Faulty car battery not replaced

I boughta new car battery a few months ago. It doesn't even turn my engin. I took it In, and they kept it for testing. They lend me a battery, and I had no pproblem with it. I went back after a few day. They told me that there is nothing wrong with my battery, and that it should be my car. I knew that it wasn't my car, but had it tested. What is the purpose of a guarantee if it is to no use?