I put a power socket on the bike. In Tesco i found a Griffin plug that goes into the socket. It has 2 usb connections and it came with a black iphone usb charger lead. Works very well and powers my satnav as well as charges the phone no troubles at all. Only cost under 12 quid. I had originally got a plug that goes in the power socket and has connection for a single usb, from ebay but that was crap and did not work.
This is what i got and has a resettable fuse
http://www.griffintechnology.com/pro...owerjoltiphone