i often sit in my car reading and listening to the radio before i go to work. i have the engine off and am only using the 12v system. about 30 mins later everything shuts off. im wondering if this is just a timer sort of thing or if it shuts everything off when the 12v is getting drained. i usually will turn on the engine at that point for a few minutes thiniking im recharging the little 12v. am i saying this right and whats going on. :?