My ipod's battery is Crap.

hey. I have a 30 gb video, which i bought in the beginning of may, and im getting pretty tired of my battery situation. I charged it monday night, and took it off the next morning. I used it that morning on the way to school, a 20 minute ride. Then i used it last night for about three hours. i used it this morning for the ride to school, again another 20 minutes, and i've been using it now since 2:15, with a couple breaks in between. The battery is almost dead, but it says on the ipod page that the battery shoudl last about 15 hours. if im adding right, i have been using it for aroudn six hours or so, and it should be lasting at least double that. And i even keep the backlight off at all times, so it doenst drain the battery. i ahvent used it for photos either. Is there any explanation for this? what shoudl i do?