I've got a new trent ~13000mAh battery. It's been rather useful, its only about the size of a very fat wallet. It has two charging ports, a 1amp and a 2.1 amp. The 2.1amp port doesnt actually charge as quickly as the wall charger, but it does well enough. I feel like I can get 3 0-100% charges out of the thing. Its pretty handy considering my wife and I can use it at the same time. It also works well for the iPad. If, however, you don't have too many other devices that would need charging, I would probably opt for an external charging dock + extra battery, makes for less bulk and less downtime.