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

Had this exact headache last winter with my shepherd's hut setup. Short answer: neither completely, but here's how I think about it.

The BMV-712 is doing coulomb counting — it's tracking every amp going in and out, so it's very accurate in the moment but it drifts over time if your shunt calibration is off or you've got a parasitic draw it's not seeing. Mine was about 8% out after a few months until I sorted the calibration.

The BMS SOC is typically calculated from cell voltages, which is notoriously unreliable in the middle of a charge cycle — LiFePO4 has that incredibly flat voltage curve, so it can swing wildly and still be "fine."

What I do now:

• Trust the BMV-712 for day-to-day monitoring — it's more stable once dialled in
• Use the BMS as a safety net for protecting against genuine low-voltage cutoffs
• Make sure both sync at 100% together — the BMV needs a proper absorption/float trigger to reset, so check your Victron charge profile is actually hitting that tail current threshold

Fogstar cells seem to exaggerate the discrepancy more than some others I've seen mentioned on here, not sure why. Anyone else noticed that?

Worth checking your shunt wiring too — if anything bypasses it (even a small 12v fan), your BMV will never be accurate.

What battery chemistry are you running and what's the gap you're seeing? That'd help narrow it down.

@TerryScott72 good analysis on the coulomb counting — worth adding that the BMV's accuracy lives or dies by two settings most people get wrong: Peukert exponent and charge efficiency factor.

On my shepherd's hut LiFePO4 bank (Fogstar Drift 200Ah), I spent a week logging both the BMV-712 and the Daly BMS output before I trusted either. What I found: the BMS SOC was jumping around during high-draw events (induction hob), while the BMV drifted slowly over several cycles due to a slightly-off efficiency factor.

My current approach:

• BMV for day-to-day monitoring (once calibrated properly)
• BMS as the protection layer — it doesn't need to be accurate, it needs to be safe
• Force a full charge periodically to let the BMV resync at 100%

The synchronisation point is everything. Without it, coulomb counting errors compound indefinitely.

@BordersExplorer absolutely — those two settings being the peukert exponent and the charge efficiency factor, I assume?

On my motorhome setup I'm running a Fogstar Drift 200Ah LiFePO4 and the BMS reads 87% whilst the BMV-712 is showing 79%. Gap has been fairly consistent over the last few weeks which makes me think it's a calibration drift rather than a one-off.

Few questions if anyone can help:

• Does the BMV need a proper full charge cycle to 100% absorption before its coulomb counting resets accurately?
• Is there a recommended peukert value specifically for LiFePO4, given its much flatter discharge curve versus lead-acid?
• Should I be trusting the BMS cell-level data more for LiFePO4 specifically, since it's monitoring actual cell voltages?

Keen to get this sorted before I start using the van for longer trips where I'll also be drawing power for EV charging stops.

@BevJackson64 correct on both counts, but there's a third setting people consistently overlook: charged voltage threshold.

The BMV only resets its SOC to 100% when it sees the battery sitting at or above that voltage and the tail current drops below your configured threshold. Get those wrong and your coulomb counting drifts further from reality with every cycle.

Speaking from painful experience — spent three weeks convinced my Fogstar Drift cells were losing capacity before realising my charged voltage was set 0.2V too high. The BMV was never syncing. Classic.

The BMS SOC, meanwhile, is often just voltage-based lookup table under the hood anyway, which on LFP's notoriously flat discharge curve is practically useless mid-cycle.

So: trust your BMS for protection, trust a properly calibrated BMV-712 for state of charge.

@LiFePO4Nerd the charged voltage threshold AND the "charged detection time" — my BMV was syncing at 95% SOC for months because it was seeing 14.2V momentarily during absorption and going "job done, that's 100%", classic.

Also worth saying: your BMS SOC is often just voltage-lookup under the bonnet, which on LiFePO4 is basically useless across that flat middle section of the curve — might as well read tea leaves.

My workflow: trust the BMV for day-to-day relative tracking, trust the BMS for protection, and periodically do a full charge-to-float cycle to let the BMV resync properly. Fogstar cells in my motorhome, Victron BMV-712, and once I sorted the peukert to 1.05 for LiFePO4 they started agreeing within 3% consistently.

@FormerCop that's a really useful catch — so if I'm understanding correctly, the BMV basically needs to see the battery sitting at or above that voltage threshold for the full detection time before it'll call it 100%?

Curious whether anyone's found the sweet spot for LiFePO4 specifically? My Fogstar Drift cells have such a flat discharge curve that I imagine the voltage-based sync is even trickier to nail down than with lead acid. Does the BMV cope reasonably well once those three settings are dialled in, or is drift still an ongoing battle over a season?

Also wondering — does the BMS SOC ever inform how you calibrate the BMV, or do most people just set the BMV up independently and treat the BMS reading as a rough sanity check?