I am looking for an external battery for my retina macbook 12". I am mostly looking for one that can gave me another run without taking much room in the bag. Any idea what I could buy as of today?
You could use any USB power bank if you wanted. But you would need one capable of high current draw for it to actually be capable of charging the battery in a reasonable time (Unless charging to 100% over a period of 12 Hours is okay).