Im stuck at a dead end please help.
 

Recently I have replaced my spark plugs. Had a fresh oil change. Replaced my alternator (and serpentine belt). Also bought a new battery. And everything was running great. I noticed my fuel economy wasnt good and proceeded to replacing the fuel filter (fuel pump tested good.) Unknowingly I did not disable my fuel pump before doing this and since then my battery light is on and my voltage wont read past 12.2 when car is on. And if i just leave the car running my voltage will begin to drop and the battery will completely die. It starts fine and once it dies and I can jump start it and it will start but the battery wont hold charge and voltage wont go past 12 and just drop from there until completely dead. Ive switched the alternator and battery again thinking it was the charging system but it runs good for a day then voltage level drops and drain battery. I need HELP!!! Please. The problem started after the fuel filter change what are the chances that this is going to be my prob. Once again I did not disable the fuel pump before the filter change. Please I hope hope someone could help im so lost.

You might have a heavy current draw. Check voltage drop from battery + to alternator. Check voltage drop from battery - to a good ground. Good luck.