Battery dead after I shut car off

My battery will not keep any charge. Everytime I cut the car off I have to jump it to get it started back up. It has not cut off on me while driving it. I have taken it to several shops and they are not able to find out what is wrong with it. They said that my alternator and my starter are good so I bought a new battery thinking that would solve the problem but it didnt. I had all the wires checked and still nothing. I need my vehicle badly. Do you know what it might possibly be?