How can I connect the RCX 2.0 to an external power source?
The older RCX 1.0 had a power plug inbuilt. However Philo has shown how to create one yourself in a fairly simple way:
LEGO® 9V Technic Motors compared characteristics, scroll down to the section "Getting maximum mechanical power from RCX output" towards the bottom of the page.

The idea is to create two fake batteries and draw a wire from each, creating a +9V and a ground from a separate source. You would need a voltage regulator though. WARNING: supplying incorrect voltage can destroy your RCX.


Image and technique by Philippe "Philo" Hurbain © 2000-2009, used in accordance to "You are free to use it for any purpose, just give me credit...".