iPhone 3GS Charging Fault

Hi,

I've got an iPhone 3GS and needed to swap the battery out. I did and it worked for a while, but I noticed that the battery wasn't charging at all. Eventually, even on the mains the battery died and all I get now is the low battery screen. It recognises that it's plugged into the wall but it never comes off the low battery screen. I can get the phone into DFU mode and restore using iTunes successfully (no error 29) but it returns to the low battery screen straight after. I checked that the battery I fitted was an exact part number match to the original so that shouldn't be an issue. Also, the phone can stay in DFU mode on the battery without any issues.

Thanks...