24" Intel iMac (2006) date and time keeps resetting on boot, even after new battery and NVRAM reset
Because I had 1% uncertainty about how I had placed the new battery in its holder and because I wanted to make sure it was firmly in place, I reopened the iMac. And what did I behold? The battery holder is supposed to be connected to the motherboard on two places, the upper and bottom side. And the upper side was loose! This must have happened while prying out the old battery, that is an even trickier operation than I thought it was.

So that was the cause. Now I have the challenge of fixing this. Duct tape (to hold the battery holder down on the board) didn't help, I can tell you that by now. It should be soldered, but I have neither the equipment nor the skill to do that :-(