I recently dug out my old Creative Zen Vision W but being circa 2006, the battery was roached. You can't get replacements anymore but since the battery is removable, I found some cheap pouch cells that were the right thickness and soldered them to the existing connector/charge control board.
The original was 1650mah iirc, the new ones are allegedly 2500. From some basic testing, I believe its more than it used to have. $15 all in, considering no one makes these anymore and NOS ones go for $190, I'm happy to have 2 functional ones again.
Nice work!
Thanks, I figured it was low risk because they were already fully dead, cheap(ish) experiment. I did watch it like a hawk while I was charging and put it in a lipo bag in case the spicy pillow got too excited. It's been running well for a week and no fires so far.