The iPhone Air has surprisingly good battery life for users with light-to-medium daily usage habits. But if you are going to be away from a charger for a long time, especially going travelling like on a plane, you’ll probably want to carry some extra juice with you.Apple has made its custom MagSafe Battery for iPhone Air, that fits precisely beneath the camera plateau on the back of the phone. But another big competitor in this space is Anker, with the Anker Nano Power Bank in a similar size promising 5,000 mAH capacity at half the price of Apple’s offering. But how much extra battery life should you actually expect to get from these power banks, and which is best? more…