Battery Drain on '02 540

I have an '02 540, purchased used 3 months ago. Three days ago, when trying to start it, all I got were clicks and dash lights. AAA said battery had dead cell, so purchased and installed new battery. Car worked fine for 1.5 days, then same problem. Mechanic has checked and OK'd charging system - thinks there must be some unknown battery drain. Thoughts?

Sound like a battery drain to me. Have you added any new accessories (electrical)?