3GS had original battery and wouldn't hold a charge.
Go the replacement battery and followed the web page instructions. After replacement, the battery wouldn't charge and the iphone would re-boot over and over. I opened the case and checked all the connections - same story.
I called Mike at ifixit and he said it sounded like I had a defective battery and they would ship me another that same day. The new replacment came in 2 days and that fixed the problem.
Thanks for the great support and service!!