Portable power: 10 small and compact external battery packs for smartphones

You've probably seen those external battery packs made for smartphones. Also known as power banks, they come with an embedded rechargeable battery and a USB port providing back-up power for our mobile gadgets. And if you do a little research, you'll discover that they come in all shapes and sizes. Here's for example a post listing some of the largest smartphone battery packs money can buy.