vettes4fun

I nevered had any problems till I replaced it with a chrome one. So I called the company back that I bought it from and they said they had shipped the wrong Alternator in the box (shipped it for a 2004-2005 vette and mine is a 97) so they sent me the right one. Now when I start my car it's charging at 13.4 - 13.6 volts but after it runs for about 5 mins it drops to around 12.2 - 12.6 volts and lower at a idle with a/c and head lights on. So I called the company back again and they sent me a 3rd one same problem. So I put my factory one back on but still the same problem. Why is it only charging at first it's not killing my battery. Putting the 2004-2005 alt on did it screwed up my car. I checked it tonight at the Alt it's about 13.2 at the battery it shows about 12.4 and on the guage it shows around 11.4 or lower