Should SOC Line up with 12v Discharge Chart?

Been scratching my head over this one with my own Victron setup — SmartShunt reckons I'm at 50% SOC but the voltage is reading lower than the discharge chart from my Fogstar cells says it should be.

Surely if the shunt is properly calibrated and the batteries are genuinely at half capacity, the resting voltage should more or less match the manufacturer's published table? Mine's consistently about 0.2–0.3v adrift and I can't decide if that's:

• The shunt just being a bit optimistic about capacity
• Temperature throwing the discharge curve off (it is a garage install in Birmingham, so "ambient" is generous)
• My Cerbo needing a proper battery profile tweak

The classic trap I fell into was trusting the voltage readout during load rather than resting — apples and oranges, obviously — but even resting overnight it doesn't quite line up.

Anyone else running a SmartShunt alongside a 12v lithium bank finding the SOC percentage and the voltage chart from the manufacturer are telling slightly different stories? Wondering whether to just ditch the voltage-based SOC crosscheck entirely and let the coulomb counting do its thing, or whether a persistent mismatch is a red flag worth chasing down.

What's your experience — do yours line up cleanly, or is a small discrepancy just par for the course? 🤔

Barry1965 | Senior Member

@Brummie84 worth checking when your SmartShunt last did a full synchronisation. If it hasn't seen a proper full charge cycle recently, it can drift quite badly and the SOC figure becomes unreliable. The voltage is arguably more honest in that situation.

Also bear in mind that voltage under load tells a very different story to resting voltage — what's your current draw when you're seeing this discrepancy? Heavy loads will sag the voltage considerably and make it look worse than the SOC suggests.

Fogstar's discharge charts are typically based on resting voltage too, so comparing them whilst the inverter's pulling amps isn't really apples to apples. Let the bank rest for a few hours with no load and see if the two figures reconcile better.

@Brummie84 had almost the exact same head-scratch moment with my static caravan setup last summer. The thing that caught me out was cell imbalance — the SmartShunt is counting coulombs in and out, so it's tracking pack energy, but if your cells have drifted apart, the weakest cell drags the pack voltage down before the shunt thinks you've hit 50%.

Worth grabbing the Victron Connect app and watching individual cell voltages under load if you've got a BMS that reports them. My Fogstar cells looked balanced at rest but one was noticeably softer once I had the inverter pulling a decent load.

The discharge chart assumes a healthy, balanced pack — if yours isn't, voltage and SOC will tell you different stories, and the shunt's coulomb counting is usually the more trustworthy narrator.

Good point from @Barry1965 on the sync, but I'd also question whether you're comparing like-for-like here.

Those voltage-to-SOC discharge charts are typically generated under a specific load condition — usually 0.2C or similar. If you're pulling heavier loads, voltage sags and won't match the chart at all. The SmartShunt's coulomb counting is actually more reliable than voltage-based SOC for LiFePO4 anyway, given how flat that discharge curve is.

Also worth checking: what's your Peukert exponent set to in the SmartShunt config? Wrong value will skew the SOC reading over time.

My own setup with a Fogstar Drift had this exact confusion initially — ditched the voltage chart entirely once I trusted the shunt had a clean sync to work from.

Load matters too — that discharge chart assumes you're barely tickling the battery, not running a kettle through a Multiplus at 2am 🔌

@BatteryJason nailed the load bit — in my shepherd's hut I've got a Victron SmartShunt and learned the hard way that SOC is coulomb counting (amps in vs amps out over time), whereas voltage is just a snapshot that shifts around depending on temperature, load, internal resistance, and whether your cells have had five minutes to settle — basically the voltage chart is like checking your tyre pressure while doing 70mph on the M6.