Perhaps the easiest question ever...

Discussion in 'The Projects Forum' started by V8Goose, Aug 29, 2008.

  1. V8Goose

    Thread Starter New Member

    Aug 29, 2008
    Hi all & thanks for looking...

    I'm a mechanical guy, electronics is voodoo to me :D

    Heres the scenario:

    2 R/c car batteries in series through a momentary press switch to a 12v mini air compressor.

    The batteries are rated at 7.2v 1500mAh each, 14.4v total.

    The mini air compressor runs off a standard car battery 12v.

    When I put 14.4v into the compressor it smells bad so I assume I am going to cook it.

    I looked for a variable resistor of some type and failed, bought some trim pots and fried a couple before giving up.

    I dont understand enough about this but guess I need to reduce the voltage to 12 via some method taking the mAh into consideration?

    What component do I use and what rating does it need to be?

    Thank you!!

  2. Audioguru


    Dec 20, 2007
    A resistor in series with a motor gets very hot and limits the current. A motor uses a high current to start running and when it is loaded so you don't want a resistor limiting the current.

    You need a voltage regulator. There are some IC adjustable voltage regulators but since you don't know how much is the max current of the compressor then we can't recommend one.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    There may be some other problem. A car battery should have a voltage output of 13.2 volts ( that is six 2.2 volts cells in series - the 12 volts spoken of is not accurate). The 14.4 volts applied should be no problem.

    Your batteries may not be capable of supplying the current the compressor needs, causing the compressor motor to bog and overheat. The motor probably need several amps of current, and the 7.2 volt cells can only supply a fraction of that.

    You might get good results with a lawn tractor battery. It would be easy to recharge from a lighter plug in the car.
  4. V8Goose

    Thread Starter New Member

    Aug 29, 2008
    Thank you both for your speedy replies.

    I will do away with the resistor idea based on your reply.

    I'm not sure of the current statement. The compressor motor runs very quickly when connected to the 14v and the smell (which may or may not be there normally) makes me think I'm going to fry the motor.

    Does the max current of the compressor determine the load on the batteries?

    Thanks again

  5. SgtWookie


    Jul 17, 2007
    Your compressor should run just fine on a lead-acid battery, or a car's charging system - which may put out up to 14.5v.

    You probably were smelling emissions from the batteries you were trying to use. They didn't have the current output capability that your compressor was trying to draw.

    If the compressor is rated for operation from an auto's electrical system, then it should be OK with a supply voltage anywhere between 11.4 and 14.5v. Naturally, it will run faster on the higher voltage. The motor generates back-EMF to balance out the input voltage vs the load. However, if you give it more than about 16v, it will likely burn up.

    When the load on the motor is heavy, the motor will reduce RPMs, and will draw more current.
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    Not Voodoo, Sir - but rather formulaic Hermetic magic.:D

    Description matches 6 cell NiCad batteries. No way can they safely source enough current for a compressor motor - not even a small one. A 12V 1/4 hp compressor needs well more than 15 Amps.
  7. SgtWookie


    Jul 17, 2007
    Psst... it's more like six 2.11v batteries. The real static voltage is close to 12.7v; not the plain "12v" that is commonly bantered about, nor 12.6v, but actually near 12.7v.

    Charge a "12v" lead-acid cell to float level (13.4v), then disconnect the charger for at least several hours, and check it again. You'll find the battery has settled to between 12.6 and 12.7v - that is, if it's any good.

    Not likely that it'll be above 12.7v.
  8. hgmjr


    Jan 28, 2005
    I would echo the advice of those that counsel against the use of a power resistor in series with a motor to control the speed of the motor.

  9. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    Since RC car batteries are expected to deliver high average currents of 10A or more, I think you should be Ok with the batteries. Take a look at the value of the in-line fuse on the compressor to get an idea of expected current consumption.

    I'm guessing that the mini air compressor is the inexpensive type for topping up car tyres? These compressors have very small displacement so do no present a particularly high load. What you probably have is a cheap motor which is marginally rated and not intended for continuous use. The hot-motor smell may go away after a while.
  10. V8Goose

    Thread Starter New Member

    Aug 29, 2008
    Once again, I thank you sirs...

    Teaching and learning is something that is rapidly disappearing from around us.

    Yes, the compressor is a small top-up type and is used for approx. 20-30 seconds each time. Just enough time to get 100psi into the air retract tank.

    I will take both the car battery and rc batteries to the field tomorrow and try them.


  11. V8Goose

    Thread Starter New Member

    Aug 29, 2008
    I have now used the r/c car battery and top-up air compressor at the field three times without recharging it successfully.

    At the end of the last day I noticed the speed of the compressor drop a little as expected but it has lasted very well.

    Thank you all again for your insights.