Device charging using power plug and USB

Question

Hi,,

I am working on windows phone c# application. I am using Microsoft.Phone.Info.DeviceStatus.PowerSource class and "External" keyword to check whether the device is connected to external power supply. But how can we know whether the device is connected to a battery charger (using power plug )or to the USB cable. I am using Battery class to get the remaining battery percentage in windows phone 8. But I couldn't find any API's for getting battery percentage in windows phone 7. Could you please help me to find a solution to my problem.

thanks in advance,
Aswathy.