This app is called Battery Charge Alarm. It's completely free, no ads.
It's only job is to play an alarm when your device charges up to (or discharges below) a configurable percentage. Changing up to 80%, and discharging above 30%, is recommended for prolonging the life of your battery.
Enjoy!
The app has been re-designed to use a foreground service to improve stability. Android Doze was prevent the alarm on certain Android versions.
9Apps 4.9
