Um, so we noticed recently that our iPods were losing their battery charges much more rapidly. Well, I actually noticed it on Sepi's iPod (iPod G1/2 to you wannabe hipsters out there) a long time ago but the battery replacement situation had not sorted itself out at that time... so I waited. I did my usual "research" (aka, geeked out on the boards) and finally found what seemed to be the most reliable source (and with the best directions) -> NewerTech. Seems that they kept coming out with higher capacity battery replacements for the iPods.
Here's the basic info you need to know:
1. The iPods (except the Shuffle) are hard-drive based and subject to fragmentation. That means as you add and remove files, they're not stored contiguously and so the drive has to search for the bits of your files. When it happens on your computer, it slows things down. On the iPod, you lose battery life. So, as part of finding out why your iPod has lost life, it might due to severe fragmentation. But this shouldn't be too much of a problem.
2. Batteries die - all of them. So eventually, they say about 1.5 years after usual use, they must be replaced. You can either pay Apple or you can try to do it yourself. I obviously chose to do it myself.
OtherWorldComputing has lots of batteries for you to chose from.
3. The iPod G1/2 came with a battery rated at 1300 mAh and you can buy a replacement that has either 1800 or 2100 mAh. That translates to significantly longer battery life than even when your iPod was new. If you want really long battery lie, go with the 2100.
4. Don't look for screws and don't do it yourself if you're not mechanically inclined. It's not hard but it sure isn't eas and you MUST read the directions before starting.
5. Final lesson - don't charge the iPod until it's fully drained. It's something about the recharge cycles that these new fangled batteries have and each time you charge it, even if it's a partial charge, it takes away "one life."