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Why take the battery out anyway? When you do take the battery out, the clock-speed gets dropped (I think it only uses 1 core too, not sure).
 
I think this is published behavior. If you want to run without battery then try cold booting the machine without the battery rather than removing while running. I was told, but never tested this, by apple that the machine will run at full speed if you do this.

Peter
 
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