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

Running into exactly this on my narrowboat at the moment — Victron BMV-712 sitting at 74% whilst the BMS on my Fogstar Drift 200Ah is claiming 81%. Seven percent difference isn't massive but it's enough to make me second-guess when I should be running the generator.

From what I've read, the BMV works purely on coulomb counting — it tracks every amp in and out from a known reference point. The BMS SOC is often calculated differently, sometimes using cell voltage curves which can be notoriously inaccurate in the flat middle section of a LiFePO4 charge curve.

A few questions I'm trying to work out:

• Which one resets its reference point more reliably after a full charge?
• Does the BMV drift over time if you never hit 100% SOC?
• Is there a way to get the two talking to each other via the Victron ecosystem to at least agree on a figure?

I've heard the BMV can be told to sync when the battery hits a certain tail current, but I'm not sure if my settings are right for lithium. Running a Victron MPPT and a MultiPlus-II if that's relevant.

Anyone else running a dual-readout setup like this? Which figure do you actually trust day-to-day?

@BerlingoConvert classic calibration drift situation — seen it on my own Vivaro setup with Fogstar cells.

The short answer: trust the BMV-712 long-term, but the BMS short-term.

The BMV uses coulomb counting, so it accumulates small errors over time. The key fix is ensuring your tail current and charged voltage settings are dialled in correctly — if it's not registering a proper full charge event to reset itself, the drift compounds.

Check these BMV settings specifically:

• Charged voltage: ~0.2V below absorption voltage
• Tail current: around 4% of capacity (8A for 200Ah)
• Charge detection time: 3 minutes minimum

Once the BMV hits those thresholds together, it resets to 100% and recalibrates. If you're rarely hitting full absorption, that's likely your culprit.

What charger/solar controller are you running?

@BerlingoConvert worth checking whether your BMV-712 has been properly configured for your actual battery capacity and the Peukert exponent. Mine was wildly off until I adjusted the charged voltage threshold and tail current settings to match the Fogstar Drift's actual charge profile.

Also — has the BMV had a proper full synchronisation cycle recently? If it hasn't seen a genuine 100% reference point (sitting at absorption/float with tail current dropping below your set threshold), it'll drift further and further from reality over time.

The BMS figure is calculated from cell voltages whereas the BMV is coulomb counting, so neither is perfect. Which one drifted first after a full charge? That usually tells you which is losing its reference.

@BerlingoConvert had almost exactly this on my narrowboat last summer — what sorted it for me was doing a proper full charge cycle all the way to absorption and float, then letting the BMV-712 sync to 100% from there. If it never hits that trigger point it just keeps drifting.

Worth checking your charged voltage and tail current settings in the BMV config — mine were slightly off from factory defaults which meant it wasn't registering a "full" charge correctly. What Victron settings are you currently running for those two parameters?

Also curious whether your Fogstar BMS is reading cell voltage directly or doing its own coulomb counting — because if it's purely voltage-based it can be wildly optimistic at higher SOC levels given how flat LiFePO4 curves are.

@BerlingoConvert the key thing nobody's mentioned yet: the BMV-712 is a coulomb counter — it tracks electrons in and out — whilst your Fogstar BMS is inferring SOC from cell voltage curves. Neither is definitively "correct," but they fail in different ways.

The BMV drifts over time if your charged voltage threshold isn't triggering the sync properly. Check your "charged voltage" setting — for a 12V LiFePO4 it wants to be around 13.4V with a small tail current defined. If it's never fully syncing, error compounds.

The BMS voltage-based reading is notoriously unreliable in the middle of the LiFePO4 plateau (roughly 20–80%) — that flat curve makes voltage a poor proxy for capacity.

Practically speaking, trust the BMV for daily usage tracking once properly calibrated, and treat the BMS figure as a rough sanity check only.

@DevonDweller nailed the coulomb counter bit, but here's the dirty secret nobody wants to admit — both are wrong, just differently wrong.

BMS SOC is typically voltage-interpolated guesswork dressed up as precision; BMV-712 drift accumulates over weeks without a proper sync point. My motorhome Fogstar/Victron setup disagreed by 11% until I sorted the Peukert exponent and charged efficiency factor in the BMV settings — most people leave those at defaults, which were designed for lead-acid. For lithium you want Peukert around 1.05 and charge efficiency closer to 99%. Once the BMV gets a clean 100% sync after a genuine full absorption, it'll track brilliantly and you can largely ignore whatever the BMS is claiming — that figure's really just for cell-level protection anyway, not accurate state reporting.