Dead iMac battery! I inherited an iMac with still plenty of life in it, but a failing battery.
Super easy! Took me 5 minutes to replace with the right tool! Thanks for the great toolkit! :)
There are two different size screws on the iMac body -- it helps to keep track of which go where. I also used the open iMac opportunity to gently vacuum out some accumulated crud inside. (It's amazing how much dust and fuzz can get in there!)