of people out.

Worth adding: the SmartShunt's coulomb counting drifts over time if it doesn't get a proper sync point. Make sure your charged voltage and tail current settings are dialled in correctly so it resets to 100% cleanly at the top of each charge cycle.

My setup (Fogstar Drift 200Ah + Victron SmartShunt) — I basically use the BMS as a safety net and the SmartShunt for day-to-day SOC monitoring. They sit within 2-3% of each other once everything's configured properly.

If yours are still miles apart after sorting the settings, check for parasitic loads the SmartShunt might be missing — anything wired before the shunt won't get counted.

@BatteryPaula both lie to you equally but in different ways — BMS is cell-level gospel, SmartShunt drifts without a clean sync point, so I treat mine like a dodgy sat-nav that needs recalibrating every time it gets confused.

My Fogstar Drift cells hit 100% at top of charge and I've got the SmartShunt set to sync there — once you nail that charged voltage threshold and tail current setting in VictronConnect, they usually agree within a couple percent.

The real gotcha nobody mentions: if your system never actually reaches full charge (garden office loads running all day, ahem), the SmartShunt never gets its sync moment and just wanders off into fantasy land percentage-wise.

Trust the BMS for protection, trust the SmartShunt for trends, trust neither when your teenager's been running a fan heater off it.

Great points all round. One thing I'd add — the SmartShunt really needs a proper synchronisation event to stay honest. Mine was drifting noticeably until I made sure it was actually reaching a clean 100% reset regularly (tail current properly configured for my specific cells).

I now run both and treat them as a cross-check rather than competing sources of truth. If they diverge by more than a few percent I take it as a prompt to investigate rather than just trusting one blindly.

@BorderVanLifer is right that they fail in different directions, which is actually useful — if your BMS is showing stress on a particular cell group but the SmartShunt looks fine overall, the BMS is telling you something important that coulomb counting would completely mask.