Cron is badly behind the capabilities of the Win XP task scheduler in some 
areas.

If the prelink monster runs when I am on batteries, then it reduces my battery 
time by about 25%. Seriously.

Hence I do not want prelink to run if on batteries. I could file this as a bug 
against prelink, but I think that cron should have this ability.

It needs to associate flags with jobs, which then decide whether execution 
should go ahead. One flag would be "do not run if on batteries". When the time 
comes to run the job, cron would then detect that we're on batteries (via 
HAL?) and refuse to run the job.

This message looks relevant too, as it suggests a wholesale replacement of 
vixie-cron with a particular alternative:
https://www.redhat.com/archives/fedora-devel-list/2006-June/msg00583.html