The screen is darker when unplugged

Question

Hello,

I have an LG laptop (LG LE50 Express). Normally the screen is very bright the laptop is plugged to the electricity, and much darker when I disconnect it from the cables (to save battary life). A few days ago it became the opposite - The screen is darker when the laptop is plugged, and much brighter when unplugged.

I mostly work with the laptop when it's plugged to electricity (faster, better to watch DVD's, fast downloading from the internet ect.). What should I do to change it back to the way it was?