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I've got a 15" macbook pro, i5 2.4ghz which I've inherited. When I use it to watch video -- with the notebooks monitor turned off, video piped to an external monitor - the battery dies in 90 - 120min.

Is this normal? Does that mean its time to replace the battery? What sort of life should I be expecting?

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Not enough info. Size of external monitor? Type (size, compression, etc.) of video?. 2 hours could be reasonable if it is CPU/GPU intensive. –  user588 Feb 26 '11 at 2:37
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3 Answers

up vote 3 down vote accepted

You can check the battery's capacity to see if there's a problem with that. I use the free CoconutBattery. It will give you the percentage of the battery's full capacity (what it had when it shipped) that it has at this point.

It's possible that you may have some errant process running that are eating up your battery. Open Activity Monitor and click the CPU column head so that it sorts by CPU usage. Is there anything there that shouldn't be?

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with CocnutBattery I can see the battery is at most of its factory capacity (tiny bit lower) ... so at least I know its behaving correctly and doesn't need a new battery. –  Erik Feb 26 '11 at 15:36
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If you're powering a large external display, it's possible that the GPU is having to work harder than it would for the internal display, which might lead to reduced battery life.

Also, if I recall correctly, since Mac OS X 10.5 it's not possible to only drive an external display on battery power - the steps required to do so (connect display, close clamshell, wake with USB) don't work unless power is attached. So if by "turning off" the internal display you just turn down the brightness, be aware that your Mac is still using GPU power to put content on that display even if you can't see it.

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I recommend CoconutBattery too. For a battery refresh (with perhaps better capacity after that) you have to run your MBP in battery mode until it turns out itself. Then you have to wait until the white LED doesn't pulse. Then you should recharge your MBP to full capacity and keep it at the power for some more hours.

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