Why won't my iPhone 3GS charge?

I bought my friends iPhone 3GS 16 gb and since I got it I have had to turn off the phone and then plug it in to start charging it otherwise it wouldn't charge. I thought it was a battery problem so I bought a battery and replaced it. This did nothing, I still had to power off the phone to get it to start charging so I ordered a new charging port with the speaker mic and antenna in one unit. I didn't change it out and just kept turning off the phone to start charging it. Just recently I did this and when I woke up the phone was at 9% so I tried another cord then another power source nothing worked. The phone will show the dead battery and can tell when it's pluged in but won't charge. I could go onto iTunes and do the restore but it still won't charge. I replaced the dock port on the phone and it made no difference. I also tried holding the reset and power button for 10 sec. Please help I am running out of things to try. Thanks for any suggestions.