He had been sitting in the car for a while waiting for us and listening to the radio . When he went to start his car the battery was dead . So we decided to take my car over there instead . I always hate having to park the car somewhere because then I have to worry is someone going to notice it or is it going to get towed or whatever .
Question: What caused the car battery to be dead ?
The answer to the above question:


He listened to the radio while the car was turned off and this caused the battery to die .