Default [Q] Red light after screen replacement

Hi Guys,

Read a lot of posts about the red light of death but not had any joy as yet.
So screen cracked, I got a replacement and changed it, got the flashing red light and after quite a few hours of changing that would randomly stop the phone started up fine and I got everything working. Phone was still on about 75% change after some usage so that night I left it connected to wall charger and when I woke up in the morning it had the solid red light of death. I have since opened it up and disconnected batt for a few seconds and connected it back up without the screws (some posts suggested it was too tight with the screws?) and still nothing, only other thing I can think is the back cover has a slight gap on the top left hand corner. I know it is supposed to be flush but I didnt think such a small gap would cause red light of death?
I'm not sure if it is bad battery (did take a very long time to charge when I did get it up and running after screen replacement) or if it is back cover. Any suggestions on how to diagnose the problem?