I couldn’t believe this, but the knowledge base entry is real: if you take the battery out of a MacBook or MacBook Pro (not sure is strictly all models are concerned, and the entry dates back to August 2008), the processor speed will be reduced.
This prevents the computer from shutting down if it demands more power than the A/C adaptor alone can provide.
I guess it may make some kind of sense, from an electrical engineering point of view, but unless you’re using a MacBook Pro on a MacBook adapter (in which case, if I remember correctly, the battery was known to take eons to charge) it seems like the problem would be pretty rare — otherwise you’d have heard by now of lots of people draining their battery by watching HD video on their MacBooks while connected to a power outlet.
And it seems to me that there could be much more elegant approaches to that edge case than forcefully and secretly cutting down 40% of your computer power when all you wanted to do was extend the life of your battery, and avoid having it explode on your desk in the middle of the night. (Although I think batteries are basically just as likely to blow up even when removed from the computer. You can’t stop progress.)
It’s just a little bit counter-intuitive that you can set your CPU to full power while on battery alone, but it will slow down without warning if you’re on A/C alone. Or maybe the system does reduce CPU speed while on battery, regardless of what energy settings you’ve chosen? After all, the Mac knows best.