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Air conditioning drains battery?

My wife, when we first married, was always in the habit of turning off the AC in our car before turning the actual car off. I didn't understand why, and she fell out of the habit after seeing that I never did it and nothing went wrong.
<br/> While we were visiting her parents, my wife did not turn the AC off on her dad's car. When he got in, turned the car on, and saw that the AC was turned on he reminded my wife that she needs to always remember to turn off the AC prior to turning off the vehicle. He seemed to say that it had something to do with Arizona's climate.
<br/> I'd never heard this (my wife and I live in Virginia and I grew up in Philadelphia). I always figured that while the car is shut off, the AC is shut off--no big deal. Additionally, it seems like the owner's manual should mention this if it's a problem.
<br/> Apparently, my father-in-law's battery went dead the other day and he thinks this is due to all the times my wife left the AC on in his car when turning it off. (I should add--this is not something particular to his car--he's always had this policy, but this car is new)
<br/> What say the experts? Does leaving the AC on when you turn a car off cause problems in any way--particularly with the battery?


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