So, this is simple, i unplug my moms dell inspiron notebook and the screen is super dark even when i bring up the darkness. Im doing this for mother dearest. This isnt my laptop.

Check the battery charge (if full) and the power settings. If the power settings are set to "Power Saver", set it higher. That will deplete the battery faster, but might be brighter. Turning the Aero off might also help.

The brightness settings are probably different for when it is plugged in and when it is not
double click on the battery icon in system tray
and tweak settings

Control Panel > Power Options > Adjust the display brightness

Turning Aero off will help save battery life so if you need to increase the display brightness turning off Aero will give you a few more minutes running off the battery.

Power options is where you change the screen brightness