This is a basic and functional widget, wich allow you to monitor in real-time what's the level of your battery power exactly.
This is not an app. Widget is a mini application you can start and fix at your home screen, pushing for 2 seconds your home screen and select widgets option. There you will find widget battery to fix in your screen.
This widget is designed to avoid draining at maximum, battery power. During sleep screen no power is drain.