Interesting, for a long time, the POWER instruction set architecture information required for designing a CPU to it was held secretive behind an expensive proprietary liccense, but now it is published openly. There is at least one Open Hardware reference design on GitHub, take a look.
20191012/https://www.nextplatform.com/2019/08/20/big-blue-open-sources-power-chip-instruction-set/
20191012/https://github.com/antonblanchard/microwatt
TODO!
Ah, so it turns out the clock does have a diode that allows it to get power from the main logic board. So when your Macintosh is powered on, the battery is not being drained.
https://68kmla.org/forums/index.php?/topic/58070-classic-clock-wont-run/
Interestingly, older Macintoshes had an externally reachable battery compartment, to make replacement easier.
https://68kmla.org/forums/index.php?/topic/57057-help-id-this-port/