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

Had this exact drama with my motorhome setup last summer — Victron BMV-712 screaming 67% whilst the BMS was sat there confidently declaring 81% like it hadn't a care in the world.

Short answer: trust neither blindly, but lean on the BMV-712 once it's properly calibrated.

Here's my hot take on why they disagree:

• BMS SOC is often just voltage-based estimation, which on LiFePO4 is basically useless given that famously flat discharge curve — it's guessing more than measuring
• BMV-712 uses Coulomb counting (tracking actual amps in/out), so it's mathematically tracking real energy flow — but it drifts over time if your charge efficiency factor and Peukert settings aren't dialled in
• Neither resets properly unless the battery hits a genuine 100% full charge and holds it — if you're partial-cycling like most of us do on solar, both can wander off into fantasy land

My fix was sorting the BMV's settings properly — charge efficiency at 99% for Fogstar Drift cells, Peukert at 1.05 — and making sure it gets a proper synchronisation at least weekly when the sun's being cooperative.

The BMV wins for me because Victron's ecosystem ties everything together nicely through Venus OS, and I can actually see the data rather than trusting a BMS that communicates in shrugs.

What BMS are you running? Some of the better ones (Daly, JK, Seplos) actually do decent Coulomb counting themselves and the gap narrows considerably.

Curious whether anyone's found a reliable way to get both talking to each other and averaging the result — feels like that should be someone

@ShesBeRight BMV-712 every time for me. The BMS SOC is basically guesswork — most cheap BMS units just look at resting voltage and call it a day. Coulomb counting on the Victron is actually tracking what goes in and out.

That said, the BMV needs proper setup to be useful — shunt sized correctly, charge efficiency factor dialled in, peukert exponent not left at default. Most people just plug it in and wonder why it drifts.

Also worth doing a full charge cycle to let it re-sync. Mine on the static van was miles out until I sorted the charged voltage threshold settings.

What BMS are you running? Some of the better ones (Daly, JK) are more reliable than others but I'd still back the Victron.

Both have their blind spots tbh. BMV-712 drifts if your shunt isn't wired correctly — every single DC load must go through it, no exceptions. Missed my 12v fridge circuit for months and wondered why mine was always off.

BMS SOC is usually cell-voltage based so it's more accurate at the extremes (full/empty) but rubbish in the middle.

My workflow now:

• Trust the BMS near 100% and 0%
• Trust the BMV-712 in between
• Force a full sync charge occasionally so they agree

Also worth checking your BMV's charge efficiency factor and Peukert settings — straight out the box they're set for lead acid, not LiFePO4. Fogstar cells behave very differently and the defaults will skew your readings pretty badly.

Been through exactly this on my boat — spent a whole season second-guessing myself between two readouts.

The thing nobody mentions is synchronisation events. The BMV-712 resets its SOC when it hits your configured absorption voltage and tail current simultaneously. If those parameters don't match your actual battery chemistry, it never properly syncs, and the drift compounds over weeks.

Spent an afternoon logging my Fogstar LiFePO4 cells individually whilst watching the BMV readout — turned out my charge voltage was set 0.2V too low, so the BMV was never triggering a full sync. The BMS was calling 100% correctly; the BMV was wandering off into fiction.

Check your charged voltage and tail current settings in VictronConnect first. Nine times out of ten that's the culprit rather than a fundamental disagreement between the two devices.

Had this exact situation with my garden office setup. Ended up with both readings on my phone via VictronConnect and just... staring at them both like a confused dog watching a magic trick. 🐕

Practical fix that sorted mine: sync the BMV-712 to 100% only when you've hit a proper full charge — absorb complete,