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

Had exactly this on my boat build last summer — 40% difference between my Daly BMS and the BMV-712 at one point, nearly drove me mad.

Short answer: trust the BMV-712 for day-to-day SOC, but use the BMS as your protection layer, not your gauge.

Here's why. Most budget BMS units (Daly, JBD, etc.) calculate SOC using a fairly crude voltage-lookup table. The problem is LiFePO4 has an almost flat discharge curve, so voltage is a terrible proxy for state of charge across the middle 80% of capacity. The Victron uses coulomb counting — it's literally tracking every amp going in and out — which is far more accurate in practice.

The catch is the BMV needs a proper synchronisation point to reset its counter. Mine was drifting because I wasn't regularly hitting 100% absorption. Once I sorted my charging profile on the Victron MPPT and let it fully synchronise, the readings stabilised within a few percent of each other.

Key things to check:

• Is your BMV's charged voltage threshold set correctly for LiFePO4 (around 13.4–13.5V for 12V)?
• Is your tail current set appropriately (I use 2% of capacity)?
• Has the BMV actually completed a sync recently? Check the history screen.

The BMS SOC is basically decorative on most units below £100 — useful for catching something catastrophic, not for trip planning.

Curious what BMS you're running and what your charging setup looks like — that'll help narrow it down further for your specific situation.

@WelshCamper had the same headache on my tiny house build with a 280Ah Fogstar Drift LiFePO4 bank.

The Daly BMS SOC is basically guesswork — it resets on every cell balance event and loses track fast. The BMV-712 uses proper coulomb counting and once you've nailed the Peukert exponent and charge efficiency factor settings it's genuinely reliable.

Key thing most people miss: you must let the BMV sync properly. It needs a proper full charge cycle to 100% to reset its reference point. After that mine stays within 2-3% for weeks.

Set your tail current correctly for LiFePO4 — I use 1% of capacity.

Great thread to land on for a first post, @WelshCamper and @BatteryRay — welcome to the forum both of you if you're newish here.

One thing worth adding: the BMV-712 is only as good as its calibration. If you haven't done a full charge cycle to absorption, held it, then set that as your 100% sync point, the coulomb counting drifts badly over time.

On my narrowboat I synchronise mine every couple of weeks with a proper bulk/absorb/float cycle. Before I got into that habit the two readings were all over the place — similar 30-40% gaps to what you're describing.

Also worth checking your battery capacity setting in the BMV matches your actual usable capacity, not the headline spec from the data sheet. Fogstar Drift cells in particular are worth measuring rather than assuming.

@WelshCamper @BatteryRay Worth adding — the BMV-712 is only as good as its calibration. If your charged voltage and tail current thresholds aren't set correctly for LiFePO4, you'll never get a proper 100% sync point and the drift compounds over time.

On my narrowboat I run a 200Ah Fogstar Drift bank and spent a good week tweaking the charged voltage to 14.2V and tail current to around 4% before it started behaving sensibly. Most people leave it on the lead-acid defaults and then wonder why it's wandering.

Also — both readings being wrong simultaneously is entirely possible if you haven't done a proper full charge calibration cycle after installation. Do that first before concluding one is more trustworthy than the other.

@FrostySailor makes a solid point on calibration. I had mine way off initially on the static caravan setup — tail current was set too high so it kept declaring "full" prematurely and everything snowballed from there.

One thing nobody's mentioned yet: synchronisation events. The BMV-712 resets its SOC to 100% when it hits your charged voltage and tail current threshold together. If either is wrong, it never syncs properly and drifts further over time.

I now manually sync mine about once a month — just do a proper full charge and watch VictronConnect until it actually registers 100%. Sorted most of my drift issues.

The Daly SOC is pretty much useless for accuracy tbh, it's really just there for cell-level protection.