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

Had this exact headache last year. Short answer: neither, fully.

Both are estimating, just differently. The BMV-712 does coulomb counting from a shunt — dead accurate in the moment but drifts over time if you don't let it sync to 100% properly. Most BMS units use voltage-based SOC curves which are notoriously optimistic, especially with LiFePO4's flat discharge curve.

What I do with my cabin setup:

• BMV-712 for day-to-day monitoring — it's the more trustworthy of the two once calibrated correctly
• BMS for protection (that's its actual job anyway)
• Let the BMV sync to 100% properly — mine needs a proper absorb cycle to reset, otherwise it wanders off

The key question is: have you set your battery capacity correctly in the BMV? I was running Fogstar Drift cells and had the capacity slightly wrong. Took me weeks to realise why the numbers never matched.

Also check your charged voltage threshold and tail current settings in the BMV — these determine when it calls the battery full and resets the counter. Most people leave them at defaults which are often wrong for their specific cells.

What BMS are you running? Some of the cheaper units have genuinely terrible SOC algorithms. If it's a JK or Daly the behaviour can vary quite a bit between firmware versions too.

Anyone else had persistent drift issues they've actually solved? Curious what worked.

@GrumpyBuilder nailed the drift issue. Worth adding — the BMV only resets/syncs when it hits your set tail current at 100% charged. If you're never fully topping off (common with solar in UK winters), it'll drift badly over weeks.

My setup: Fogstar Drift 200Ah with a Daly BMS. I trust the BMV for day-to-day, but I also watch cell voltages on the BMS when it matters. If they disagree by more than 5-6%, something's off with your BMV calibration — check your charged voltage and tail current settings first.

Honestly the bigger issue is most people set the tail current too low. Victron recommend 4% of capacity as a starting point.

Good thread. One thing I'd add — what's your battery chemistry? I've got LiFePO4 (Fogstar Drift cells) and the flat voltage curve makes the BMV's voltage-based sync almost meaningless. It'll sit at 13.3v whether you're at 40% or 85%.

In practice I've ended up using the BMS SOC as a rough guide for day-to-day, but watching the BMV's Ah consumed figure as the more trustworthy real-time number. The actual percentage readout on both I basically ignore unless I've just come off a full charge cycle.

Has anyone found the Victron VictronConnect SOC calibration settings make a meaningful difference? Wondering whether tweaking the charge efficiency factor away from the default 95% would help the drift on LiFePO4 specifically.

@George1972 raises the chemistry point and it's the crux of it really. LiFePO4's flat discharge curve makes voltage-based SOC readings next to useless across most of the range — your BMS is essentially guessing from 20% to 80%.

In my motorhome I run a BMV-712 and learned the hard way that the real answer is to trust whichever one you've calibrated properly. Spent a winter in the Brecon Beacons relying on a poorly configured BMV and nearly killed my Fogstar cells through over-discharge because the sync thresholds were all wrong.

Set your tail current and absorption voltage correctly in the BMV, let it sync on full charges religiously, and it'll earn your trust. The BMS is your safety net, not your fuel gauge.

@ExmoorCamper this is exactly my situation on the boat — LiFePO4 (Victron Smart Lithium) and I've been pulling my hair out trying to work out why they drift apart over time.

Quick question though: does the BMS SOC reading even mean the same thing as the BMV's? My BMS seems to reset to 100% based on cell voltage thresholds, whereas the BMV needs that tail current sync. Are they effectively measuring different endpoints?

Also wondering if anyone's had success adjusting the Peukert exponent on the BMV-712 for marine use — I'm seeing quite variable loads (bow thruster spikes vs. slow overnight drain) which I suspect is making the coulomb counting less reliable than it should be.

The BMV wins the daily trust war in my van, but I give it a "sync" every few weeks by letting the bank hit 100% absorption properly — Victron calls it a synchronisation point and it yanks the coulomb counter back to reality before the drift becomes embarrassing.