Here is a very easy way to use a battery to power a Raspberry Pi.
The instructable assumes the Raspberry Pi is configured and running.
Step 1: Gather Parts
Get parts and tools (prices in USD):
- Working Raspberry Pi
Step 2: Put It All Together
Charge the battery pack
Shutdown raspberry pi and unplug from wall outlet
Plug USB to microUSB into battery pack and into Raspberry Pi
The Raspberry Pi should run for 8-10 hours