By mains charger recently stopped working so I rigged up a cable from a USB plug to a 2mm power jack. Although, my 770 reported that it was charging, it never actually got to the point where it reported being fully charged. The mains charger is rated as 890mA, so I suspect that the 500mA maximum available from a USB port is not enough. I may try to see if two USB ports in parallel does the trick.