How to disable low display brightness when power fail?

[Unga]

Hi all

I have a laptop running FreeBSD 9.2 and KDE4. 


When main A/C power fails, this laptop immediately reduces the display brightness. When power restores, it also immediately restore the full brightness.

Because of this I cannot show a presentation without power.

Therefore, how to disable becoming low bright when no main A/C power?

I prefer a xorg.conf entry.

Best regards
Unga