With most stock versions of Android, there is one missing feature that has been seen on other factory skins of Android like OnePlus, LG, and Samsung. That is the Bluetooth device battery life indicator. I used to use a pair of Jaybirds X interchangeably with an iPhone 6S and some other Android phone. Every time I switched back to the Huawei Mate 8 I was using at the time, I would lose the ability to see how much battery I had on my Bluetooth headphones.
According to a report from XDA-Developers, Google is finally including support to get battery information from a Bluetooth device in an API that would be part of a future version of Android, perhaps Android 8.1. Google has added a new “method” called “getBatteryLevel()” as part of the “BluetoothDevice” class.
What this means is that Google is now going to make it possible for Android to display a percentage from 0 to 100 for a supported Bluetooth device.
Check out the source link for a more technical breakdown of what was found in Google’s APIs and source code and when you can expect to see it on your Google Pixel or Nexus device.
There is an app called " BatOn " in which can read the battery level of the bluetooth connected devices. But sadly it doesn't work all the time on Android, at least it is free and deserve a try
Its Samsung Galaxy S8+