My ipod classic (6th generation 80GB) was only getting a couple hours playback on a full battery charge. I was charging constantly everyday. I listened to one album and half the battery was gone.
Once I got the new battery, I started dissembling it. It took a while, un doing the pins was frustrating, but I finally got it open. The new battery came the glue on it, so swapping it was very easy.
Take your time opening it. The device will get scratched, but pay extra attention when putting it back together. Make all the pins are back in the original positions. Go on youtube and look up how to videos before starting.