I test the plane’s battery capacity at each annual inspection using the test suggested by the manufacturer.  The test is to apply a constant current load equal to the amp-hour rating.   So, a 24 volt, 12 amp-hour battery is tested at 12 amps for an hour or until it reaches the limit of 20.0 volts.   If it teaches 20.0 volts in exactly 60 minutes it’s at 100%.   If it only takes 50 minutes to reach the limit voltage, it has a capacity of 50/60 x 100% = 83.3%.  If you have a 12 volt syst