Hi AlanRussell5,
Thank you for reaching out Microsoft Community. We apologize for the inconvenience this has caused you.
By the way, what happens when you remove the power supply unit from the device? Is it going to shutdown your computer automatically? If it does, this kind of behavior indicates a possible hardware issue but we'll have to consider few things. Is everything up to date on your Surface? To check for the current OS build and version of your Surface device, simply go to Settings > System > About and see Windows Specification for details. If it's not updated, then check for updates under Update & Securityand install it. If it's the most updated version, it should show Windows 10 1909 and what needs to be done since it's already the most updated version is to reset the device to really determine if it's already a hardware problem, because if after the reset it still shows the low battery warning, then, this really confirms that your Surface device is having a hardware issue.
Before doing the reset, you may try the following:
Open Device Manager
Expand Batteries
Right-click on the drivers available in Batteries and click uninstall. (If a pop-up shows, don't put a check mark on the box provided, just proceed uninstalling the driver and restart the Surface. After restarting, if issue persists, then reset the Surface device)
To reset:
Go to Settings > Updates & Security > Recovery > click Get Started under Reset This PC option and select remove everything. Make sure to back up all important files so you won't be having issues retrieving it.
If this answers your question, please let our Community know by marking this post as answer. Otherwise, don't hesitate to respond to this thread for further assistance.
Kind Regards,
Crest_Z