Well, I plan on using it as a cheap way to play old games on the go (Doom, the original StarCraft, Battlefield 1942). I meant how long I would get on a charge.
What type of battery were you thinking of using?
I believe the Pi by itself will use 700mA – if you take a worse case, standard AA cells (Alkaline) are 1000mAH = 1 hour 25 minutes of use of just the Pi itself.
You"ll need 4 in series which is 5v5, which i think is 0v25 higher than the maximum allowed. A Schottky diode between the battery pack and the Pi +V terminal would bring that down to approx 5v2 (schottky = 0v3 drop approx) . Diode terminal with the black or silver band around it would connect to the Pi end, battery + end to the other.
If you intend to use a rechargeable battery then depending on the chemistry the amount of batteries and the mAH figure will differ.
If you want to use a Lithium mobilephone battery then something like this http://www.mikroe.com/eng/cate.....ly-boards/
recharge and regulate your battery power.
Sparkfun/Dontronics/RapidOnline/Maplin will possibly have something similar.