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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.



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.



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.



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)



What say the experts? Does leaving the AC on when you turn a car off cause problems in any way--particularly with the battery?

Comments

  • edited July 2008
    I've been driving for 20+ years and have never turned the a/c off before shutting the motor down and never suffered any negative consequences as a result. This is one of those old wives tales that get passes down from generation to generation. Kind of like my grandmother used to keep the top of her console TV clean, would dust it several times a week because someone told her that the TV would ovwrheat and blow up if dust accumulated on the top of the cabinet.
  • edited July 2008
    I cannot explain the reason for the dead battery but failing to turn off the A/C had nothing to do with it.

    I too have your FIL's habit of turning off all switches prior to shutting down the engine, but this habit comes from the olden days when you needed full battery power to start (especially in winter). Anyhoo, leaving accesory switches on overnight does not tax the battery at all. The electric power is controlled by the ignition switch. No ignition, no drain on battery.
  • edited July 2008
    No related. You can turn it off before or after turning to engine off or just leave it on. Nothing will be harmed. The Battery had other problems. Make sure the charging system is checked before replacing the battery.
  • edited July 2008
    Thanks for the confirmation of my thoughts, guys.

    I think this is one of those things I'm going to keep to myself, for my own curiosity/informational purposes (I just wanted to make sure I wasn't doing something wrong). The "I told you so" is never safe.
  • edited July 2008
    I would like to add a couple notes. First, sometimes accessories are wired directly to the battery. This is most likely to include the power outlets/cigarette lighter (depending on the model). I also know people who wire their radios directly to the battery so they can play them without the keys. So although I don't see how the air conditioner could drain the battery, other electrical accessories can, in specific circumstances.
  • edited July 2008
    The only way the a/c could drain the battery with the ignition off is if the clutchh relay were to stick, keeping the clutch engaged. But I doubt that's what happened here.

    Love your Everlasting Gobstoppers, BTW.
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