>Unless your car is 20 years old (and maybe it is) if they leave a door open, your car will automatically cut the power and all the lights will go off and stop using your battery.

I wasn't going to comment because I was a mechanic back when cars were not rolling computers and there weren't so many power-sucking components. I haven't kept up on the technology over the years.

I did the math once (back then) and calculated you would have to run your radio and dome light for something like a week to use the same amount of power you use to start your car once. I don't remember the exact numbers, but it was a long time.

Most cars can start the engine multiple times in rapid succession. Think about when you park at a store, shut off your engine, and find yourself too close to a pole to open your door so you start it up again and move a few inches and try to get out again. Then you get back in the car and start the engine to drive to the gas station next door, shut the car off and fill up, then start the engine again to drive somewhere else. If your car won't do that, you need a new battery.