This is just an idea because I've had this kind of issue before. Have you checked your backlight setting? Hopefully, your ppc is set up the same way as mine and you can follow this path to the backlight setting - Start - Settings - down at the bottom are three tabs, click on the System tab - Backlight - down at the bottom are three tabs, click on Brightness. What is your Brightness Level set to for when it's on Battery power? Mine was set to the very bottom once and every time my hx2495 wasn't plugged in, my screen was virtually black.
Let us know if this helps!
