Battery won't charge even though charge port and battery replaced

I have an iPad 3rd Generation with a power problem. The battery simply will not charge. When I installed a different working charged battery in the iPad, it did turn on but once the battery was drained it wouldn't charge either. I tried replacing the charge port with a known good one, and I checked the data cable and a/c adapter; all were fine. If there is a charged battery installed, everything on the iPad works until the battery is drained, then the iPad is dead. Can anyone tell me what else could be wrong?