I've thought my battery was dead several times, but you can do things to get it to hold a charge again. Try charging the battery in a different controller if you have one. Try cleaning the gold connectors on the battery, there can be a lot of buildup on there. Try shaking the battery pack a bit, I don't know if that really does anything, but I like to think it does. And finally, try keeping it plugged in and charging often while you play. Sometimes they just need some tender care and love.
I've been on the same battery pack for the past 2.5 years.