I use the Kensington Portable Power Pack..... It powers at 5 volts...... Charges fom USB and Wall AC and car lighter..... Has worked great for me..... I think I payed roughly 25 but could be wrong...... Bought at Best Buy but is sold in many stores...... Can be used for Cell Phones, IPods, Disk Players and many other small electronic devices.....
When on the go, I use my 510, with Pass Through connected to my Kensington...... Great power and no worry about new batteries...... It is small, light and easily carried in my shirt pocket......
The only problem I have with it is, everyone wants to know what that wire is coming from my shirt pocket....... Easy price to pay.......