Battery replaced, phone has power but battery will not recharge.

Hi,

My old iphone 3G all of a sudden packed up charging, the battery died and I got no response from it at all. I got hold of a new battery & flex cable, as well as a new power button (this was a another problem that had developed) and opened it up and switched them all. Put it all back together again and tried charging it just now and I can see that when it's plugged in to a power source it is working ok. It seems that the phone is running off the battery's power supply. However, when I try to charge it again the battery doesnt recharge. The phone senses the power and does everything that it should do but just fails to charge up again. I have since tried another battery and it has the same problem. When the phone is not connected to a power supply it is still powered up ok, that is until the battery runs out, then of course it won't charge and won't turn on or anything. It just displays the low battery sign that all iphones do when you plug them in with no battery but as I said, it doesnt charge. I don't think it is connector 4 within the handset as this seems pretty well connected whenever I shut up the phone.