Why does my fan rev up whenever my battery is installed?

Hi. Great site! Whenever my battery is installed, the fan on my MacBook will rev up pretty loudly. However, if I simply remove the battery and only work on AC power, the fan is quiet. This occurs with any battery I have tried. However, I have not tried a brand new out of the package battery, as I don't have one handy.

The battery will charge, though. So that's not an issue.

As I type this, I'm trying to think of what I have on my MacBook that could cause the problem, and the only thing I can think of is I have Caffeine version 1.1.1 running.

Oh, and I have Mac OS X Version 10.6.5.

Thanks for any help.

-Brian.