MacBook says ‘no battery’ but works anyway?

Its a Macbook 13in with Leopard installed. When its plugged in, it books fine, but has a black x on the battery. When I disconnect the AC adaptor, the computer still works, and still displays a black X. I reset the PMU and Zapped the PRAM. Is there anything else I can do other than buy a new battery. PS: Applecare expired 5 days ago and I didn't renew quick enough. Smart huh? haha Thanks for your answers!

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Tags: 13, apple, applecare, battery, black, display, Mac, MacBook, RAM