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

Had exactly this situation on my narrowboat last year — Daly was showing 74% whilst the BMV-712 was reading 61%. Drove me mad for weeks.

Short answer: trust the BMV-712 for day-to-day SOC, but use the BMS as your safety net rather than your gauge.

The Daly calculates SOC using a fairly basic algorithm and it can drift badly if it doesn't get a proper full charge to reset itself. It's also not accounting for Peukert losses the same way Victron does.

The BMV-712 wins because:

• It's measuring actual coulombs in and out via the shunt
• Peukert compensation is configurable
• Tail current synchronisation means it resets properly at full charge

A few things worth checking though:

1. Is your charged voltage threshold set correctly in the BMV? If it's syncing at 95% rather than 100%, your readings will always be off
2. Shunt sizing — are you running everything through it, or have you got something wired directly to the battery bypassing the shunt?
3. Capacity setting — does the BMV actually know your real usable capacity?

On my setup I've got 200Ah of LiFePO4 (Fogstar cells) and once I sorted the tail current settings the BMV barely drifts between cycles now. The Daly still occasionally shows something wildly optimistic after a partial charge.

What BMS firmware are you running and have you tried recalibrating the Daly's SOC manually to match? Sometimes a hard reset helps temporarily, though it'll drift again if the root cause isn't sorted.

What's your charging source — solar, alternator, shoreline?

@JulieAllen great thread — one thing worth adding is that the BMV-712 is only as good as its calibration. Make sure you've got your battery capacity, Peukert exponent, and charge efficiency factor set correctly for your specific cells. A lot of people leave these at Victron defaults and then wonder why the coulomb counting drifts over time.

Also worth doing a proper full charge synchronisation — let the battery reach absorption voltage and hold it until tail current drops below your configured threshold. That's when the BMV resets to 100% and gives you a reliable baseline to count down from.

The Daly BMS is primarily protecting your cells rather than tracking SOC accurately — it's doing a different job entirely.

@VoltDoug dead right on calibration — and the bit people always skip is setting the charged voltage threshold and tail current properly in the BMV settings. Factory defaults are often way off for lithium.

My shepherd's hut setup ran screwy SOC readings for months before I realised my tail current was set for lead acid. Bumped it to around 4% and suddenly the BMV started syncing properly at full charge.

Also worth checking — is your Daly actually reading cell voltage and extrapolating? That'll drift all over the place depending on your specific cell chemistry. The BMV counting coulombs in/out is just more reliable in practice.

One more thing: make sure nothing else is pulling current outside the shunt. Classic wiring mistake that throws everything off.

@JulieAllen @PeakVanLifer good points all round. One thing I'd add that catches people out — the BMV-712 resets its 100% sync point when it sees the battery hit that charged voltage threshold and holds it for the tail current duration you've set. If your charger is undersized or you're cutting shore power too early, it may never actually complete that sync, so the drift compounds over time. I had this exact issue on my setup last winter. Worth checking your history data on the BMV app — if your "synchronisations" count isn't ticking up regularly, that's your culprit right there. On a narrowboat especially, with engine alternator charging being fairly inconsistent, you can go weeks without a proper sync without realising it.

@Ken1999 raises a good point on the sync trigger — worth emphasising that the tail current setting is just as critical as the charged voltage threshold. Default is 4% but on LiFePO4 that's often too high and you'll get premature syncs.

From my own garden office setup with a Fogstar Drift 200Ah — took me a good 3–4 full cycles before the BMV-712 really settled down and tracked accurately. The Daly BMS SOC is basically a voltage lookup table dressed up as coulomb counting. It's fine for protecting the cells but I wouldn't rely on it for real-world SOC.

One thing nobody's mentioned: check your shunt wiring. Any loads bypassing the shunt will silently corrupt the BMV readings and you'll spend weeks blaming calibration when the wiring's the actual culprit.

@JulieAllen this mirrors exactly what happened when I set up my motorhome rig last spring. One thing nobody's mentioned yet — Peukert's exponent in the BMV-712 settings. If you're pulling heavy loads, an incorrectly set Peukert value will cause the BMV to drift pessimistically, making that gap with the Daly look even wider than it actually is.

For LiFePO4 the Peukert exponent should be set very close to 1.05 (some people go as low as 1.02). The default is tuned more for lead-acid and will skew your readings noticeably under load.

Worth checking what yours is set to — mine was still on the factory default when I first noticed my readings diverging. Quick Settings → Battery → Peukert Exponent in VictronConnect sorted it.

Does your Daly BMS report coulomb counting or is it just inferring from cell voltage?