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I have a 2011 MacBook. Along the left edge (looking at it from the normal typing position), near the front, there is a small silver button which is flush with the rest of the case. When pressed, a small series of tiny lights beside it flash green before going dark again. Nothing else happens.

Can anyone explain what this button does?

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up vote 29 down vote accepted

This is your battery capacity indicator. The number of lit dots indicates how much battery you have left.

enter image description here

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I can clearly see by the votes these days that the contest has ended :) – R. M. Apr 5 '12 at 23:49
@R.M My thoughts exactly... – Kyle Cronin Apr 6 '12 at 0:03

It's a battery capacity indicator, as noted in this Apple Support KB:

The battery indicator lights are located along the left side of the computer. This allows for quick and easy verification of the battery's charge status while the computer remains solidly in-place.

When you press the battery indicator light button, the corresponding flashes (or pattern of flashes) presents either the current charge status or a diagnostic status.

The KB also has a chart of the behavior shown when the indicator is pressed.

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On older macs it was on the battery which was pretty awesome, you could have two or more batteries and easily see which one had the better charge. (New macbook rant suppressed--barely) – Bill K Apr 4 '12 at 7:47

This allows to check the charge of your battery. When the battery is full all the green lights should flash.

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