2001 Cadillac DeVille DTS Question: 2001 Cadillac DeVille DTS the Service Charging System says battery not charging?

My 01 Cadi Deville DTS has been unable to remain running. It will start, but I must keep my foot on the gas, to keep it running, soon as I remove my foot off the gas, it will cut off. During all of this,the Battery voltage will decline. Also the Service Charging System will appear. I took it a repair shop for this problem, and replaced the map sensor, and it seems to have made the problem worst. Any help would be very appreciated_thank you kindly!