i use the Anker® Astro3 10000mAh Batterie and i am Happy! running without problems. And You can connect other devices like a little Monitor to 12V or something additional to the 2. USB 5V outlet! Also i can charge the Battery using the outlets for powering the devices in the same time.
Its a little bit expensive, o.k. you can buy 4 x 3000mAh batteries with Charger for 9.99 to 40.00 usd!
Li-Ion Battery have 3.7V you need 2 and you have 7.4 V , i think too much for the PI .
Normal Alkaline 1.5V you need 4 6V ok for PI, and the Trash Mounten grows up!
NiMH Batteris have 1.2V you need 5 for 6V ! The most Charger charging only 4 !
Your Charger never will stop running !
or the Bio Battery http://en.wikipedia.org/wiki/Lemon_battery
you need a sack full Lemons