Hi There: The problem you have sounds like an issue Apple found a while back where the Battery updater was not right and caused the battery to become unknown to the computer. They had people update their computers with the Battery Updater 1.2 from their web site. I have had four clients with this problem and only one was fixed by doing this update. Apple replaced the other three batteries for free.
Here is the Apple fix : http://support.apple.com/kb/HT1359?viewlocale=en_US
They also have a 1.4 updater you may want to look at: http://support.apple.com/kb/HT3421
Each time I was able to get Apple to replace the battery the machine had to be less than three years old. If you meet this time frame call them and talk only to a supervisor. The regular folks on the phone do not know this issue. If you are outside of this time period or the second owner, try the updater mentioned above but be ready to buy a battery.
Try : http://store.fastmac.com/product_info.php?cPath=10_4&products_id=150 for the battery
Good luck