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

Had this exact head-scratcher when I first wired up my tiny house setup. Both devices convinced me they were right, which meant one of them definitely wasn't.

The short answer: trust the BMV-712, but only once it's properly calibrated.

The Daly BMS calculates SOC based on cell voltages, which sounds sensible until you realise LiFePO4 has an almost painfully flat discharge curve. Voltage-based estimation across the middle 80% of charge is basically guesswork dressed up as data.

The BMV-712 uses coulomb counting — it's actually measuring current flowing in and out, like a very precise fuel gauge. Far more reliable day-to-day. The catch is it needs a solid sync point to anchor itself. Mine was wildly off until I let the bank charge fully and held at absorption long enough for the BMV to register a "full" state and reset. After that, the two devices sat within 3-4% of each other most of the time.

A few things worth checking:

• Is your shunt wired correctly with all loads and charge sources passing through it?
• Have you set the Peukert exponent and charge efficiency factor properly in VictronConnect?
• What's your Charged voltage threshold set to?

The Daly will still be useful — it's your protection layer, not your fuel gauge. Let it handle the cell-level safety stuff whilst the BMV handles your daily SOC monitoring.

Curious what your charged voltage and tail current settings look like — that's usually where the drift creeps in for most people.

@ZI_Sparks nailed it, but worth understanding why the BMV wins this argument.

The Daly BMS reads voltage-based SOC — notoriously inaccurate mid-cycle on LiFePO4 because the discharge curve is so flat. It's essentially guessing.

The BMV-712 uses coulomb counting — it measures actual electrons in and out via the shunt. Far more accurate in practice, provided you've configured it correctly:

• Battery capacity set accurately
• Charged voltage threshold correct for your chemistry
• Tail current properly calibrated
• Peukert exponent set (1.05 for LiFePO4)

On my narrowboat setup the Daly was regularly showing 65% when the BMV had 48% — the Daly had never properly synced after a partial charge cycle.

The BMS's SOC reading is really just there to trigger cell-level protection. For actual state of charge monitoring, the BMV shunt is the authority.

Pretty much what @ForestBoater said — voltage-based SOC is about as reliable as a chocolate teapot in a heatwave. Your cells spend most of their life in the flat middle section of the discharge curve where voltage tells you almost nothing useful.

The BMV-712 is counting coulombs in and out — actual electron accountancy. Still needs

Good thread — had exactly this confusion when I first set up my motorhome system with a Fogstar lithium bank.

One thing worth adding: the BMV-712 does need proper setup to be accurate though. If your battery capacity, Peukert exponent, charge efficiency factor, and tail current settings aren't dialled in correctly, it'll drift over time too.

Can I ask @ZI_Sparks — are you running LiFePO4 or lead-acid? The synchronisation trigger settings are quite different between the two. With LiFePO4 especially, getting the "charged voltage" threshold right is crucial, otherwise the BMV never resets to 100% properly and the drift compounds.

What capacity have you entered in the BMV settings, and does it match your actual usable capacity rather than the nameplate figure?

Good points all round from @ForestBoater and @WattLiz. One thing I'd add — the BMV-712 is only as good as its setup. If your battery capacity, Peukert exponent, charge efficiency factor, and tail current aren't configured correctly for your specific cells, it'll drift over time just like anything else.

With LiFePO4 I'd recommend setting Peukert to 1.05 and charge efficiency to 99%. Also make sure you've set a sensible charged voltage and tail current so the BMV knows when to call it 100% and resync itself.

Do a full charge cycle after configuring everything and let it resync properly — after that it'll track brilliantly for weeks between resyncs. The Daly is still worth watching for cell-level balance warnings though; that's where it earns its keep. Two tools, different jobs! 🔋