I'm hoping to use my iphone as a bicycle gps for very long rides (+18 hours) and would like to have a backup battery to extend its use. I would like a backup battery that is... 1. relatively small and lightweight. 2. able to give me the most additional hours to my iphone. 3. able to charge with the minimum number of hours. I know that I won't be able to get everything I want, but what is the consensus here as the best portable backup battery for the iphone. I will also do things like turn off wifi and talk to 2 hours to extend the battery life. Basically, the iphone will be on standby mode except for the cycling app running in the background to record my rides. Thanks in advance.