The Raspberry Pi can easily be powered by rechargeable battery packs – most dedicated to be backup power packs for tablets and smartphones. There are a number of packs available com vendors such as Maplin , all depending on budget and what your are running of your Raspberry Pi. The minimum recomended requirement is that the Battery Pack can deliver a steady 5V @ 1A, but you will find a lot of people experimenting with less in the community.
A word of warning – NEVER power your Pi directly from GPIO ports – the possibility of causing permanent damage to your Raspberry Pi is to great. Pick a battery pack with a standard USB connector – the Raspberry Pi requires a male USB Micro-B connector .
I have also managed to power my Raspberry Pi with a 4xAA 1.5v battery pack with a voltage regulator, I will post details of this at a later stage.