Stop in for a cup of coffee

Jeep died on me at McDonalds today, had to jump it. On the way home, volts dropped from 11.2 to 9.1 in 8 miles. That was at 10am. Verified with my voltmeter when I got home. 9.1.

Just walked out, threw the voltmeter on the thing. Before anything else, voltmeter shows 12.7…. Same with the plug meter. Start the thing, fires right up. instantly dropped to 11.07 and with lights and blower on, continued dropping a tenth of a volt every 3-5 seconds. Shut the thing off, try to restart instantly, dead. Get out, put voltmeter on, 10.1 Walk around the Jeeps, get my tools to change the battery to the brand new truck one, hit it with the voltmeter again, battery back up to 11.77. Without it running, how the hell does it charge with it off?