I would disagree with Pyree. There are two classes of battery problem - the battery won't charge, and the battery won't HOLD a charge. If a battery says it is charged fully, then dies within a few minutes of you unplugging your laptop, the battery is dead and you need to buy a new battery (as Pyree said).
However, it sounds like you are having the same issue I had. The battery is fine, but it won't charge past a certain percentage (and that percentage keeps getting lower). That is actually a software issue, where it 'intelligently' stops trickle charging so that it doesn't burn out the battery - but obviously that isn't working right. Do your system updates with Windows, and go to Acer for software updates/patches - that usually does the trick. But hold off on buying a new battery - save yourself the $50 before going down that path, you probably won't need it.
....then again, if the new battery trick works, post back and let us know.