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

Been through exactly this on the narrowboat last winter — drove me absolutely mad for about three weeks.

Short answer: trust neither blindly, but understand what each one is actually measuring.

The BMV-712 is coulomb counting from a shunt — it's tracking electrons in and out, and it drifts over time if your shunt calibration or the "charged voltage" / "tail current" settings are off. Mine was wildly optimistic because I hadn't set the tail current correctly for my Fogstar Drift 200Ah cells.

Your BMS, on the other hand, is typically estimating SOC from cell voltages, which on LiFePO4 is notoriously flat across most of the usable range. It's essentially guessing between about 20% and 80%.

What I ended up doing:

• Got the BMV-712 settings dialled in properly (there's a brilliant Victron community thread on LiFePO4-specific settings)
• Let the BMV sync at full charge regularly — it self-corrects when it hits your "charged" parameters
• Stopped obsessing over the BMS percentage mid-cycle

After a full charge cycle where both reset, they typically agree within 3-4% now. If they're wildly different after a full charge, something's genuinely wrong — either a drifting shunt reading or a cell imbalance the BMS is reacting to.

What BMS are you running? Some of the cheaper units have really poor SOC algorithms. Also, what are your BMV sync settings currently set to?

Would be good to hear if others on LiFePO4 have cracked this — particularly anyone with Daly or JK BMS units, which seem to behave quite differently from each other.

@PeteJames nailed the framing there. Here's what pushed me to finally understand it properly on my own setup:

The BMV-712 is coulomb counting from a reference point — so any error in your shunt calibration or your synchronisation SOC compounds over time. Mine was reading 4% optimistic after a fortnight without a proper full-charge sync.

The BMS, meanwhile, is typically estimating from cell voltages — which are notoriously flat across the middle of LiFePO4's discharge curve. So at 50-80% it's essentially guessing.

The answer I landed on: use the BMS as your low-voltage protection and the BMV-712 as your day-to-day gauge, but religiously let it sync at 100% during absorption. Victron's charge detection settings matter enormously here — tail current threshold especially.

Get those wrong and both devices will confidently lie to you simultaneously.

The BMV-712 counts coulombs like a paranoid accountant, while the BMS is basically just checking its bank balance at the voltage ATM — both useful, both lying to you in different ways when it suits them. 🏦

For my garden office setup I run them together and let the Victron MPPT data act as the tiebreaker when they disagree — which on a Fogstar Drift 200Ah is more often than I'd like to admit. Calibrate your BMV-712 at a proper full charge (absorb → float transition, not just "looks about right") and suddenly the two numbers get a lot friendlier with each other.

Right, so this is exactly my situation on the narrowboat — two different numbers staring at me and no idea which to act on.

Quick question for the thread: does the discrepancy tend to get worse over time, or does it stabilise once the BMV-712 has had a few full charge cycles to calibrate against?

I've got a Fogstar Drift 200Ah LiFePO4 and the built-in BMS, paired with a BMV-712 that's only been running about six weeks. Currently showing roughly 8% difference between the two readings — BMV lower than BMS.

Also wondering whether the charged voltage and tail current settings on the BMV are worth tweaking first before assuming one is more accurate than the other. Mine are still on defaults which I suspect might be the actual problem here rather than anything fundamentally wrong with either device.

Been there with my van conversion — here's the practical bit nobody mentions:

The BMV-712 drifts over time because it relies on a accurate shunt calibration and a correctly set charge efficiency factor. If yours is reading low, check your Charge Efficiency Factor in the settings — LiFePO4 wants it at 99%, not the default lead-acid figure.

The BMS SOC is only as good as its voltage-to-SOC curve, which on flat-topped LFP cells is notoriously unreliable between 20–90%.

My actual workflow on the motorhome:

1. Let the bank reach a proper full charge (absorption complete, tail current hit)
2. The BMV-712 resets to 100% — this is your sync point
3. From there, trust the BMV for coulomb counting day-to-day

@FinnTaylor78 — is your Victron actually hitting that sync point regularly? That's usually where the drift originates.