Jason Cipriani, writing for CNET, talks you through the process of enabling iOS 9’s battery widget:

When enabled, the battery widget will display the current battery percentage of your iPad Pro, plus any Bluetooth devices connected to it. With the Apple Pencil using Bluetooth to communicate with the tablet, it naturally shows up in the widget.

This is worth looking at, even on your iPhone. Swipe down to bring up the Notification Center, make sure the Today tab is selected, then scroll all the way to the bottom to see the Edit button. My guess is, this interface will be new to a lot of people.