This app simply provides a readout of the battery voltage.
Batteries may suffer overcharge for various reasons and sometimes devices indicate the wrong state of charge. A fully charged Lithium Ion cell will have a voltage of around 4.3 Volts under charge - the voltage should ( generally speaking ) not exceed that.
For monitoring of Battery Temperature freely use this app: Battery Temperature App
For monitoring of Battery Level with text to speech readout alerts freely use:Battery Level Monitor App