I know this might belong in the wiki but, I do not have access.
My Gigabeat F40 was suffering from a ballooned failed battery and I had stopped
using it with the intention of doing the ipod battery exchange listed.
I accidently cracked a battery case for an EN-EL5 for my digital camera and inside I found a battery that is nearly the same size. The orignal was 850mah I believe. The camera battery is 1500mah.
I took scissors and clipped off the + tab so it would align with the camera battery tab. I clipped the - that went around the side and just taped the control panel onto the battery and then covered the battery with tape. I had to lightly sand the inside of the cross touchpad on the front of the case because the battery was a little taller.
It worked very well.
It is a source for a replacement battery if you cannot solder.

Thanks.