Originally Posted by
noglider
People often ask about running electronics off a dynamo hub, and you'd be surprised how complicated and expensive it is. Basically it's not worth even trying unless you are going to be riding away from a power outlet for several days at a time. Dynamo hubs put out AC with a widely varying voltage, often going down to zero (obviously). A system that converts that to a relatively constant DC supply is not trivial and requires a battery. I've done the easier thing and mounted a USB battery pack to my bike. A $20 battery can keep my phone running with the screen on all day. I charge my battery once a day, though I could get away with doing it every three or four days. Charging once a day is better for me because it's an easier habit to keep.
My favorite battery is the Anker Astro E1. It has proven to be reliable and durable.
Yup, I've put an oscilloscope on my hub and (with a lot of help... er, with me watching back in '09) built a circuit to create a constant 5V... maybe 1 or 2 amps out?. I didn't use a battery. I'm sure it had a capacitor but I certainly don't remember off the top of my head what the circuit looked like (and I'm a few thousand miles away from it at present). Even back then there was a slick chip that could do buck/boost on a wide variety of voltages and output a... different voltage, although I only wanted 5V so I could plug USB stuff into it.
Nowadays it's packaged much better into pieces that fit into your steering tube.
I recognize that it's 'easier' to just charge the darn thing. I don't care as much about the 'easy' I care about the 'reliable'. No, i'm not away from power for more than a few hours at a time, but if I need to plug it in and turn it on every time, I'll forget at least 20% of the time. I have to remember enough things as is, I don't want to remember more things. To me that's worth putting in more as an initial effort. I'm looking into some gopro stuff now and I'm sure more people are home modifying them.