Why is Windows reporting a battery charge with my desktop?

You can run the following command at the cmd to list devices Windows thinks are batteries:

wmic path Win32_Battery get Caption,Description,DeviceID,Name

You can also go to the Device Manager under Batteries. However, the command will provide you with a description and model number to work with a bit faster.

The battery icon appearing on a desktop is usually caused by a USB-connected UPS battery.

This could be because you have a UPS connected.

It does not seem to happen all the time, though. From personal experience, on one desktop, when I installed Windows 10, it seems to have detected it as a laptop. Oddly enough, my other desktop also has a UPS but no battery icon and Windows Mobility Center is not installed.

If you do not want the icon there, I suppose you could try to remove or disable Mobility Center. I just learned to ignore the icon. You could also hide it from the notification area.