I want to measure the Android battery capacity and do something specific when there is only a certain amount left.

Read the Monitor battery-level critical changes section of the document below to receive the event as a broadcast receiver when entering a low battery state.

However, if you want to implement it so that you can set the battery capacity, use the service to periodically check the remaining battery capacity. For basic information about the service and how to implement it, please refer to the link below or google "Android Services" for more information.

Please refer to the code below for the remaining battery charge. The code below was referenced in Check current battery level on the first link.

IntentFilter ifilter = new IntentFilter(Intent.ACTION_BATTERY_CHANGED);
Intent batteryStatus = context.registerReceiver(null, ifilter);
int level = batteryStatus.getIntExtra(BatteryManager.EXTRA_LEVEL, -1);
int scale = batteryStatus.getIntExtra(BatteryManager.EXTRA_SCALE, -1);

float batteryPct = level / (float)scale