You haven't specified your OS, but in 10.3.9:
If Automatic is showing in the Optimize Energy Settings dropdown menu of System Preferences/Energy Saver, and the Settings for: dropdown shows Battery Power, it's very likely that this is the iBook's idea. That is, it's the way that energy conservation is programmed to be.
Select Custom instead of Automatic, and do what you will, or use the Options tab and the dropdown menu on that page to select Highest Performance.
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