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

Had exactly this headache last summer with my static caravan setup! 🤦

So my Fogstar Drift 200Ah was showing 87% on the BMS app whilst the BMV-712 was confidently telling me 74%. Drove me absolutely mental for about a fortnight trying to work out which one to believe.

After a fair bit of digging around, here's where I landed:

• BMV-712 is Coulomb counting — it's measuring actual current in/out, so it's only as good as your calibration and how recently you did a proper full charge sync
• BMS reading is usually voltage-based under the hood, which can be wildly optimistic especially mid-discharge on LFP with that famously flat voltage curve

The BMV won me over eventually, but only once I'd let it fully sync at 100% SOC after a proper absorption cycle. Before that it was drifting all over the shop.

The real villain in my case? My Victron SmartSolar wasn't quite hitting absorption long enough, so the BMV never got a clean sync point. Sorted the charge profile and the two readings came much closer together — within 3-4% now.

Has anyone else found their BMS app tends to read high compared to the BMV? Wondering if this is a Fogstar thing specifically or more universal. And what sync thresholds are people using on their 712s?

@EcoFlow_Nerd this is pretty common and usually comes down to calibration rather than either device being outright wrong.

The BMV-712 works by coulomb counting — it tracks every amp going in and out, so any small measurement drift compounds over time. If it hasn't seen a proper full charge cycle to reset its "100%" reference point, the SOC will wander.

The BMS reading is more likely to be voltage-based, which has its own inaccuracies mid-cycle (especially with LiFePO4's flat discharge curve).

Personally I trust my BMV-712 more once it's been properly synchronised — make sure your tail current and charged voltage settings match your battery spec. Fogstar publish those figures.

If the two are diverging by 13% that's a big gap — worth checking your shunt connections are solid and there's no parasitic draw bypassing it.

@EcoFlow_Nerd the divergence you're seeing is almost certainly because the BMV-712 is coulomb counting from a reference point that doesn't match where the BMS thinks full actually is.

The critical thing people miss: the BMV only resets its 100% SOC when it hits your configured tail current and absorption voltage simultaneously. If your charge profile is even slightly mismatched to what Fogstar considers a genuine full charge, the two will drift apart progressively over weeks.

On my cabin setup I had ~15% divergence until I tightened the BMV's "charged voltage" to within 0.1V of my actual absorption setpoint and dropped the tail current threshold considerably.

Trust the BMV long-term once properly calibrated — coulomb counting from a solid reference beats the BMS's voltage-based estimation, particularly mid-cycle where voltage sag makes SOC estimation genuinely unreliable.

@EcoFlow_Nerd had almost the same thing on my boat last year. The BMV-712 will drift over time if it never hits a proper 100% sync point — basically it needs to see a genuine full charge (absorption and float completed) to reset itself.

Check your charged voltage and tail current settings in the BMV. If those aren't dialled in correctly for the Fogstar Drift's specific parameters, it'll call 100% at the wrong point and everything cascades from there.

Short answer: trust the BMV-712 long-term once it's properly calibrated, it's far more accurate than the BMS percentage which is often just a rough voltage-based guess dressed up as coulomb counting.

What are your current tail current and charged voltage settings? That'll tell us where the problem actually is.

@BayTim is on the right track about the sync point. The thing most people miss is that these two devices are measuring fundamentally different things — the BMS is typically doing voltage-based SOC estimation with some cell chemistry curves baked in, whilst the BMV-712 is integrating current over time (coulomb counting). Neither is inherently "truth."

On my narrowboat with a 400Ah Fogstar bank, I trust the BMV-712 day-to-day but only after ensuring it's properly synchronised. The critical setting is your charged voltage threshold and tail current in the BMV config — get those wrong and it never resets to 100%, drift accumulates, and you end up with exactly the 13% gap you're seeing.

Set tail current to roughly 2-4% of your battery capacity and charged voltage about 0.2V below absorption endpoint. That'll sort the calibration issue without relying on the BMS app figures.

@LH_Marine is spot on about the measurement fundamentals, but there's another angle nobody's mentioned yet — charged current threshold settings in the BMV itself.

By default the BMV thinks the battery is full when current drops below 4% of capacity, but LiFePO4 absorbs differently to lead-acid. If your charge parameters don't match what Fogstar's BMS considers "full", they'll never agree on the sync point.

On my boat I had weeks of this nonsense until I actually sat down with the BMV configuration guide and adjusted:

• Charged voltage threshold
• Tail current percentage
• Peukert exponent (less critical for LFP but still worth checking)

After that my BMV-712 and BMS were within 2-3% consistently. Still trust the BMV more for day-to-day coulomb counting once it's properly synced — the BMS SOC readout on most budget units is frankly a bit optimistic in my experience.