When I switch to battery power, my screen randomly goes blank on my HP DV6150US laptop.

I have to put the laptop to sleep and wake it up again to be able to see or do anything. This usually happens when I watch a video. The sound is still audible, but the screen goes completely blank. It doesn't shut off, just becomes black. I usually do a fn F5 to put it to sleep and then wake it up for the screen to come on again. I have an Intel centrino duo, T5500@ 1.66GHz, 2GB RAM, 32-Bit OS Vista upgrade, and Mobile Intel (R)945 Express chipset family. I am not a computer tech, but I try to tinker around to see what's up. I hope someone out there can help me! This only happens when I switch to battery power. My screen/sleep/shut-off settings while on battery power are not the cause. Thanks!