when I use my laptop without mains plugged in the screen is too dim. On mains power it's much brighter.
What do I do to brighten up the setting on battery power?
I use windows XP.
There's usually a key with Fn in blue on it and one of the other keys will have a brightness symbol with an ^ (up arrow). press Fn and then the symbol key together.
Is the laptop an acer? if so there is an icon in the system tray showing the battery symbol(when on battery) right click to bring up the brightness menu for the energy profile you are working in.