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Since I have heard that keeping on the charger plugged while the battery is at 100% will degrade battery life, I would like to find a way to make the Mac switch off by itself when it reaches 100%. This is mainly for when I want to charge the laptop overnight.

Is the first claim true? Also, if it is, how can I get the Mac to shut down by itself when it reaches 100% charge?

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Keeping a battery charged to 100% all of the time is not good for the battery. Charging it overnight won't hurt it, assuming that you occasionally take it off a/c and use it in battery. – TJ Luoma Feb 3 '14 at 22:49

The claim is not true.

The only thing that has been proven to degrade battery life is heat, which also means overnight charging with your Mac sleeping is better than charging it while in use.

Switching it off doesn't make a difference.

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This is correct. Playing a lot of power intensive games while plugged in for a long amount of time is an example of something that can kill a battery. However simply charging it over night won't hurt it a bit. – James Manes Feb 3 '14 at 21:37
@JamesManes s/plugged in/unplugged/? – moala Feb 4 '14 at 10:05

Straight from the source:

For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her notebook on the train, then plugs it in at the office to charge.

And for more information this is a useful page:

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