Not so. Apps cached in memory do not use CPU cycles or battery power, except for the tiny amount required to load them into memory. Of course, if you use a task killer to constantly kill them, forcing them to constantly reload...well, those tiny bits will mount up amazingly.
In the atypical case that you truly have a runaway app that is eating battery because it is poorly coded, you should identify it with something like System Panel, and then delete it.