What should I do if my Apple laptop wont on battery power?

When it is plugged in to a power outlet, my 6 month old 13" Apple MacBook runs just fine. As soon as I unplug it from the outlet, it shuts down and wont turn on, unless it's plugged back in. When I look at the power level of the Apple Laptop Battery , it says, "Laptop Battery is not charging," even though it is plugged into external power; it also reads that the battery is charged at 85%. What is wrong, and what should I do? How about buying a new battery such as Asus Laptop Battery?