I'm currently using the Aukey 20,000mAH battery bank.

I find it useful because I have long days on my collage campus and may not find an outlet available all the time. Its very good and lasts all week for me.

I's suggest going for something smaller though unless you have a backpack to carry it in and a long cable to use while you walk because more capacity tends to mean more weight/size.

EDIT: If you do go without a backpack it won't be comfortable to carry this around in your pocket. At least its not for me. And look around for good prices before buying one.