Very much like this. I carry 2 batteries for my Samsung S5 now because i cannot swap the iPhone ones and external USB ones are too big/heavy. This would solve it and I can use my iPhone again.
Very useful for camping, traveling, music festivals, etc. You just keep a few cheap batteries in your bag and don't have to worry about finding the nearest wall outlet.
One of the makers here. Let us know if you have any questions or comments. :)