Please don't use a usb charger connected to the mains. It will drain your battery.
Very flat batteries can take an extremely long time to charge via usb and computer. In fact, the computer may go to sleep or hibernate and nothing will happen.
A long mains charge is the first thing to try when there are battery problems. A PSP charger, 5v, will work fine.
Here are some threads about the uselessness of trying to use usb+mains:
https://www.mobileread.com/forums/showthread.php?t=56170
https://www.mobileread.com/forums/showthread.php?t=50169