2002 battery light flashing

I have a 2002 with a 4.8
My alternator started to make some noise, and my battery wasn't holding a charge very well so i decided to change the alternator today before it went some were out in the boonies. After i put the new alternator in, the battery light started flashing in the "message center" but everything seems to be running fine. The voltage gauge in the dash is reading about 15. So from there I turned everything on in the truck and the gauge barely dropped. At this point I'm not sure what to do, I'm going to put the old one back on and see if it goes out, oh I also put some tension on the belt to make sure it wasn't slipping. CAN SOMEONE PLEASE HELP ME ON THIS. Thanks.

Ok I went back out and put the old one back on and the battery light didn,t come back on. does this mean the the new one is crap, but if it is why is everything work right except for that damn light or maybe something else is going on.

You need to get the charging system tested using volt ammeter and load tester.

I went out today and bought a multimeter and tested the old alternator and the new one, alone with the battery and here is the readings

OLD = 14.78
NEW = 14.89
BATTERY = 12.56

I tried the old one first and got 14.56 with no battery light blinking, then I put the new one on and had to use my battery pack booster to start the truck, but before I started the truck I took a reading of the battery to see what the volts were and they had dropped to 11.95, so I then put on the head lights and it dropped to 11.80 and continued to go down 79,78,77,76 etc. Maybe i need to replace the battery, but if that is the case why would the battery light blink with the new alternator and not the old one, I don't understand this problem and i have to drive 2500km in 5 days.