Wont Recognize Battery At All

Hey Everyone i just bought a used black macbook from a friend. its running a 2 gig core 2 duo,1 gig ram, and a 120gig hd. Anyhow the problem i m having with it is that the battery dosent charge or anything, when i check the system profiler it says there no battery plugged in at all. if i take it out and put it back in while its on the charger light turns amber for a few seconds and then back to green yet it still dosent recognize the battery as being installed. Any ideas if it might be something more then just the battery?

also how dose apples warranty work? my friend that sold it too me dosent have the receipt and said he paid cash so i doubt they can pull it up. is there anyway that they would replace the battery under warranty?

Sorry for asking so much, but thats in advance.

The mac rocks btw (10yr pc user) i dont plan on ever going back to PC's

Go here
https://selfsolve.apple.com/GetWarranty.do
enter your serial number. It'll tell you how much warranty time you have.