Most likely the reason is because you're not using cpu scaling on the processor. Almost all laptops use cpu scaling to reduce the voltage, hence the power the processor uses. Either get a kernel or recompile yours to enable the cpu scaling driver (should be the one for Intel Centrino I believe). I'm surprised that you said the battery time went from 4 hours to 1 because I believe Intel's mobile Centrino processor on the average uses relatively very little power (~7 watts according to intel's website) compared to AMD's (~25 watts).
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