Did you buy the battery new? If so it may be under warranty. Also check the battery's serial number at apples web site to see if it is a lemon. Apple has had recalls for certain batches of batteries. Also if it is one of the recalled batteries, it shouldn't matter if it is under warranty or not.

good luck to ya
2.4gHz 15" MacBook Pro, 1.66gHz Core Duo Mac Mini, 2.5gHz G5 QUAD, 733mHz Quicksilver, 450mHz G4 Cube, 700mHz G3 iBook, 350mHz Sawtooth G4, 350mHz Revs. A and B B&W G3, 16mHz Powerbook 100, 8mHz Macitosh Classic.