Windows already does this, for all supported devices. (older BT devices don't support this feature)
Go to Settings/Devices and you should see a list that looks something like this:
[REQUEST] Program that shows the battery life of connected Bluetooth devicesIf you don't see your device in the list, it hasn't been paired with your PC.
If you see it in the list but no battery indicator, then your device doesn't support it, and no 3rd party program will make it support it. (it most likely is using an older version of BT)