Recently I was using a small dust buster-type vacuum cleaner for my SUV which I plugged in to the cigarette lighter in order to use it. I only turned the car on (so that the radio could be turned on, etc.) but not the ignition/engine. I was told that doing so would put pressure on the battery and drain it.
Is this true? When using my vacuum cleaner or portable tire inflater, should I only turn the car on, or should I turn the key all the way to actually start the engine?
In theory - yes. But I seriously doubt that vacuum is going to be that much of a draw. maybe if you ran it continuously for an hour or so it might.
In short, it depends on how much power your vaccum draws and how long you take to vacuum the car.
As an example, if you have a 100AH battery and your vacuum draws 5A, you could theoretically run the vacuum for 20 hours before fully depleting the battery (assuming it is in good shape to start with).
So if you run that same vacuum for 1/2 hour, that is something like 2.5% of the battery capacity. Not much to worry about. The charge will be restored the next time you run the car.
In the end, it depends on the specified capacity of your battery, the power being consumed by the device being used and the condition of the battery.
That being said, I do what you do all the time without running the car to replenish the charge immediately.
I find that, with my compressor, it works better when the engine is running. I bet you find the same is true of your dust buster.
@MikeInNH @TwinTurbo : I just checked the details of the vacuum, and it is 12V/75 Watts. What effect would that have?
@Whitey : Actually, yes, I kinda did notice a difference when I tried it with the engine running. 
That’s 6.25 A so not too far off from my example. You forgot to answer the second half- how long do you run it?
I left off the second part because I only recently purchased the vacuum, and have only used it twice so far – the 1st time was maybe about an hour or so, the 2nd time was a bit less than 30 minutes. It really all depends on how dirty the vehicle’s interior is.
An hour? Dang, that’s a lot of vacuuming! I might be inclined to run the engine for at least part of that time. You’d be better off with a 120V shop vac if you have power available. Much more suction and no issues no matter how long you run it…
Or another option is to do the deed more often, for shorter periods…
Don’t worry. With a good battery a 70W vacuum will be OK to use for an hour. People wash and clean their cars and have their 100W, 500W or more stereo running while they do that without issues. The battery will be re-charged during your next driving cycle.
1 hour… that’s a lot of vacuuming…
Keep in mind, it’s not a continuous hour-long vacuum session. I did have to stop at some intervals to clean out the filter and hand clean stuff that the vacuum couldn’t pick up.
But yeah, I do plan on cleaning the interior often which would be short periods of time each time.
Having the ignition on and the doors open draws 15 to 20 amps, this is a larger draw than your vacuum cleaner.
Still it’s not doing either the battery or the alternator any good. For a dollar or two you can use the car wash vacuum and be done in about 5 minutes or get a real shop vac to use at home. I have one with about a 12 foot hose mounted on the garage wall and works great. Those little Dust Devil vacs are really not very effective.
What’s it say in your owner’s manual?
It sure seems wasteful to run an engine to keep a battery charged that’s powering a radio and a 12V vacuum cleaner. I have a 100-foot extension cord (picked up cheap at a garage sale) and use my shop vac. Some battery-powered tool sets come with vacuum cleaners; I’ve never used.
@the_same_mountainbik:
It says "These supply 12V electric power to operate electric accessories or equipment only when the key is in the “ON” or “ACC” position. So I guess I don’t need to, or shouldn’t, turn the key all the way to turn on the engine.
It will differ by car maker. I run the pump for bicycle tires and it is always live from the rear powerpoint in my suv.
With a fairly new battery in the summer, I wouldn’t be concerned. When the battery is nearing the end of its life, and it is the dead of winter, then would be the time to be more cautious about running the portable vacuum cleaner. One thing is certain: If you run it too much, you’ll soon know.
There is no need to start the engine to operate the vacuum for 10-15 minutes however if I need to have the ignition on for more than 10 minutes I connect a power supply to the battery to avoid discharging it. The ignition and dome lights draw much more current than your vacuum.
When a battery is discharged in a short amount of time it draws a lot of current from the alternator after start-up, this can shorten the life of the alternator and battery.
I think you are mis-reading the maual. Under caution it says to use when the engine is running. Seems pretty clear to me. The part about turning the key is just telling you the outlet is off until you turn the key. Different from my VW where the radio was on whether the key was on or not so you had to make sure to turn the radio off when the car was shut down. They are just telling you its not powered when the car is off.
That’s a good point. OP, read what the label says under the word “CAUTION.” It clearly states, “Use when the engine is running…”
