Thx for your replys, Whistle and Metal,
your info is really helpful. The energizer is a bit expensive, but there are also some battery packs which will do the job. The problem is, that these packs only got an USB out an no output port like the energizer. Do you know the name of USB Adapters (cable) for USB to (I don't know...) power, AC, output port? I think on Whistle's Pic, there is something like that. Would be grateful for help. :)
THX!