Alternator/generator/regulator problems of the 2001 Chevrolet 2500

Alternator does not charge after startup on days when outside temp. <30 deg f. Takes up to 90sec. Depending on temp. To start charging. . . I am afraid it won't charge one of these times, and leave me stranded. Dealer says this is normal for new vehicles, although I or other mechanics I've brought it to have ever heard of this. I am the second owner of this vehicle (40k mi) and my warranty is up in 1000 mi. So I would like this to be taken care of now, before something happens, but Chevrolet dealer won't acknowlege the problem. Although they did replace one of the batteries when my mechanic found it to be bad. I had to bring it back twice to the dealer before they admitted one of the batteries was leaking, corroded, tested bad , and replaced it, thought that might solve the charging problem but did not.