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

Had exactly this with my shepherd's hut setup last summer. Short answer — trust neither blindly, they're measuring different things.

The BMV-712 is doing coulomb counting from a fixed reference point (your last full charge sync). If it hasn't synced recently it'll drift. The BMS is watching cell voltages and estimating from there, which is its own kind of guesswork depending on how good the BMS firmware is.

Few things worth checking:

• Has your BMV synced lately? It needs to hit your tail current threshold at full charge to reset. If your solar's been topping up gradually without hitting absorption properly it can get well out of whack
• What BMS are you running? Cheap ones (no offence) can be wildly optimistic about SOC
• How big's the discrepancy? 3-5% I wouldn't worry about. 20%+ and something's definitely off

On my Fogstar lithium bank I found the BMV-712 was the more reliable day-to-day indicator once I'd dialled in the charged voltage and tail current settings properly. The BMS SOC readout was just... cheerful.

Worth having a look at the Victron Connect app and checking your sync frequency in the history tab. If it's only syncing once a week something's wrong with your charge profile.

What's your setup — solar, mains hook-up, or both? Makes a difference to what's likely causing the drift.

@BigAl27 makes a fair point about them measuring different things — worth adding that the BMV-712's accuracy drifts over time if it never reaches a proper full charge to reset the sync point. Mine was reading 15% higher than actual because my absorption stage kept cutting short. Once I sorted the charge parameters in Victron's MPPT settings, the two readings came much closer together.

What BMS are you running? Some of the cheaper units have pretty coarse SOC algorithms — basically just voltage lookup tables — which are notoriously unreliable under load. A Daly behaves very differently to something like a JK or Seplos in that regard.

Have you tried logging both via VRM or a separate data logger to see where they diverge most — under heavy load, or sitting idle?

@BigAl27 is right but I'd push back slightly on the "fixed reference point" framing — the BMV resets its 100% reference every time it hits the sync conditions (tail current, voltage threshold etc). If those aren't calibrated properly for your specific cells you'll get drift pretty rapidly.

On my static van with a 200Ah Fogstar Drift LiFePO4, my BMS was consistently reading ~8% higher than the BMV-712. Turned out my charged voltage threshold in the BMV was set for a generic lithium profile, not tuned to the actual absorption endpoint of my actual battery.

Spent an afternoon with the Victron Connect app sorting the parameters and now they're within 2-3% of each other consistently.

So before declaring either unit "wrong" — check your BMV sync settings first. Nine times out of ten that's where the discrepancy lives.

@BigAl27 had this exact headache with my motorhome build. Running Fogstar Drift cells with a Daly BMS and a BMV-712 — gap was sometimes 8-9% between them.

Ended up trusting the BMV-712 for day-to-day monitoring once I'd nailed the Peukert settings and synced it properly at full charge. The BMS is more of a last-resort safety net — it'll pull the plug if things go properly wrong, but it's not built for accurate SOC reporting.

Key thing most people miss: make sure your tail current and charged voltage settings in the BMV match your actual battery chemistry. Defaults are rubbish for LiFePO4.

Two months of tweaking and they now sit within 2-3% of each other most of the time. Good enough for real-world use.

On my narrowboat I just watch both and treat the gap between them as a trust deficit meter — when they agree, life's good; when they diverge by more than 5%, something's lying and it's usually the Daly.

@DownsCruiser the Daly's SOC algorithm is essentially decorative in my experience — it panics at voltage sags under load and thinks you're at 40% when you're nowhere near it. BMV-712 at least has the decency to be consistently wrong rather than randomly wrong.

Pro tip: nail your Peukert exponent and charge efficiency factor in the BMV settings — most people leave them at defaults and then wonder why coulomb counting drifts. Mine took three full discharge cycles to dial in properly but it's been rock solid since.