There are lots of discussions about charging from a dynamo hub. It can be done but not cheaply, and it's worthwhile only if you are going to put a lot of miles on between power outlets, such as on a multi-day tour where you are camping. Little external batteries are cheap, and you can carry more than one.

The dynamo produces A/C rather than DC, and the voltage fluctuates widely. You need circuitry to compensate for that. Not only that, it doesn't produce much power, either.