I tend to switch every few weeks between Apple's current wireless keyboard and the Extended Keyboard II with an iMate attached. I've had the iMate for about 25 years, and I've also been wondering about the battery. Thanks to the author for finding out what it's for! I do have the same gripe as the author regarding the iMate not distinguishing between left and right modifier keys, though. Anyhow, after all those years, it's indeed amazing how the whole ancient setup still works flawlessly with my current ARM Mac.