This app is called Charge Alarm. It's completely free no ads and it just works.

The short pitch:

It's only job is to play an alarm when your device charges up to a configurable percentage. 80% is recommended for prolonging the life of your battery.

The long pitch:

When your device is unplugged the app schedules a job that doesn't start until the device is plugged in. Only when it is plugged in and charging it will check once every minute whether the battery level has charged to X% or more.

Once the battery level has reached X% or more an alarm is played. This includes a sound of your choice and vibration. Unplugging the charger will stop the alarm and reschedule the job which will not start until the device is plugged in again.

The app doesn't have any permanent foreground / background services or ads. It works when the app isn't running. It works after you restart your device. It won't ironically drain your battery like some of the other apps.

Enjoy!