A free app for Android, by Nexamuse.

This is a free app that helps you to monitor the battery level of your smartphone, tablet, or other mobile devices. It can be used as a source of constant notifications and alerts regarding the battery charge status of your device. It allows you to set the percentage level of the battery charge and also the time when the alarm should go off.

The application can be used in a simple way. You can set the alarm to go off when the battery level reaches 100%. You can also set the alert level. The app is based on automation, so it will do everything automatically and give you the alarm.