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I've got a MacBook with a Core 2 Duo CPU. I've got CoreDuoTemp installed which can show the current speed of the CPU. It appears to always show:

Mini : 1.0 GHz
Maxi : 2.0 GHz
Current : 2.0 GHz

I believe my laptop would run longer on battery if it were to run at a maximum of 1 GHz. Is there a way to configure this, or is the CPU speed adjustment completely automatic?

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5 Answers

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You could try the pmset command for power-management settings:

pmset -b reduce 1

-b       for battery mode only
reduce   reduce processor speed (value = 0/1)

But I'm not sure it will have the desired effect.

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Thanks, this appears to be the most specific answer. I tried pmset but it didn't seem to change the observed behaviour. – Greg Hewgill Jul 19 '09 at 21:49
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up vote 3 down vote

You cannot manually limit the CPU speed to 1 GHz.

You can optimize for battery life.

Go into your System Preferences, and pick the Energy Saving Settings (the light bulb). Set the first dropdown to Battery, and the second to optimize for saving energy (my Mac is set to Dutch so I don't know the exact wording for this option in English. That should optimize your CPU settings.

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I've done that, and have the display dim reasonably soon when on battery, and shut off the display after a few minutes, and sleep the hard disk when possible, etc. I just thought that since I rarely do CPU-intensive stuff especially while on battery, I would hardly notice if the CPU was speed-limited to 1 GHz. – Greg Hewgill Jul 19 '09 at 6:34
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up vote 1 down vote

Short answer: No.

The processor will automatically adjust its speed according to usage. The only way to limit the CPU speed is to remove your battery (at least on older Macbooks that works) and rely on the Magsafe, although that does defeat the purpose of what you are trying to do.

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Can't you underclock the CPU? – Ivo Flipse♦ Jul 19 '09 at 5:59
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up vote 0 down vote

Even if you could, I really doubt this will help with your battery life usefully..

The CPU will throttle itself automatically, I would guess CoreDuoTemp is simply reporting the current (throttled) speed incorrectly..

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up vote 0 down vote

In windows Vista/Seven you can do this. You can set minimum/maximum CPU % usage for every power scheme, plus you can set passive/active cooling method.

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I suppose I didn't actually say so, but my MacBook runs OS X and I intend to keep it that way. – Greg Hewgill Apr 9 '10 at 10:02
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