ipod classic losing charge very quickly

hi

Ive had my ipod a few years and never bothered to update the firmware, for some reason I stupidly decide to update about two weeks ago. I immediately noticed my battery was draining very quickly. Previously I would prob average 15-16 hours on a full charge, I was now lucky to get 2. I restored the ipod to firmware 1.0.1 but the problem was still there. After a week of searching the web and finding no answers I decided to change the battery, but I still have the same problem. Does anyone have any ideas?

thanks