This 'feature' has been noted many times on the forum here; a quick exercise of the search function will help.
In summary, the most reliable way of charging a Reader is through the PCP jack (the round yellow hole), and *not* through the mini-usb connector. You can use any generic Sony PSP charger, or a usb-to-psp cable, which can be plugged into a computer or any usb wall-wort or auto charger.
The mini-usb port requires some level of communication with a computer to charge a Reader. And, the PSP jack charges more quickly anyway.