Battery won't charge yet Sync works and icon shows connected to power

Hi

I have been repairing an iphone 3G that was damaged and given to me. I replaced the power flex cable as the old one was broken. I also replaced the back casing and battery, as well as a cracked screen.

My problem is that now the phone will not charge. Initially I had placed the 3Gs battery in the phone with the 4.51 watt hours. I beleived this was the problem so I have since replaced the old batter. Same problem. Iphone syncs with PC no problem and battery was powering phone until it went flat. Also the battery icon changes to the lightning bolt when connected, but like I said it does not charge.

Appreciate any suggestions.

Perhaps I have damaged the mainboard with the 3Gs battery, yet I'm presuming it is all connected correctly as sync works and battery was powering phone.

Thanks