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

Had this exact situation last winter with my static caravan setup. The BMV-712 was showing 74% whilst the BMS on my Fogstar Drift 200Ah was reading 61%. Drove me absolutely mad for a couple of weeks.

After a fair bit of digging, I landed on trusting the BMV-712 for day-to-day monitoring but keeping a close eye on the BMS for protection events. Here's my thinking:

• The BMV does proper coulomb counting — it's tracking every amp in and out
• Most built-in BMS displays use a simpler voltage-based estimate, which is notoriously wobbly mid-charge
• That said, the BMV needs a good synchronisation point (a genuine full charge to 100%) to stay accurate over time

My culprit turned out to be tail current settings. My BMV was syncing to 100% too early because the tail current threshold was set too high. Once I dropped it to around 1–2% of battery capacity, the two started agreeing much more closely.

Worth checking:

1. Are your charged voltage and tail current settings in the BMV actually matching your battery's spec?
2. When did you last do a proper full absorption charge to let it sync?

Curious whether anyone else has found the BMS to be the more accurate one in certain setups — particularly with newer smart BMS units that do their own coulomb counting. Would be good to compare notes.

@SparkySparky — classic discrepancy, and it's more common than people realise!

The root cause is usually that the two devices are measuring fundamentally different things. The BMV-712 uses coulomb counting from a reference point (your last "full" synchronisation), whilst the BMS is typically doing voltage-based estimation with its own internal model.

Key question: when did your BMV-712 last synchronise? If your battery never quite reaches the tail current threshold for a proper full-charge sync, the coulomb counting drifts progressively over time.

My experience with my own Fogstar/Victron setup — I'd trust the BMS's lower reading as the more conservative figure until you've done a proper calibration cycle. Better to have capacity in reserve than to over-discharge.

Check your BMV's "charged voltage" and "tail current" settings in VictronConnect — they're often slightly off from Fogstar's recommended charge parameters.

@SparkySparky — interested in this because I'm running a similar setup in my motorhome. Quick question: had you done a full calibration cycle on the BMV-712 recently?

I ask because I noticed mine drifted noticeably over about 3 months without a proper sync point. The BMV needs to see a genuine 100% full charge to reset its Peukert calculations, otherwise the coulomb counting just accumulates error.

What are your absorption/float settings on the charger side? If the BMV isn't registering a proper tail current cutoff, it'll never sync correctly.

Also — are both devices measuring at exactly the same point in the circuit? Even a small difference in shunt placement versus where the BMS measures internally can cause persistent discrepancies like that 13% gap you're seeing.

@SparkySparky the BMV-712 is only as good as its calibration. If it hasn't seen a proper full charge cycle to reset its 100% reference point, it drifts — sometimes badly. The BMS is reading cell voltages directly, so it's generally more honest about actual state.

My workflow on the boat: I trust the BMS for real-time protection and the BMV for trend monitoring. When they disagree significantly, I do a full absorption charge until the BMS shows cells balanced, then let the BMV resync.

Also worth checking:

• Peukert exponent setting on the BMV (Fogstar LiFePO4 wants this close to 1.00)
• Charge efficiency factor — should be around 99% for lithium
• Tail current threshold for the "charged" detection

Wrong defaults on those three will cause exactly the discrepancy you're seeing.

@RayWatson81 makes a good point about calibration, but I'd also look at the shunt placement. Had a similar headache with my setup — turned out I had a small parasitic load bypassing the shunt entirely, so the BMV never saw the full picture. The SOC drift just compounds over time.

In my experience with LiFePO4, I'd lean toward the BMS reading for actual cell-level state, but neither is gospel if the fundamentals aren't right. The BMV is brilliant once properly configured — make sure your Peukert exponent and charge efficiency factor are set correctly for lithium, not left on lead-acid defaults. Victron's defaults are way off for LiFePO4 out of the box.