Victron BMV-712 showing different SOC to my BMS — which one do I trust?

Had this exact situation on my narrowboat last winter — BMV-712 reading 67% whilst the BMS was showing 54%. Drove me a bit mad trying to figure out which to trust.

Short answer: neither blindly, but the BMV-712 is generally more reliable for day-to-day SOC tracking if it's been properly configured.

The BMV does coulomb counting, so it's measuring actual amps in and out. The problem is it drifts over time if your charged voltage threshold isn't set correctly — it won't reset to 100% at the right moment. Check your Peukert exponent and charge efficiency factor settings in VictronConnect, they're often left at defaults that don't suit lithium.

Most BMS units (depends heavily on which one you've got) use voltage-based SOC estimation, which is notoriously wobbly mid-cycle with LiFePO4 — the discharge curve is so flat it's almost useless for accurate readings between roughly 20–90%.

What I'd do:

• Fully charge to whatever your top voltage is, let the current tail off properly
• See which one reads 100% first and how the other responds
• Recalibrate the BMV from there

Worth mentioning what BMS you're running — some of the better ones (Daly, JK, Seplos) handle this differently. Also, how old is your BMV and has it ever been reset?

Anyone else had success getting the two to agree, or just accepted the discrepancy?

@YorkshireVanLifer had the same headache in my motorhome last year.

Main thing I'd add — the BMV is only as good as its calibration. If you haven't nailed down your Peukert exponent and charge efficiency factor, it'll drift over time. Mine was way off until I tweaked those settings properly.

Also worth checking: is your BMV seeing all your loads? Any current bypassing the shunt will throw the coulomb counting right out. Took me ages to spot a fridge feed that was wired upstream of mine.

Honestly I lean towards the BMS for a rough state, but use the BMV trend data to spot problems. Neither's perfect — they're measuring different things fundamentally.

...is calibrated, which is why I spent three hours arguing with my shepherd's hut BMV-712 before realising I'd set the Peukert exponent wrong — basically had it convinced my Fogstar Drift 200Ah was a potato 🥔

@BatteryPaula finish that sentence — I suspect you set Peukert's exponent wrong, which is exactly where most people go wrong with LiFePO4.

The BMV-712 defaults assume lead-acid. For lithium you want Peukert set to 1.05 (effectively flat) and charge efficiency factor at 99%. Most BMS SOC calculations are voltage-curve based and are notoriously unreliable mid-charge — they're good at 100% and 0% but hopeless in between.

On my narrowboat setup with Fogstar Drift cells, I trust the BMV-712 after a proper sync cycle, but only because I've verified the shunt is the only path current takes — no parasitic loads bypassing it. That's another common culprit.

Run a full charge to absorption, let the BMV auto-sync to 100%, then discharge steadily and compare. If they still diverge significantly, you've got a shunt wiring issue, not a calibration problem.

@LH_Marine is almost certainly right about Peukert, but there's another culprit that took me an embarrassingly long time to diagnose on my narrowboat: the charged voltage threshold.

If that's set even slightly below your actual absorption voltage, the BMV never registers a proper "full" event and the coulomb counting drifts progressively further from reality. Mine was out by 0.2V — sounds trivial, but over a fortnight of daily cycles the SOC reading had wandered completely.

The BMS cell-level data is genuinely more trustworthy for catching an imminent problem, but for day-to-day state of charge the BMV-712 wins — provided the synchronisation parameters actually reflect your charging profile. I cross-referenced both against my Victron Cerbo logs before concluding the BMV had simply never been told what "full" looked like.

Get that threshold right first, then revisit the Peukert figure.

@SophieFisher go on then, don't leave us hanging — what was your culprit?

Mine on the boat was the charged voltage threshold being set slightly too low, so the BMV-712 never registered a "full" cycle and the SOC drift just compounded over weeks. Once I corrected that and did a proper sync at 100%, it tracked the BMS much more closely.

Worth mentioning: if you're on LiFePO4, the BMS often has a more accurate picture during the flat middle section of the discharge curve because it's reading cell voltages directly. The BMV is doing coulomb counting, so any accumulated error since the last sync matters a lot. Neither is infallible — they're measuring different things.