Probable Cause

• If your laptop screen dims when you connect it to the power adapter, it is likely that your current power management plan is set to have a lower screen brightness when plugged in than when on battery. If this is the case, your screen will automatically dim or brighten, depending on the charging status of your laptop. If you accidentally bump your power cable, you may see the screen brighten as it moves from plugged in to not. By default, most power management schemes keep the screen dimmer when not plugged in, to save power.
  
  

Temporary Resolution

• You can temporarily adjust the brightness of your screen, regardless of a connected power cord, with the brightness keys on your keyboard. These keys vary by manufacturer but, generally, you press the "Function" key and either the brightness up or down key to adjust your screen. The changes will take effect until you plug or unplug the laptop and the default settings override your temporary changes. Right-click the battery icon in your taskbar and choose the option to adjust screen brightness. This will bring up the power plan settings section of the Windows control panel and, at the bottom of the screen, is a slider that controls your screen's brightness.
  
  

Changing Plans

• From the "Power Options" screen of the control panel, you can permanently alter the behavior of your screen when plugged in or when running on battery. The Power Options screen shows your current power plan and click "Change Plan Settings" to alter your computer's behaviors, including screen brightness. Adjust the slider bar to the left (dimmer) or right (brighter) for your computer when plugged in or on battery. Depending on your preferences, you may use two different settings or the same for both. When you save the changes, they become permanent (until you next change the settings). Click the "Restore Defaults" link to change your plan back to default settings.
  
  

Power Plans

• Since the release in Windows Vista, Microsoft has included three default power schemes with its operating system. You may have more than the default if your laptop's manufacturer has included a scheme. The default plans include high performance, which utilizes more of your processor's capabilities and keeps your screen bright. The power saver plan exists to prolong the life of your battery, thus your processor performs more slowly and your screen dims, especially when unplugged. The middle scheme, called "Balanced," is a happy medium between hardware performance and energy saving.