In one of the future versions of Android will appear and charge indicator connected Bluetooth devices. It is reported by XDA.
Now to find out the amount of charge is connected with Bluetooth device, you need to download additional app, and new versions of Android it will be a built-in feature of the system.
The developers from the forum found in the code of the latest test build of Android 8 the mention of a special API, which will be responsible for the charge indicator connected devices.
The user can choose to display pictogram or interest. Hope to see this feature with the release of 8.1 and Android smartphones Pixel 2 and Pixel 2 XL.