1997 Cadillac STS battery no charge help

1997 Cadillac STS Northstar eng 100,000 miles - battery no charge problem

Battery went dead - after recharge went dead again - assumed alternator bad - changed alt - problem not fixed - now the battery symbol lights and the display reads 'battery no charge' - OBDII scanner shows no codes found - please give me your suggestions- Dick Schulz dickschulz@charter.net 507 254 3287