Have you tried a different USB cord or port?

Check your dock connector for dirt/dust debris..it might be that the pins are dirty and not making a good contact. clean with a toothpick or something non metallic. Next recheck your soldering is good, no bridges between contacts/wires. I had to use 3 batteries before I got a good one, I think they are so old and sat on shelves that it's a distinct possibility that you've just got a duff one.

I'm not sure how you'd go about changing the dock connector on this ipod but there's a clever guy on here who will, so perhaps he'll chime in if those things don't work.