Why does the laptop battery indicator show the battery 100 percent when plugged in but when it out it shows the real percent?

Laptop batteries have a fairly short running time - depending on how you're using it. Typically, you'll get around four hours work done between charges - provided you're not constantly accessing the hard-drive (by playing music while you're working etc). If the percentage instantly drops the moment you remove the power cord - then the battery may be faulty.