Many features that are missing on stock Android are implemented on custom ROM’s and devices from other manufacturers. One such feature is the battery life indicator for Bluetooth devices.
The feature is supported by many OEMs such as LG, Samsung and OnePlus etc, however, it is missing on the stock Android. But, thankfully, the future Android versions, perhaps starting from Android O, would support battery life indicator for connected devices without the need of any third party application.
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Although many third party apps support battery life indicator, there are many limitations to them and most of them are quite buggy. So, indeed, this feature will be a welcome feature for Android devices.
To implement this feature, Google has added a new method to the BluetoothDevice API, which is called as “getBatteryLevel()”. With this new class, Android devices will show the battery life percentage from 0 to 100 for connected Bluetooth devices.
Source: XDA Developers