battery drain when powered off

Basically as the title says.

Phone had a new screen & charging port added. Then about 2 weeks later, the battery drain was crazy, so changed battery for a "Genuine" one.

Still the same, however, when the battery dies, you can't charge it up, the apple logo flashes up, then off again.

Only way to charge the battery initially is to plug it into another iphone, then put the battery back in, and it will charge as normal.

I have seen elsewhere, that a charging port can cause the U2 power chip to cause problems? Is this toast if it is, or would another charging port solve this issue?

Phone has been rebooted, and also reset.

Any ideas guys?

Thanks