Do I really have to shut off the ac in the car?

Pure superstition. I am a mechanic, FWIW.

I'm just not going to get into an argument about car batteries in India, mate. See, the quality of the car battery is a total distraction. You attempted to justify the existence of an urban myth using the following logic: AirCon units add load to an engine. When a car starts, the load on the battery is substantial and increasing that load is a 'bad thing'. Older style batteries (most likely still in extensive use in India, I agree) are more prone to wearing out and need more maintenance than modern batteries. Therefore it makes some sort of sense that this urban myth may have some basis. This is why urban myths exist, and this is precisely why they are perpetuated - by someone with seemingly good levels of knowledge (the information about relative battery types is 100% correct) directly or indirectly adding credence to the myth with faulty logic. Yes, I can see how the link has been made, and the advice created, but it is flat out wrong despite all three of the statements above being completely true. The main stumbling block of the link between AC load and cranking causing battery drain being made entirely false is that the air con, through the cranking circuit only leaving the starter and ignition systems of the engine only live through starting, means that the car itself turns the AC off while it is starting - the position of the switch inside the car is entirely arbitrary at this point. Turning the AC off is to deny it electrical current - without that, it is just a spinning pulley with no/trace loading on it. It's as off as it can be while the engine is cranking. So any discussion on battery types is a complete distraction as it is irrelevant to the core perceived 'issue' that created the myth. The bottom line is that I read part of his question to be whether there is any historical context for the advice and well, there just is -- as misguided as it may be. This is why I clarified. There is way too much noise and misguided advice on automotive (and medical, from what I read in MeTa) AskMe's, and this is very seriously not helpful. Your answer, while based on a reasonable level of knowledge, has become noise. While it may be enjoyable for you to wax lyrical about why these myths come about, you have directly implied in your answer that it will have a tangible effect on battery life: If in this scenario one had an A/C compressor being powered by its belt, which is in turn powered by the starter, which is in turn powered by the battery -- there is absolutely a material effect on the life of the battery (though it's admittedly small) given that it will have to pump out more power to turn the A/C compressor. Does that make sense? No. It doesn't make any sense if you know how the electrical system is wired in every single car I have worked on or seen. If the AC is part of the car (rather than an external, fully electrical system, as mentioned above) then your argument is plain wrong as there is no power to the AC when cranking. Feel free to start with a 'pissing contest' as to who knows more about cars, but please do so with some sort of fact-based position for your argument, and please stop insisting that your position is correct without evidence. The idea of this site is to answer questions accurately, not have fun playing with urban myths. (as a complete aside, when I was in Egypt, I experienced the pant-wetting moments of the "lights off unless a car is coming" thing. The explanation given to me was that it ruined your night vision if the stars/moon were out, and the driver insisted he could see more with them off. He didn't even turn his lights on when the cars approached, just put his indicator on on the side he wanted them to pass on. Scary as all hell. And the french lights being yellow was, as I understand it, because some government bright spark decided that yellow lights dazzle oncoming traffic less. I haven't heard any research anywhere that either discounts that, or whether it was discarded over the increased safety element of better vision for the driver at the risk of dazzling).

The question wasn't "is this advice erroneous but understandably psychologically". It was "Do I need to do this". The starter circuit on most cars since 1970 (or even before, in many manufacturers) cuts off all unnecessary electrical drains while cranking the engine - this is why everything stops when you turn the key to start. This is not a modern thing by any means. In other words, everything that turns on when you turn on the ignition will be turned off while you are cranking (and starting) the engine. There is no extra battery drain. Hence, there is no drain on the 'system' when the engine is cranking from the AC. It is a compressor that is not turned on until electrical power is allowed to it - ie after the engine starts and the starter is released. The AC, the blower, the lights.. nearly everything is disabled while the engine is cranking to give it the best chance of starting. This advice is badly misguided and stems most likely from either a complete misunderstanding of car electrics (at worst) or on knowledge from 1950/60's cars (at best). Hence either way it is plain wrong for modern cars. There is no truth at all in the AC 'adding load' to a starting engine. Manually operated AC systems? That's just made up faux-technology. They are electrically operated systems using mechanical pumps to compress and cool the air through an interchanger. There is no such thing as 'manual AC'.

thanks, all. I think we can put this to bed.

I would point out that when I lived in Singapore, many years ago, most taxi's were equipped with after-market, pure-electric AC's. They aren't driven by the engine torque at all, but work more like a window-mounted AC at home. These guys would definitely kill your battery if you leave them running without the car going, and IIRC, required an upgraded alternator for best effect, even with the engine running.

So - if you take your car on repeated short trips, over time it *may* discharge your battery and ultimately lead to its early demise. This is entirely independent of the AC - and the trips would have to be extremely short for the alternator not to recover the lost voltage. Short as in 'to the end of the road and the car running for less than 5 minutes'. The rest of the logic in that answer is flawed as a result. To heap on conformation, your girlfriend is utterly wrong, and unfortunately, there is sufficient additional 'wrong' available on the internet to cloud all automotive questions. There is absolutely no requirement to shut off AC before turning off the car. All the moisture is external anyway, as the AC runs a dehumidifier in the system as part of the cooling, so Corvette man is hideously wrong - the AC actually helps the issue he is complaining about (and I echo the lack of trust of judgment for corvette owners). Oh, and a 2004 car is not, by any means, an 'old car' in terms of air conditioning. You'd need to go back to the 1960's to find anything mych different. There is, incidentally, no such thing as a 'digital' or 'analogue' AC system. They are all electrical systems of similar style, the only difference is the display for the temperature that makes you think they are different. The computer control in later models just controls when to turn on the AC and how hard to blow the fan. The presence of this control is not necessarily reflected by having a digital display.

Heat exchanges never rot out, do they - ever owned a SAAB?. There are external parts to consider - and I believe this is what the Corvette thread refers to, I'd never heard of doing this prior to finding that post, FWIW.

I would again look to what kind of car it is, and how high the AC is set. Some of my less fortunate friends who live in warm climes have cars that will die, actually stall out then and there, if they AC is turned up to the max. God only knows what the strain would be if their weary vehicles were started with AC on full blast. If your girlfriend has cars that are smallish, and were purchased somewhere mild, and the moved to blistering heat, she may have a point.

Hold on here, gang. There is definitely truth to this but as the OP mentions, it's related to *starting* the car with the A/C on versus shutting it down (to which knave and TomMelee alluded). On cars with manually operated A/C systems, if the A/C is on when the car is in the process of starting, there will be more strain on the starter motor since the belt turning the A/C compressor will need to turn it with the compressor clutch engaged. Given that the alternator is not running when you engage the starter motor, the starter must draw direct power from the battery (which is not getting recharged by the alternator), and hence if there is more strain on the starter, it will need to draw more power from the battery which over time could shorten its life. A couple things can help us sort of "anthropologize" the advice she's getting from her mechanic. If you think about older cars (esp. those with carburetors) it takes relatively longer for the engine to start combustion. If you're old enough to remember cars with carburetors, sometimes it took a few seconds or even longer -- even on a well-tuned and well-running car -- for the engine to start up. If in this scenario one had an A/C compressor being powered by its belt, which is in turn powered by the starter, which is in turn powered by the battery -- there is absolutely a material effect on the life of the battery (though it's admittedly small) given that it will have to pump out more power to turn the A/C compressor. Does that make sense? Now if her mechanic is older and worked on older cars back in the day it would have been *totally* sound advice to tell people to start their cars with the A/C turned off, especially in a country like India where car parts are relatively more expensive (compared to wages, lets say). In a way, it's not unlike the situation you'll find in many African countries (and maybe other places) where people drive with their lights off at night in order to lengthen the life of the bulbs. They then turn them on just just before encountering another car. (From personal experience I can tell you that it's pants-shittingly scary sometimes if as the driver of the "other" car you experience bright lights pop on out of seemingly nowhere as you're speeding down the road.) So why would the advice be to turn off the A/C when you shut down the car? Simple psychology. It's when you're turning off the car, stopping things from running. One is far more likely to remember to turn off the A/C when turning off the car than one would be to turn off the A/C when turning on / starting the car. Indeed for this very reason, on many modern cars with climate control systems there is actually a delay of 10-15 seconds from when you engage the ignition to when the compressor clutch engages. Is it going to make a gigantic difference in the life of the battery? Probably not but if you have a bum battery that's barely got enough juice to get the car going (like I imagine many cars in India might have) it could make a positive difference in how long the battery lasts. To conclude: if you have an older car -- or maybe better stated, one that takes a bit of time to start up -- it's not a bad idea to have everything not related to starting the car turned off when you're starting it. Lights, wipers, radio, A/C, etc. It really is better for the life of the battery. You'll also be more likely to remember to do it when you're turning the car off.