Kia Sonet Battery problems

I took my car to the dealer last year for my 1st service and I mentioned to them that my battery light comes on and off while driving. They said that they tested the battery and all was fine. It kept on doing that since. I took my car for it's 2nd service and told them again. Now they said my battery life is only 23% and I need to replace the battery. But I have to pay for this. How can a brand new battery barely last 2 years? I emailed them and more than a week later no response from them. I like my car but the service is not good enough.