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

Had exactly this issue when I first set up my system. Short answer: trust the BMV-712, but only if it's been properly configured.

The Daly BMS SOC is notoriously unreliable — it resets on every charge cycle and doesn't account for Peukert losses properly. It's really just there to protect the cells, not to give you accurate state of charge readings.

The BMV-712 tracks coulombs in and out continuously, so as long as you've set these correctly it's far more accurate:

• Battery capacity (Ah)
• Charged voltage threshold
• Tail current %
• Peukert exponent (for your specific chemistry)

One thing that catches people out — the BMV needs a proper full charge cycle to "sync" and set its 100% reference point. If you've never let it reach the charged voltage + tail current condition simultaneously, it may never have synced, which throws everything off.

What battery chemistry are you running? LiFePO4 flat discharge curve makes coulomb counting essential since voltage alone tells you almost nothing useful between about 20–90% SOC.

Also worth checking your shunt is on the negative terminal with all loads and charge sources on the battery side of the shunt — any stray currents bypassing it will cause drift over time.

Others here probably have thoughts on this too — @anyone running dual BMS + BMV setups, have you found a sweet spot for the sync settings?

@CotswoldNomad makes a solid point about configuration being everything with the BMV-712.

Had a similar situation on my boat build last spring — the Daly was confidently telling me I had 67% remaining whilst the BMV was screaming at 31%. Trusted the Daly. Ended up with a very flat Fogstar battery and a rather embarrassing call to the marina.

The key thing worth adding: your BMV-712 is only as trustworthy as its synchronisation history. If it hasn't hit a proper full charge recently and logged that as 100%, it'll drift over time.

A few things to check:

• Charged voltage threshold set correctly for your chemistry
• Tail current configured (typically 4% of capacity for LiFePO4)
• Peukert exponent set to 1.05 for lithium

Once those are dialled in, the BMV becomes genuinely reliable. The Daly is really just protection circuitry with a guess bolted on.

Great points from @CotswoldNomad and @WattAMess25. One thing worth adding — the BMV-712's accuracy hinges heavily on nailing your Peukert exponent and charge efficiency factor for your specific battery chemistry. LiFePO4 users often leave these at lead-acid defaults, which throws the SOC right off.

Also worth doing a full synchronisation cycle — let the battery discharge to your low voltage cutoff, then charge completely until the tail current drops to whatever you've set (typically 1-2% of capacity). The BMV will sync to 100% at that point and recalibrate itself.

The Daly's SOC is really just a rough guide between resets — fine for basic protection duties but not something I'd rely on for day-to-day capacity monitoring. Once your BMV is properly dialled in, the two readings will still differ, but the BMV will be the one to trust. 👍

The BMV-712 is only as clever as the Peukert exponent and charge efficiency factor you've fed it — wrong numbers in, garbage SOC out, same as any spreadsheet you've ever trusted with your tax return.

On my narrowboat I run both but treat the Daly figure the same way I treat weather forecasts: mildly interesting, ultimately fictional. Key thing nobody's mentioned yet — synchronisation events are everything. Your BMV-712 resets to 100% SOC when it hits your configured absorption voltage and the tail current drops below threshold. If those two parameters aren't dialled in for your specific Fogstar cells, it'll declare "full" about 20% too early and the whole calibration drifts from there.

Check your Charged voltage and Tail current settings in VictronConnect before blaming the hardware itself.

Really useful thread — just what I needed.

Can I ask a practical question though? My BMV-712 is fitted in my van conversion and I'm still dialling in the settings. How do I actually know when it's done a proper full synchronisation? I've read it needs to hit the "charged voltage" threshold and hold it for a set time, but I'm never quite sure if it's actually triggered or not.

Also, does the tail current setting make a big difference for LiFePO4? I've got Fogstar Drift 100Ah cells and I've seen wildly different recommendations — some say 1% of capacity, others go lower.

@ExPostie86 — you mentioned Peukert exponent; for lithium I've seen people just set it to 1.05 or even 1.00. Is there a meaningful difference in practice with a relatively small bank like mine?