will apple replace my phone with bad battery iPhone

I have an iphone 3gs and still have warranty on it. I don't talk on it much at all. I use the internet and text and listen to music. the most i use the internet is about 30-35minutes a day, sometimes less. I have everything turned off like bluetooth and fetch data and even 3g. I can go probably 6 hours and do nothing to loose all of my battery.  I literally watched my phone go from 90 to 79 in 30 min. just listening to music. Would apple replace/fix with my warranty or would they just laugh at me?