Still under warantee and they want to charge me

I have a Samsung s3 mini and have had it since Sep 2014. I took it into the Vodacom Gateway repair shop on Saturday and they have called me to say that the battery is faulty and want me to pay for the replacement. How can this be possible that I am liable for a faulty battery that came originally with the phone. i am still under my warrantee period and the phone has never been dropped or had any water damage. This is not right. A supervisor needs to please call me and explain to me how this is acceptable. I will not pay for something that was faulty from the beginning.