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

Been through exactly this with my van build last spring. Had a Fogstar Drift 200Ah LiFePO4 paired with a BMV-712, and the two were regularly disagreeing by 8-12% — maddening when you're trying to judge whether you've got enough juice to run the diesel heater overnight.

Here's what I eventually worked out:

The BMS is watching voltage and cell-level data in real time, but its SOC calculation is often quite basic — many budget BMS units just use voltage curves, which are notoriously flat on LiFePO4 and therefore nearly useless mid-range.

The BMV-712 uses coulomb counting — it's literally measuring every amp flowing in and out. Far more accurate provided you've set it up properly. The critical bits:

• Correct battery capacity entered
• Charged voltage threshold set right (I use 13.8V for a 12V LiFePO4)
• Tail current set around 4% for LiFePO4
• Peukert exponent dropped to ~1.05

Once you've nailed those settings and let it complete a few full charge cycles to synchronise, the BMV-712 will track accurately and essentially self-correct at top of charge each time.

The BMS SOC reading is more of a safety-net figure — trust it when it screams (low voltage cutoff, cell balancing alerts), but for day-to-day energy management, the Victron wins every time.

Anyone else found the sync interval makes a big difference? Curious whether others are tweaking the tail current percentage or leaving it default.

Hey @MarinePhil, I had almost identical grief with my narrowboat setup last year! One thing worth checking that nobody mentions enough — have you set the Peukert exponent correctly in the BMV? For LiFePO4 it should be very close to 1.05 rather than the lead-acid default. Also, your charged voltage threshold and tail current settings massively affect when the BMV resets to 100%. If it's not syncing properly at full charge, the error just compounds over time.

Generally I'd lean on the BMS for cell-level protection decisions, but trust the BMV-712 more for accurate capacity tracking once it's properly calibrated — it's doing proper coulomb counting rather than just estimating from voltage.

What's your charge absorption voltage set to? That's often where the mismatch originates.

@MarinePhil the disagreement you're describing is almost certainly a synchronisation issue. The BMV-712 is coulomb-counting from a reference point, so if it never sees a proper full charge (tail current dropping to ~4% of capacity), its 100% anchor drifts. Meanwhile your BMS is reading cell voltages directly.

On my narrowboat I trust the BMS for instantaneous state and the BMV for trend — but only after forcing a manual sync by holding at absorption voltage until the tail current genuinely collapses.

Worth checking in VictronConnect:

• Charged voltage threshold set correctly for LiFePO4 (13.4V-ish, not a lead-acid figure)
• Tail current percentage appropriate for your 200Ah bank
• Peukert exponent — set it to 1.05 for lithium, not the default

Once those three are dialled in properly the two readings should converge within 2-3%.

Really useful thread this — exactly the kind of thing that trips people up when first pairing a quality coulomb counter with a smart BMS.

One thing worth adding: peukert exponent settings in the BMV-712 are often overlooked. LiFePO4 behaves very differently to lead-acid here — the default Peukert value of 1.25 is wrong for lithium and will skew SOC calculations under load. Set it to 1.05 for LiFePO4.

Also check your charged voltage threshold and tail current settings in the BMV. These determine when the BMV decides the battery is "full" and resets to 100% — if they're misconfigured, synchronisation never happens properly.

From my static caravan setup with a 400Ah Fogstar bank, once I dialled those parameters in, my BMV and BMS rarely disagree by more than 2-3%.

@MarinePhil — welcome properly to the forum, great first technical thread!

Had this exact drama on my boat last winter — nearly discharged down to nothing because I trusted the wrong number at the wrong moment. Classic.

The thing that sorted it for me was being really pedantic about the tail current setting in the BMV. My Fogstar BMS was calling "full" at a higher current than the BMV recognised, so they never agreed on the sync point. Once I dropped the tail current threshold to around 1-2% of capacity, they started shaking hands properly.

Also worth checking — is your BMV peukert exponent set correctly for LiFePO4? It should be very close to 1.05, not the default lead-acid value. Had mine at 1.25 for an embarrassingly long time. The BMS won't care about peukert at all, so that gap can look massive under load.

@MarinePhil the key thing people miss is that the BMV-712 needs a properly configured charged voltage threshold to sync correctly. If your Fogstar's BMS is declaring "full" at say 3.45V/cell but your BMV is set to sync at 3.65V/cell, it'll never catch up and drift gets worse over time.

Check your Charged voltage setting in VictronConnect — for LiFePO4 I run mine about 0.2V below the actual absorption voltage. Also nail down your Tail current setting; too high and it syncs prematurely.

In my setup I actually trust the BMV long-term but use the BMS as the safety floor. They're doing different jobs really — the BMS is protection, the BMV is tracking. Once I got the sync parameters dialled in they agreed within 2-3% consistently.