A number of different charging devices that run off your dynamo hub have appeared in the last few years. Here is a well written summary of them. I was planning a trip where I may not see an outlet for several days, so I thought it was time to try one of them.
The Sinewave Revolution was designed by somebody who wanted a simple device to allow use of the dynamo power to charge his devices. The USB plug goes into a waterproof port on the right side (not visible in this picture). Power is always there at the dynamo hub to charge the phone, but current offerings to accomplish this are complex/clunky/or include a light system that may be unnecessary. The Revolution is an elegant, small and lightweight device that converts the AC power coming from your dynamo hub into 5 volt DC power that charges your device via the USB cable. It does not have a cache battery, which may or may not be a good thing.
I’m not asking for much. I just want to keep my phone charged. It’s an iPhone 5. While I ride, I have the Gaia mapping/location app sleeping in the background. I check my position at intersections if I am unsure of where I am, or just to see my progress. I may have a podcast running in the background, or music or Audible book going, using earphones. Unless I need to manage a phone app or see my position with GPS, I keep the phone asleep (dark screen), but not turned off. I will pull it out to take pictures now and then. I use it in camp before sleep, draining 20 to 50% of the charge. That means I may start the next day with a 50% charge or less. I will turn the wifi off unless it is available and I need it. If I am away from all hope of a cell tower signal I will turn off the SIM card as well. No need to have those running and using power for no reason. I also leave the bike lights off if they were not needed, so that the phone would not have to share power with the lights.
The phone recognizes and begins to use the power when you pass about 3.5 MPH. At 9 mph it is supposed to be the same as being plugged into a wall outlet. It has overload protection, meaning it will continue to supply power, but not damage the phone even if you go 50mph or more.
Over a week on tour, I always ended the day with the phone fully charged, or in the 90% range. It works! The only issue is that if I go less than 4mph, the phone give off the “charging” tone. It will continue to do so until I exceed that speed. A long slow hill can issue a continual string of these phone alerts. If I turn on the “silent” button, the phone will vibrate instead, using quite a bit of power. The phone settings allow you to turn off the vibrate feature, but it does not apply in this situation. After a 20 minute hill with my phone silently vibrating, I found I had lost about 8% of the charge!
So I tend to unplug it on on long, steep hills. On short hills, it is a bit of a game to try to go fast enough so that the charging tone does not go off.
On flat terrain I can get about 20 to 30% charge in an hour of riding. It is nice to be free of the dependence on finding an outlet or carrying along an external battery. One solution would be to have it charge an external battery, and then charge your phone from that. I imagine the efficiency would suffer but maybe not enough to matter.