If question is about Bluetooth HFP feature: HF indicators feature is optional for the both sides. If the both sides support it, BluetoothHeadset will broadcast BluetoothHeadset.ACTION_HF_INDICATORS_VALUE_CHANGED with BluetoothHeadset.EXTRA_HF_INDICATORS_IND_ID equal 2 (Battery Level) and BluetoothHeadset.EXTRA_HF_INDICATORS_IND_VALUE with scope 0..100. Do not remember Android version were it was implemented, you should check it.

Also battery level can be implemented in device using vendor specific HFP AT commands (especially for old handsfree devices) and maybe BLE.

I found a solution, but it only works on android 8 and above

I took this code from here

Kotlin

 fun getBatteryLevel(pairedDevice: BluetoothDevice?): Int {
    return pairedDevice?.let { bluetoothDevice ->
        (bluetoothDevice.javaClass.getMethod("getBatteryLevel"))
            .invoke(pairedDevice) as Int
    } ?: -1
}

The first thing to register BroadcastReciver by "android.bluetooth.device.action.BATTERY_LEVEL_CHANGED"

and you can receive this action by the broadcast receiver then get extra data by "android.bluetooth.device.extra.BATTERY_LEVEL"

and if you want to trigger this action, you need to reconnect your Bluetooth device or Bluetooth device battery level happened to change.

Good luck for you.

Connected AirPods Pro to OnePlus 5T with Android 9. None of those registered events happen:

"android.bluetooth.device.action.BATTERY_LEVEL_CHANGED"
"android.bluetooth.headset.profile.action.AUDIO_STATE_CHANGED"
"android.bluetooth.headset.action.HF_INDICATORS_VALUE_CHANGED"

I am Able to achieve the handset battery Level in Java

   try {                      
          BluetoothDevice device = bluetoothAdapter.getRemoteDevice("Connected device ID");
          java.lang.reflect.Method method;
          method = device.getClass().getMethod("getBatteryLevel");
          int value = (int) method.invoke(device);
          result.success(value);
       } catch (Exception ex) {
          result.error("invalid_argument", "'deviceId' argument is required to be string", null);
          break;
       }

This is @Kirill Martyuk answer as an Extension variable

val BluetoothDevice.batteryLevel
    get() = this.let { device ->
        val method = device.javaClass.getMethod("getBatteryLevel")
        method.invoke(device) as Int?
      } ?: -1

Usage would be something like

val manager = context.getSystemService(Context.BLUETOOTH_SERVICE) as BluetoothManager?
val adapter = manager?.adapter
val devices = adapter?.bondedDevices.orEmpty()
devices.forEach { device ->
    Log.d("DEVICE_NAME", device.name)
    Log.d("CHARGE_LEVEL", device.batteryLevel.toString())
}