I do know some laptops underclock when on battery. I know i have mine set to go to 800MHz. Why don't you get CPUz(easy find on google) and post what type of processor she has and what speed it's running at both on and off A/C.
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Desktop * Athlon X2 4200 | 2G RAM | 160G HD | 7600GT
Laptop1 * C2D T5550 | 3G RAM | 120G HD | Intel x3100
Laptop2 * C2D T5250 | 2G RAM | 160G HD | Intel x3100
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