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

Had exactly this situation last year with my 280Ah Fogstar Drift LiFePO4 bank. The two readings were sometimes 8-10% apart which drove me mad initially.

Short answer: trust the BMS for protection, trust the BMV-712 for day-to-day SOC management — but only once the BMV is properly calibrated.

The BMV works by coulomb counting from a known full charge point. If you haven't nailed down your:

• Charged voltage threshold
• Tail current setting (I use 1-2% of capacity)
• Peukert exponent (LiFePO4 is typically 1.05)

...then it'll drift over time. Mine was way off until I tightened up those settings.

The BMS SOC is usually calculated differently — often just voltage-based, which is notoriously unreliable in the flat middle section of a LiFePO4 curve. That's probably why they disagree.

What I do now is let the BMV be my primary readout but do a full charge every couple of weeks to give it a proper sync point. After a full charge cycle where the BMS hits top balance, both my readings usually land within 2-3% of each other.

What does your charge setup look like? Victron MPPT or shore power? And has the battery ever been fully charged since you configured the BMV? That's usually where the problem starts.

Would be good to hear what others are doing — especially anyone running a Daly or JK BMS alongside Victron kit.

@Tel1968 trust the BMV-712 for actual state of charge — the BMS is just a bouncer that cuts power when things get lairy, not a maths genius.

@AlexHobbs has it roughly right but I'd nuance it slightly — on my narrowboat I run both a BMV-712 and a Daly BMS on my 200Ah LiFePO4 bank, and I've found the BMV drifts if you don't nail the Peukert exponent and charged voltage settings properly.

The real trick is syncing them periodically. Once your BMS confirms a genuine 100% full charge (top-balanced cells, all hitting absorption voltage), tell the BMV that's 100% too. After that the coulomb counting stays honest for weeks.

The 8-10% gap @Tel1968 mentioned is almost certainly the BMV never getting a proper sync point — common on boats where you're rarely hitting a full charge cycle.

@LisaKelly66 and @AlexHobbs are both dancing around the real issue here — neither device actually measures SOC directly, they both infer it, just via different methods.

The BMV-712 does coulomb counting (integrating current over time), which drifts unless it gets a proper synchronisation event — i.e., a genuine full charge. On a narrowboat in winter with limited solar, mine was wandering 12% off after three weeks without hitting absorption properly.

The Daly and similar budget BMS units typically use cell voltage for SOC estimation, which is notoriously unreliable in the flat middle section of a LiFePO4 curve.

Practical answer: calibrate your BMV-712 correctly (tail current set right, charge efficiency factor dialled in), make sure it's synchronising regularly, and use the BMS purely as the safety net it's designed to be. The BMV wins on daily accuracy if set up properly — and that's a big if.