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This is new, just started this month:

  • Usually it would last until 0%. For a few days it had been shutting off at around 4%. Last night it shut off at 5%... really hope tomorrow night it's not 6% and so on :/

  • It doesn't just go to sleep when this happens, it actually shuts off.

  • I bought this MacBook around August, 2008.

  • My battery is OK according to system profiler. And its health is at 85% according to iStat Pro widget. It's at 703 cycles and 3 years of using it 24/7 as my main computer.

  • OS 10.6.8, all up to date.

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up vote 2 down vote accepted

Battery metering simply isn't that accurate for the battery controller to be able to reliably tell 5% from nearly dead. Likely your battery is showing its age and it goes from a charge state that "looks like" 5% to being dead fairly rapidly.

You can try disconnecting/reconnecting the battery -- this sometimes "resets" the battery metering circuitry. But I'd recommend that you set your controls (if you can) to hibernate or whatever at 10% or above.

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It's at 703 cycles and 3 years of using it 24/7 as my main computer. Maybe it's just age. But why is it shutting down? — Sidenote: is it better to leave it plugged in all day, or to charge it to 100% and then disconnect until it goes to 0%? – iDontKnowBetter Aug 16 '11 at 22:18
    
703 cycles is a lot. You've already gotten very far with this one. – slhck Aug 17 '11 at 7:21

Try taking your battery out and putting it back in if it happens again. Then check the system preferences to make sure everything is unchecked.

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To make sure all of what is unchecked? – nhinkle Aug 16 '11 at 21:27
2  
> check the system preferences to make sure everything is unchecked. How does that make sense at all? Which system preferences? In OS X energy saving preferences, there are only: 1) Put hard disks to sleep (> irrelevant), 2) Wake for network access (> irrelevant), 3) Automatically reduce brightness (> irrelevant). /cc @nhinkle – slhck Aug 16 '11 at 21:27
    
I agree @slhck, it's confusing. Let's see if he comments to clarify what he meant, and if not, we can just remove the answer. – nhinkle Aug 16 '11 at 21:30
    
well you don't have to remove any answer the battery and energy settings – Jggreenblatt Aug 17 '11 at 0:07
    
What exactly in the battery and energy settings? I mentioned all there is, you don't need to uncheck those as they are irrelevant to the problem. – slhck Aug 17 '11 at 7:16

How long has it been since you calibrated your battery? It may just need to tweak it's measurements of full-charge and functionally-empty.

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