batter not charging message

Have a 2002 trailblazer ltz. After starting the trailblazer i got a battery not charging message in the odometer screen. The battery is only a year old. So i took the battery back they tested it said it was undercharged and the put a load on it and it didnt hold. So they gave me a new battery and i put it in and still getting the same reading. The battery light stays on and the gauge reads around 12 amps. Just wonder if anyone else has had this message. Going to look into the altenator.

Um, how about checking if the alternator is charging?

Both AutoZone and Advance do an analysis of your charging system free.