The laptop screen goes darker when the power adapter is plugged in. When the adapter is unplugged, the screen becomes brighter.
Wrong setting in the power management software or BIOS.
Possible troubleshooting steps and repair solution:
Most likely the problem is related to the settings in the power management software or BIOS.
Find the power management utility installed on your laptop and check the LCD screen brightness settings. Apparently, the LCD screen brightness settings are higher for the battery mode. Increase LCD screen brightness settings for the AC mode and the problem will be fixed.
In some laptops, you can change LCD screen brightness in the BIOS menu. Enter the BIOS menu and make sure the LCD brightness is set to maximum for the AC mode.