A USB charger fed by a dynamo hub MIGHT be worth it, but extra batteries ARE worth it, so get them first, and then decide about a charger. And even if you use a charger, you should also bring the extra batteries. The charger is a lot of expense which doesn't really yield a lot of watt hours. If they made them so that they provided a lot of energy, they would also cost you a lot of energy from your body, and that's no good, so they don't make sense for most people. My favorite extra battery is the Anker Astro E1. It's $26, and it's rated at 6,700 mAH. You can get a bigger one, but if you need more than that capacity, get two, because you can charge two at once, and you're not carrying any more weight than you would with a larger battery. If two aren't enough, get four. I got a gigantic battery that weighs a pound (450 grams) and it didn't work out so well: the charging time is much too long, and one of the connectors failed, and it's undependable, so the Anker Astro seems like the ideal choice.