I have a 2004 duramax LB7 . Battery light came on this morning and display read battery not charging . Went to auto zone tested batteries one was bad . Replaced the bad battery and still displays battery light and battery not charging . Disconnected both negative battery cables and disconnected alternator as I was going to change it in the auto zone parking lot . Didnt have proper tools. Put it back together and it starts for a second and dies . Any ideas ?