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

Had this exact argument with my boat setup — spoiler: trust the Victron.

The Daly BMS is great at protecting your cells but its SOC estimation is basically a rough guess dressed up in electronics clothing. It resets on voltage thresholds which goes completely sideways once your cells age a bit and those voltage curves flatten out.

The BMV-712 does proper coulomb counting — it's actually measuring current in and out over time, so as long as you've got your battery capacity, Peukert exponent, and charge efficiency factor dialled in correctly, it'll run rings round the Daly for accuracy.

Key things to check on your Victron setup:

• Charged voltage set correctly (not too high or it won't sync properly)
• Tail current — I'd go around 4% for LiFePO4
• Battery capacity — be honest, don't just punch in the rated figure

The BMV will drift a bit if it never gets a proper full sync, so make sure it's actually hitting that charged voltage threshold regularly to reset itself.

Anyone else on here running dual SOC displays and just... picking the one showing the higher number when they fancy it? 😅 Curious whether others have found a workflow for cross-referencing the two when something looks off — particularly on LiFePO4 where the voltage plateau makes everything look like 80% for an annoyingly long time.

@PartnerCamper — Daly's SOC is basically that one mate who confidently gives you directions and gets you lost every single time. 🗺️

My Fogstar cells paired with the BMV-712 sorted this exact headache — once you've properly set the charged voltage, tail current, and Peukert exponent in the Victron, it's genuinely uncanny how accurate it stays even after a week off-grid in the van.

The Daly's doing its real job keeping your cells from going pop, let it crack on with that and leave the actual bookkeeping to the BMV.

@ExBrickie94 that analogy is painfully accurate 😄

Right, story time — I spent about three weeks on the cut last summer watching my Daly and BMV-712 argue like an old married couple. Daly was insisting I had 67% left, Victron was showing 41%. Trusted the Daly, ran my inverter hard charging the EV overnight... and woke up to a low-cell alarm at 6am.

The BMV-712 wins because it's doing proper coulomb counting with a calibrated shunt — it's measuring actual current in and out. The Daly is essentially making educated guesses based on cell voltage, which is notoriously flat through most of a LiFePO4 charge curve.

One tip — make sure you've got your battery capacity and Peukert settings dialled in on the Victron side, otherwise even the BMV will drift over time. Victron's own documentation covers this really well.

@PartnerCamper is right, but worth understanding why the discrepancy exists rather than just blindly trusting one over the other.

The Daly calculates SOC primarily from cell voltage, which is notoriously flat across the middle of a LiFePO4 discharge curve — you can swing 40% actual capacity with barely 50mV movement. It's genuinely poor resolution.

The BMV-712 uses coulomb counting — it's measuring actual current in/out via the shunt, so it's tracking real energy throughput. Combine that with Victron's charged detection algorithm and it self-corrects at every full charge cycle.

Key thing to nail down: ensure your BMV shunt is positioned correctly (between the negative busbar and everything — no loads or charging sources bypassing it), otherwise the coulomb counting drifts badly. Learned that the hard way with my own Fogstar Drift 200Ah build — had a bilge pump wired the wrong side for months.

@Taffy29 nails it — the why matters enormously here.

The Daly's SOC is voltage-based, which on LiFePO4 is almost meaningless across that brutally flat discharge curve between 20–80%. You're essentially trying to read a ruler that's 90% the same measurement.

The BMV-712 does Coulomb counting — it's literally tracking every amp-hour in and out, calibrated against a proper shunt. Mine's a 500A Victron shunt wired tight to the negative busbar with nothing bypassing it. Get that installation right and the BMV stays within 2–3% across a full cycle.

Key caveat though: the BMV drifts if it never sees a proper full charge to re-sync. My Fogstar Drift cells hit absorption and the BMV resets — that's your anchor point. Without that periodic sync, even Coulomb counting accumulates error over weeks.

Both devices together tell the full story. Neither alone tells the truth.