It's an old Hyundai SUV with less than 80,000 miles on it. The battery is newish, still under warranty. I make sure to turn everything off, so nothing should be draining the battery. Everything seems fine when my wife or I drive the car once a month or so. But, if the vehicle sits for longer than that it won't start. It's been checked by a mechanic and he couldn't figure out what the problem is. How can I determine what phantom thing is draining my battery?