In reply to Siward:
> If you are reading, that means having the screen on for extended periods which means battery drain.
P.S. While I'm thinking about the subject...
The biggest cause of battery drain is actually low cell signal, so the single best thing you can do to extend battery life is to put your phone into airplane mode. As of the latest iOS software update, both iPhones and Androids can now access GPS in airplane mode if required. Needless to say, also switch off wifi, bluetooth, all background app refresh, unnecessary location services, and anything else you don't need. Brightness should also be on the minimum usable level. Another important tip: delete the Facebook app, which is a battery hog even when you aren't using it (can't overemphasise this point!)
With everything switched off, I've been able to use the Kindle app on my iPhone like this for extended periods of time. Battery drain can be as low as 5% per hour. That's actually better mileage than some of the more recent Kindle models.