Battery Light Remaining On - NEED ASST.

[mjj9d8]

I have a 04' Mercury Sable GS 3.0 12V (Engine Vin. "U"). My vehicle was having trouble maintaining proper voltage. When the car would sit running idle with nothing on it would get about 13.2v. With it running idle with all accessories on, it would come down to about 12.5v. I went to the auto part store got a new alternator. Went to start it and would not crank. I just got the clicking of the solenoid/starter with very little power. Jumped it and got it started and then would die about 30 seconds later. I replaced the alternator, Very little power. Jumped started it and ran at 14.0v. Turned it off and went to re-start, completely dead. I replaced the battery with the same CCA of 540 (which I got off the original motorcraft battery). Installed it and vehicle started with no problem. About 2-5 minutes of running the battery light comes on and the information center comes up "CHECK THE CHARGING SYSTEM." I have load tested both the battery and alternator.
WHAT COULD BE THE PROBLEM???????
Please need a little help. Thanks Matt