Samsung r530 brightness problem

eveytime i unplug the laptop the brightness goes down to half or more
the battery is fully charged, in device manager power options i have set the brightness to full for both plugged in and on battery, everytime i reboot it still goes down to half brightness, there is no option in the bios to alter it either.
i have to use the fn plus up key to up the brighness everytime i reboot.
if i reboot with it plugged in it stays bright im using win7 64bit ultimate.
i tried the samsung display manager within device manager power options
makes no difference, the original hd in the laptop died so i had to get a new one
and do a win7 reinstall from scratch, with drivers from samsunfs own support page.
i wonder if there's a reg fix or something.