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

Had this exact headache last year with my Fogstar Drift 100Ah. Both were reading differently and I didn't know which to believe.

Short answer — trust the BMS for protection, trust the BMV-712 for day-to-day SOC tracking, once it's properly calibrated.

The BMS is hardwired into the cell-level data and will cut off when it needs to regardless of what the BMV thinks. But for actually knowing where you are between those limits, the BMV wins hands down — it's doing proper coulomb counting rather than just voltage-based guessing.

Few things to check though:

• Has the BMV been through a full charge cycle to sync? It needs to hit your absorption voltage and hold it before it resets to 100%
• Peukert exponent — worth tweaking if you're seeing drift over time
• Tail current setting — this tells the BMV when to call it "full", needs to match your battery spec

The two readings will likely never match perfectly because they're measuring different things in different ways. My BMV-712 and BMS were about 8% apart initially, got it down to 2-3% after proper setup.

What battery and BMS are you running? Makes a difference to the advice tbh. Some cheaper BMS units are notoriously optimistic with their SOC figures.

Anyone else had success syncing these up more tightly? Curious if there's a better method I'm missing.

@BorderOffGrid — really useful distinction there, cheers for clarifying.

Quick question though: does the BMV-712 accuracy depend heavily on having the shunt correctly sized and the charge efficiency factor dialled in? I'm running a 200Ah Fogstar setup in my shepherd's hut and I suspect my BMV might be drifting over time because I haven't touched the default settings since install.

Also — does either reading actually sync up at 100% SOC when the battery hits absorption/float? I've read the BMV resets its coulomb counting at full charge, which presumably means if your charger parameters are slightly off you could be compounding errors every cycle.

Wondering if this is why my two readings are perpetually about 8% apart regardless of charge state.

@BorderOffGrid nailed it really. From my experience with emergency backup setups, the BMV-712 is only as good as its calibration — if your shunt isn't sized right or Peukert's exponent is off, it'll drift over time.

Few things worth checking:

• Charged voltage set correctly in BMV settings
• Tail current configured for your specific cells
• Self-discharge rate — easy one to forget

My BMV needed a full sync cycle before it started reading sensibly. Basically run it down, charge to 100%, let it hit the tail current threshold and it resets itself.

The BMS is always going to be more conservative — that's its job. It's protecting the cells, not telling you how much capacity you've actually got left.

Good points all round. One thing worth adding from my van conversion days — the BMV-712's SOC will drift over time if it never sees a proper full charge cycle to reset its synchronisation point.

I had this exact problem with my motorhome setup. The BMV was reading 78% "full" because I'd been running solar topping-up without ever hitting the absorption/float transition cleanly. Meanwhile the BMS was happily reporting cells balanced at 100%.

Fix is straightforward:

• Set your tail current correctly (typically 4% of capacity)
• Let the bank charge fully until synchronisation triggers
• Then both readings should converge

@LDVSolar is right about calibration being everything. Get your charged voltage and Peukert exponent dialled in — Victron's documentation is actually decent on this. Once synchronised properly, my BMV-712 is remarkably accurate, often within 2-3% of what the BMS reports.

@MarinePhil raises a good point about drift — the synchronisation trigger is your friend here. I've got mine set to sync at 99% absorption on my static caravan setup, which keeps the BMV-712 honest after a full solar day.

One thing nobody's mentioned yet: if your BMS is a "dumb" one that just cuts on voltage thresholds rather than doing proper coulomb counting, the BMV-712 will actually give you more useful day-to-day SOC data, not less. My old Fogstar setup had exactly this — the BMS was purely protective, the Victron was doing the actual bookkeeping.

The two aren't really in competition — different jobs entirely.