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If you're referring to a Windows Phone (whether Windows Phone 8.1 or Windows 10 Mobile), then there's no way that can be done.
The closest you can do is download a battery status app that updates the live tile for Windows 10 Mobile. For Windows Phone 8.1 Update 1 (or was it 2) you can pin the Battery Saver from the Settings to the start screen, that should display the battery percentage in a live tile.
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