Been through exactly this with my garden office setup. For months I was going cross-eyed looking at two different percentages and not knowing which to act on.
Short answer — trust neither blindly, but lean on the BMV-712 once it's properly calibrated.
Here's my thinking. The BMS is protecting the cells and working from voltage readings at pack level, which can be fairly crude for SOC calculation. The BMV-712 is doing coulomb counting — measuring actual current in and out — which is far more accurate over time if you've got the parameters dialled in correctly.
The critical bit most people miss:
- Peukert exponent — get this right for your specific chemistry
- Charged voltage — must match your actual absorption voltage
- Tail current — set this so the BMV knows when a full charge has actually landed
I'm running Fogstar Drift cells and it took me a good few charge cycles to get the BMV syncing properly. Once it did, the two readings came much closer together — now they're usually within 3-4%.
If yours are miles apart (10%+), I'd suspect either the BMV hasn't synced a full charge yet, or your battery capacity figure is off. The BMV will drift if it never sees a proper 100% reference point.
What cells and BMS are you running? That might help narrow it down — some cheaper BMS units are genuinely just reading voltage curves and calling it SOC, which is pretty hopeless on LFP where the voltage curve is so flat.
Interested to hear what others do when the two disagree in practice — do you override, or just watch both?
