Battery Monitor value is wrong

I use high capacity battery 50AH for my rover with Mauch electrical power module PL4-14S/4x
How to calculate voltage divider and amp pre volt.

I follow this video, but how to make it accurate.

Finally figured out the battery monitor setup. But it looks not accuracy.
I keep machine running for 24 hours, I have a power meter connect to battery.
After 24 hours, it shows down to 22.7V from 28.1V. I use power meter measure the battery, it is 27.3V that is same as the attached digital power meter
it shows wrong value on MP.

I keep machine turn on for one more day, then open MP, it shows 20.22V, actually power meter measures 26.9V. The voltage divider is same as before.
It looks Ardupilot reduces voltage value faster then real voltage value.

What is reason for this?

In MissionPlanner go to the Setup/Optional Hardware/Battery Monitor section with your vehicle connected.
Sensor should be 0: Other

  • You need to connect a stable voltage and measure it with a digital multimeter while it’s connected - use a voltage that is about as low or lower than what you expect to see from a depleted battery system.
  • Enter that measured value into “Measured Battery Voltage”

Job done for voltage !

Now for current… You should have the Amps per Volt value that Mauch supplied. The way to check it is some repeated constant current draws and battery charge cycles.
I am assuming you cant drive easily while doing these tests

  • Set LOG_DISARMED in parameters and reboot
  • Set up some system (like bank of high power resistors, or a constant running motor) to draw a reasonable amount of current, I would say 3 Amps or more.
  • Connect a fully charged battery, fresh off the charger
  • You can either measure the current with an ampmeter in series and enter that value into the “Measured Current” field, or
  • continue to draw the battery substantially, but not dangerously low or anything crazy.
  • At some point disconnect the battery and put it straight onto a charger - when it finishes note the charged mAhr value.
  • Examine the .bin log for the total current used value Batt CurrTot
  • In the spreadsheet (link supplied below) look at the current multiplier section
  • Enter the existing Amps per Volt in column H
  • Enter the charged mAhr in column I
  • Enter the consumed current from the log in column J
  • Copy the new calculated multiplier (Amps per Volt) into MissionPlanner battery monitor Amps per Volt

Repeat until there’s no significant change between old and new Amps per Volt.

Thank you so much @xfacta, I will try it. looks very hard.