First, thanks a lot for your answers.

The solar-stuff looks cool. But: too expensive and bad for cloudy days.

A lead acid battery with 12v and 17 Ah is too heavy (about 6kg). Even two of the 6v batteries I supposed and a car-charger are much - I`ve lot of other gabbage to transport. But if theres no other solution its a good idea! I even have a car charger here - but no Idea if it`s efficient... but I don`t think so... its a cheap 4€ part from ebay.

What if I use an 1A low drop regulator like this: http://www.national.com/ds/LM/LM2940.pdf, together with 2 capacitors as voltage regulator between the battery and the device? Then there are 5V and max. 1A Output - enough for charging?