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?

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.

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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.

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.

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.

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.

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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So I made an account just to reply to this post cause I think this is very useful information, so I own a 2001 ford ranger and my ac only worked on max power. I then replaced the resistor and my ac started working just like new again, the resistor does exactly what the name suggests, it resists power coming to the blower motor which changes the speed of air blowing through your ac. In my truck it goes low, medium, high, and max. Also my ac only worked on max because the resistor has a failsafe on it which if the module breaks or fails then the batter then sends power directly to the blower motor, which is why there’s a max option and why it worked and all other ranges didn’t. Now just recently I would go outside and turn my truck on and it wouldn’t turn the starter motor, aka the engine wouldn’t start, but all my electronics would turn on including my ac and when I’d look at the battery gauge it would read at 1/4 battery power and when I’d look at the ac control it’d be on high (not max). Now I don’t believe my battery was draining while it was off, what I think is happening and could be the reason to this old wives’ tale is that when I would park my truck with the ac on, and specifically on the high setting (I’m unsure if the other modes apply yet and I doubt the max setting affects this at all), that puts an excess load on your battery, sure if the alternator is on it’s trying to charge the battery back up but since you have something “resisting” current, you’re technically still lowering the volts being recovered, so when you shut the vehicle off you already killed it from turning on again, I doubt this is an issue with new models of cars due to technological advances and less power needed to crank an engine. So I hope my story can help somebody out there, there’s always a method to the madness.

When you turn the ignition switch to Off, the power to most of circuits, including the HVAC blower motor, is cut off. That’s with a switch that is working correctly.

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I know of no issues this will cause with the battery, but living in AZ- leaving the blower on in the summer is a sure fire (pun intended) way to get a blast of extra hot air in the face when the blower first kicks on.

Yes I know that, I meant when the car is on and I turn it off without turning the ac to the off position before, it doesn’t let my alternator charge my battery back to 12 volts because like you said the power to most conduits is cut off when the ignition is
in the off position. So when I go back the next day to turn it on, the battery doesn’t have the full 12 volts to power the starter. Solution, turn the ac off before I turn off the car, to let the battery get back to 12 volts so I can crank my engine over the
next day.

Couple things to make note of here:
-Your alternator does not charge your battery when the engine is off, nor is it supposed to.

-If your a/c blower is killing your battery over night, then you have an issue that needs to be addressed.

-Your situation sounds like you have a parasitic draw on your battery, or you have a weak battery. Get this looked at before it leaves you stranded.


I saw your response on this thread. I’ve found that my a/c clutch was causing my battery to go dead. Does this mean the relay switch is bad? Or need replaced?

Probably would have been better to start a new thread so that people don’t have to read through stuff that is over 12 years old

The relay is in the fuse box under the hood. Inside the lid you have a map that identifies all the fuses and relays. Switch the A/C relay with another identical relay from inside the fuse box and see if that solves it. If yes, it’s the relay.

How do you know it is an A/C component that drains the battery?

I just found this forum and see something in your reply that applies to my 2011 Buick Enclave right now. There is a parasitic draw on my battery and our trusted mechanics that have looked at it are at a loss as to what it is. They are very sure it’s NOT an alternator issue, that it’s something else. One thing my husband and I have noticed is that for sure, at least 2 of the 3 times we’ve tried to start it and couldn’t get it to turn over, the a/c light came on and started blowing. However, in neither of these instances was I using the a/c the previous time I drove it. (I was adamant about this and remember it clearly because my husband fussed at me for NOT turning the a/c off before I turned the car off! I told him with absolute certainty that I had NOT used the a/c!) So if it’s not the alternator, is there any way my car’s battery is being drained by the a/c, even though it wasn’t on when the car was turned off?? HELP!!

The AC is not your problem . You can start your vehicle or turn it off with the AC on . Vehicles with Auto Temperature Control do this all the time. You really need a better shop .

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good way to test this:

Find the A/c fuse, the blower fuse, A/c relay- and pull them all out when you park your car at night. Then, before reinstalling them, try to start your car. If your problems consistently stay away, then your a/c may be drawing power overnight. (A stuck relay could possibly cause this)

Thank you! We will give that a try.