[MENTION=281631]Tandem Tom[/MENTION], what kind of riding do you do? I've determined that charging from a dynamo is too expensive and not worth it. I strap a $20 external battery to my top tube, and it provides power for a long time. I once powered my phone on a six-hour ride with the screen on the whole time. At the end of the ride, the phone was still at 100% charge. I had brought a second external battery but didn't end up needing it. Dynamo charging seems useful only if you are away from a power outlet for a long time.