As someone who knows nothing about electronics I really welcomed the guys at Dexter publishing http://www.dexterindustries.com/BrickPi/getting-started/power-up-the-brickpi/ .
One option is not described there though: would there be any advantage in using USB y cables like http://www.amazon.co.uk/StarTech-feet-Cable-External-Drive/dp/B0047AALW6/ref=sr_1_2?ie=UTF8&qid=1383646542&sr=8-2&keywords=dual+usb+micro , providing the Raspberry Pi with double the power?
E.g. can double the USB power - without the battery pack - provide enough power for the motors?
Or: in the scenario in which we used both the battery pack and USB power, would a Y USB cable make the battery pack last even longer?