Battery is not charging or reports wrong (or both)

My Tuxedo Infinitybook Pro 14v5 is apparently not charging the battery anymore. In fact, it won’t power on anymore without being plugged in.
On the other hand, the KDE battery widget always reports a max. of 11% when I charge it, aging 94% - I think 11% should be at least enough to boot up, if accurate.

How can I diagnose why/how charging is affected, or the battery is actually broken, or the reporting by OS is wrong?

Please provide Information: