It's normal for a laptop screen to be dimmer on battery than on mains power. This is done to extend battery life. You can change this behaviour if you want to by going into your computer's power saving options. You should find that there are alternative settings for running on mains power and battery power.
While you're looking at this setting, you may also find that the screen brightness setting on mains power has changed - you could reset it to full brightness.
Most laptops also have a brightness adjustment on the keyboard. Normally this involves holding down the CTRL or the ALT key and hitting one of two keys that have symbols like the sun on them - one with an up arrow and one with a down arrow.