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

Had exactly this with my motorhome setup last summer. Short answer: both are lying slightly, but in different ways.

The BMV-712 is coulomb counting — it's tracking every amp going in and out, but it drifts over time if your charged voltage threshold isn't set right. The Peukert exponent and charge efficiency factor in the settings make a massive difference. Most people leave them at defaults and wonder why it's off.

Your BMS is typically just looking at cell voltages and making a rough guess from that. Fine at the extremes (0% and 100%) but pretty hopeless in the middle of the curve, especially with LiFePO4 which is famously flat.

What I do in practice:

• Trust the BMS at the top and bottom (it'll protect the cells regardless)
• Trust the BMV day-to-day for consumption tracking
• Make sure the BMV gets a proper full sync regularly — if your system never hits absorption properly, it never resets

Worth checking your BMV settings. On my setup with Fogstar 200Ah cells I had to tweak:

• Charged voltage down to 13.6v
• Tail current to about 4%
• Charge efficiency to 99%

After that mine were within 2-3% of each other most of the time.

What BMS are you running? Some of the cheaper ones are genuinely terrible at SOC estimation. If it's a JK or Daly there's a big difference in accuracy between them.

Others got thoughts on this? Curious whether people generally favour one over the other long-term.

@WildTinker nailed the coulomb drift issue — my Fogstar 200Ah LiFePO4 in the garden office does the same dance, so I just let the BMV-712 reset at 100% absorption tail current and treat the BMS SOC as a "don't go below this or bad things happen" floor rather than an accurate gauge 🎯

Key Victron tip: set your Charged voltage threshold correctly in the BMV app — most people leave it at default and wonder why it never syncs properly. Get that right and the two readings will stop arguing like my neighbours over a fence.