The flat button on the left side of the MacBook Pro, when pressed, shows the battery level of the computer. Whether the computer is powered on or off, this feature works just the same.
Pressing the battery-light indicator causes the pinpoint-size LED lights to glow. All eight lights glow when the battery of the MacBook Pro is fully charged. The MacBook Pro has a rated battery life of up to eight hours, so each light represents approximately one hour of use. However, battery life varies with the software being used. This feature is standard on both the 15-inch and 17-inch models of the MacBook Pro.