Disclaimer: This information is provided for academic curiosity and as a testamonial to what I was able to do. It should not be followed unless you are aware that you may permanently damage your iPod and are willing to risk losing your iPod. I'd love to hear from people who successfully make batteries for their iPods, but if you fail, don't come asking me for a new iPod.

The image files are quite large, feel free to open them in their own tab or window to see them full scale.

The first battery I built actually only puts out 9V because Fry's was out of 8 AA holders. I haven't had a chance to test it for recharge life yet, but will update this site after the numbers are in.
You can see the design quite easily here: note the female firewire plug available from outpost.com see below for a wiring diagram of the plug I purchsed. I provide no guarantee that yours will be wired the same way, be sure to check yours with a volt meter. I found out which should provide the power from frontx.com. For me it was pin 1 (ground -) and pin 4 (+,) good luck.