Dell Latitude D820 batteria shuts down

I’ve had this laptop Dell Latitude D820 shut down with less than five minutes between shutdowns or more than two hours, so I cant figure out what the problem could even almost be. It has shut down with me running one internet browser tab, as well as running multiple applications. I was told it could be overheating, due to where the fan is located; so i have the laptop elevated, giving MUCH more airflow. Im more than willing to take suggestions. Logical, Sensible, Insane, and Dangerous; I’ll try anything once.
http://www.batterietutti.com/dell-latitude-d820.html