In Short

Apple's Smart Battery Case for iPhone 6s and iPhone 6 is pretty much the silicone case with a power pack embedded in the back. While it's not much to look at, it's light, svelte, and does some really intelligent things with how it manages charging.

Buy Smart Battery Case at Apple

Available in white or black for $99, the Smart Battery Case keeps your iPhone 6s charged up on the go. You can charge the case and your phone at the same time using a standard Lightning connector, and view the battery status of both the phone and the case within your notifications at any time.