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

Had this exact situation with my setup last winter. The short answer is: neither one is perfectly right, but understanding why they differ helps you decide which to lean on.

The BMV-712 uses coulomb counting — it tracks every amp in and out from a known reference point. Problem is, if that reference point (your 100% sync point) is slightly off, the error compounds over time. It's also sensitive to your Peukert exponent and charge efficiency settings.

Your BMS, on the other hand, is typically estimating SOC based on cell voltages, which is notoriously unreliable in the middle of the charge curve where lithium is basically flat. It's much better at the extremes.

What I actually do:

• Trust the BMS near the top and bottom (it'll catch overvoltage/undervoltage reliably)
• Trust the BMV-712 in the middle range for day-to-day consumption tracking
• Make sure the BMV syncs to 100% properly — mine only triggers when charge current drops below 1A at absorption voltage, took me ages to dial that in correctly with my Fogstar Drift cells

Also worth checking your BMV settings match your actual battery capacity. I had mine set to 200Ah on a 230Ah bank for ages and wondered why it was always pessimistic 😅

What BMS are you running? Some of the cheaper ones are genuinely terrible at SOC estimation. If you're on something like a Daly vs a JK or Seplos, that'll make a real difference to which reading you should weight more heavily.

@MoorLee this is exactly the situation I'm in with my garden office build. My Fogstar Drift 100Ah is reading about 8% higher on the BMS app than the BMV-712 is showing.

Quick question — does the discrepancy tend to get worse over time, or does it self-correct after a full charge cycle? I've noticed mine seems to drift further apart during prolonged cloudy periods when I'm not hitting 100% regularly.

Also, is there a recommended way to force a BMV-712 sync? I know you can manually set SOC to 100% when the battery is genuinely full, but I'm not confident I'm actually reaching a proper full charge through the solar alone at this time of year.

@Macca97 I had the same head-scratching moment with my motorhome setup — Victron BMV against a Fogstar Drift BMS telling me completely different stories.

What sorted it for me was doing a proper full charge cycle and letting the BMV resync. The BMV's coulomb counting drifts over time if it never sees a genuine 100% charge — so if your solar's been keeping it topped almost full but not quite triggering absorption properly, the BMV slowly loses track.

Meanwhile the BMS is just reading cell voltages at that moment, which can look optimistic under light load.

I now trust the BMV for trends and daily usage patterns, and the BMS for low-voltage warnings. Together they tell the full story — neither alone is gospel.

Check your charge profile settings in VictronConnect too. Wrong absorption voltage will compound the drift problem considerably.

Good thread this. One thing worth adding that nobody's mentioned yet — calibration opportunities matter a lot here. The BMV-712 gets more accurate over time, but only if you're letting it see genuine full charge cycles and proper tail current cutoff. If your system rarely hits absorption properly (common with smaller solar arrays in British winters), the BMV drifts pessimistically over time.

@Macca97 with a garden office setup, you're probably not doing full cycles regularly, which could explain that 8% discrepancy right there. Try manually synchronising the BMV after a confirmed full charge — hold it there until tail current drops right off — then see if the gap closes.

I'd use the BMS as your hard limit safety net, but trust the BMV for day-to-day SOC monitoring once it's properly calibrated. Best of both worlds. 👍

Exactly what I've been wondering about for my garden office setup. Quick question though — does the peukert exponent setting on the BMV-712 make a meaningful difference here, or is it fairly negligible for LiFePO4?

I've got a Fogstar Drift 200Ah and I've left the peukert setting pretty much at default because I wasn't sure what to change it to for lithium. Wondering if that's been skewing my BMV readings all along and partly explaining the discrepancy between it and my BMS.

Also — does the charge efficiency factor setting matter as much for lithium as it apparently does for lead-acid? I've seen figures like 99% recommended for LiFePO4 but never confirmed it.

@Smithy98 the Peukert exponent does matter but honestly for LiFePO4 it's nearly negligible — set it to 1.05 in the BMV and leave it alone. The bigger issue with a garden office setup (same as mine) is that your loads are probably quite variable. The BMV drifts when it can't find a clean sync point to recalibrate to 100%.

What I found helped massively was setting the tail current and charged voltage thresholds properly so the BMV actually sees a full charge regularly. Mine was just never hitting the sync condition because the absorption phase kept getting cut short by my Renogy solar controller.

Once I sorted that, the BMV and BMS agreed within 2-3% most of the time. I trust the BMV for daily monitoring, BMS for protection limits.

@Smithy98 worth knowing that on LiFePO4 the voltage curve is so flat between roughly 20–80% that your BMS voltage-based SOC estimate is essentially guessing during that middle range — it's only reliable near the top and bottom of charge. This is precisely where the BMV-712's coulomb counting should outperform it, assuming your shunt installation is clean (no parasitic loads bypassing it — common oversight in shepherd's hut builds where the 12V lighting circuit gets wired before the shunt).

In my garden office setup I trust the BMV during normal cycling but always defer to a full charge reset to resync it — once the Victron MPPT signals absorption complete, that's your calibration point. Without regular full charges the coulomb counter drifts noticeably over weeks.

@Trigger nails it on the flat voltage curve — that's the crux of the whole debate really.

What I'd add from my own cabin setup: sync them both at full charge and let the BMV-712 run coulomb counting as your primary SOC reference day-to-day. The BMS is doing cell-level protection, not precision bookkeeping.

The BMV drifts over time though, so set a synchronisation point (I use 100% at absorption end on my Victron MPPT) to periodically reset it. Without that, errors compound and after a few weeks you're flying blind.

Fogstar Drift cells especially behave predictably enough that once you've dialled in the BMV settings, it's remarkably accurate.