The Anker PowerCore is on sale on Amazon for $40 today, and you can get it for $35 if you click the link that says "3 Applicable promotions" to the right of the product photo. This is the one I use when I'm walking around an unfamiliar city all day and need to use my phone's map to help me get around. I keep it in my daypack with a charging cord dangling out. With 20000mAH it'll last for a very long time without having to recharge it.
