Yesterday, I rode for 6-1/2 hours with my phone mounted to my handlebars, using two GPS apps. I told my phone to leave the screen on forever. I plugged the phone into a small external battery which I had strapped to the bottom of my top tube, right behind the head tube. The phone's charge stayed at 100% the whole time, and the battery was run down halfway. The battery cost about $20 and weighs about three ounces.

I love my dynamo hub for running lights but not for charging. It's not practical for me. If I rode long bike tours without being near an outlet at night, I would consider a charging system that runs off the dynamo. But if I had that, I couldn't run the lights in the day. I could use it for lights or charging but not both, which is another thing to consider if you are thinking of getting a dynamo-powered charging system.