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

Running into exactly this at the moment with my own setup, so watching this thread closely.

My BMV-712 and the BMS on my Fogstar Drift 100Ah are regularly disagreeing by anywhere between 8–15%. Frustrating when you're trying to work out whether you've got enough juice left to top up the van battery or run the evening charge on the car.

From what I've read, the BMV does its coulomb counting from a fixed reference point, so any error in the shunt calibration or an incorrectly set Peukert exponent compounds over time. The BMS meanwhile is reading cell voltages directly, so it should know what's actually in the pack — but voltage-based SOC on lithium is notoriously flat in the mid-range, which makes it pretty unreliable too.

A few questions for the more experienced folks here:

• Does periodically syncing the BMV (letting it hit 100% at a proper full charge) bring the two closer together over time?
• Which figure do you actually use for day-to-day decisions?
• Is there a recommended Peukert value for LiFePO4 in the BMV settings? I've seen people say 1.05 but I'm not sure if that's universal

I've got a Victron MPPT and a Multiplus on the same VE.Smart network, so in theory everything should be talking to each other — but the discrepancy is still there.

Would be really useful to hear from anyone who's resolved this rather than just learned to live with it.

Trust the BMS, @NeilEdwards — it's sat right on the cells watching voltage chemistry like a paranoid mother-in-law, whereas the BMV-712 is essentially just doing maths and hoping you remembered to tell it your battery capacity correctly.

Classic culprit on my static is the Peukert exponent being slightly off in the Victron settings — mine was making the BMV think my Fogstar was a tired old AGM from 2003. Dial that in properly and they'll start agreeing with each other like old friends who've had a pint.

Also worth checking your charged voltage and tail current thresholds in the BMV settings — if it's not hitting a proper sync point to reset to 100%, the drift compounds over time and you end up chasing ghosts.

@NeilEdwards both are lying to you, they've just agreed to disagree — recalibrate the BMV-712 at a known 100% full charge and let it relearn, because a drifting shunt is basically a maths teacher who's forgotten how to count.

There's a chapter in my boat-build story that sounds exactly like this. Spent three weeks convinced my Fogstar was dying because the BMV-712 and BMS were telling completely different tales.

Turned out my Peukert exponent was set wrong in the BMV — it was configured for AGM, not lithium. Lithium wants something much closer to 1.05, whereas the default often sits around 1.25. That single setting was responsible for most of the drift I was seeing, especially under heavier loads when the Peukert correction really bites.

Worth checking your charged voltage threshold and tail current settings too — if the BMV isn't recognising a genuine full charge event, it never gets a clean synchronisation point to anchor itself to.

Both @MoorRuss and @GlenSimon make fair points, but sometimes the disagreement is the BMV miscalculating rather than either device being fundamentally wrong.

@NeilEdwards had this exact tension with my static caravan setup for a good month or two. The piece nobody mentions is tail current — if your BMV-712's charge detection threshold is set too generously, it'll call 100% before the Fogstar's BMS agrees the cells are genuinely topped off. That drift compounds over every cycle.

Worth checking your charged voltage and tail current settings in the BMV configuration. Mine needed dropping to around 0.5% of capacity before both devices started telling roughly the same story.

@GlenSimon's point about recalibrating at a known full charge is the right instinct — just make sure you're actually achieving absorption completion first, otherwise you're locking in the error rather than clearing it.