I recently tested the RAVPower 16750mAh Portable USB Charger similar to this one here :
This external battery pack is useful when you need to have your phone charged and usable but will not be near any wall outlet to charge your phone. For example, if you are a vendor at a trade show and need your phone to process credit card payments, and external power pack is indispensable. This USB battery backup pack performs well, charges quickly and has plenty of capacity to charge devices multiple times. It’s an idea for a Christmas gift. Below is my video review.