Suppose I start the engine of my car, but keep it stationary. At this point, I start the ac or use a vaccum cleaner from the power outlet (engine still running). Then will the power be consumed from engine or battery?


3 Answers 3


The alternator powers the electrical system while the engine is running providing 13.5 to 14.5 volts.

The battery is primarily for starting the engine and to provide backup power for when the engine is not running / turning the alternator.

To answer your question, power would be provided by the alternator.


  • 1
    I believe the voltage you state, but have you measured amps at various engine speeds such as tickover or idle 800rpm, then 1500 rpm etc?
    – Solar Mike
    Jul 2, 2017 at 19:36
  • 1
    Considering this is hypothetical, why would I? The voltage regulator is responsible to maintain voltages at various engine speeds, unless I suspect it to be defective, which is a problem the OP did not ask about.
    – CharlieRB
    Jul 3, 2017 at 11:54
  • If you check out what speed alternators need to be at to start delivering power (approx 2300 rpm alternator not engine) and they deliver max power around 6000 rpm again alternator rpm, then you may follow that alternators don't do much charging when the engine is at idle.
    – Solar Mike
    Jul 3, 2017 at 13:14
  • @SolarMike Thanks for your input. Why haven't you taken the time to add all of this detail to your answer? This is great information that could make your answer outstanding. Linking to sources is also a great way to validate.
    – CharlieRB
    Jul 3, 2017 at 18:55
  • Because I did not need sources to support the point that I made, others however, may not believe what I said - and still don't. It may be that the OP will come back and let us all know what happens.
    – Solar Mike
    Jul 3, 2017 at 20:31

yes, electrical power would be being used, but with key on/engine running (engine running, but vehicle in park or neutral), the alternator will be charging the system (battery), so battery will not 'die' like it could if vehicle ignition was in key on/engine off. yes, 'gasoline' power is also being used, and may be used less efficiently with a/c on.


While the alternator may be producing enough at tickover or idle to put the warning light out - if you run a vacuum cleaner you would be most likely taking charge out of the battery.

To understand the effect of engine speed and why an alternator won't produce much power at engine idle but will start to charge above 1500 rpm (engine) this is a good link : http://www.hotrod.com/articles/0206sr-understanding-your-alternator/

  • Depends on what kind of vacuum cleaner it is. If it is run through the cigarette lighter socket (like the OP said), it consumes probably at most 10 amps. Turn off the lights and you save yourself 10 amps that can be used by the vacuum cleaner. So, even if the system is not able to support vacuum cleaner with lights on, it will definitely be able to support it with lights off.
    – juhist
    Jul 3, 2017 at 19:25
  • @juhist Check out the alternator power output v engine speed before you make definite statements. Do not confuse volts, amps and Watts.
    – Solar Mike
    Jul 3, 2017 at 20:32
  • I don't understand what the alternator power output has to do with this statement, assuming it is enough to run the lights (which it usually is). The lights consume about 10 amps total (55 watts = about 4 amps per side plus then the parking lights and rear lights which add up to about 10 amps). Turn off the lights and suddenly you will find yourself that the spare capacity you used to run lights can be used to run the vacuum cleaner.
    – juhist
    Jul 4, 2017 at 13:42
  • Oh, and about alternator output: most alternators spin at about 2400RPM at idle as they're overdriven (source: hotrod.com/articles/0206sr-understanding-your-alternator). Here's the first alternator curve I found: vortexbuicks-etc.com/CS144.jpg ...and all alternators in the curve produce at least 70 amps at 2400 RPM. Plenty for a vacuum cleaner AND the lights. So, you don't need to even turn off the lights! You may have been confused by the fact that output is zero below 1000 RPM, but that's the alternator speed, not engine speed.
    – juhist
    Jul 4, 2017 at 13:45
  • Are you sure alternators are overdriven - then they tend to fail as is explained.... Perhaps you have mis-read something...
    – Solar Mike
    Jul 4, 2017 at 16:48

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