In recent Windows 10 updates, Microsoft lets you check the battery of your connected Bluetooth device from the Settings app. To do so:
Pair Your Bluetooth Headset
Make sure your Bluetooth Headset is already paired and connected to your Surface.
Open Windows Settings
First, you need to access Windows Settings by going to Start and clicking the Gear icon (or by using the Win + I shortcut key).
Go to Devices
In Windows Settings, select “Devices “.
Go to the Bluetooth & other devices.
Under the “Mouse, keyboard, & pen” section, you will see the battery percentage of the Surface Mobile Mouse.