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car battery as an oil-filled capacitor

Discussion in 'General Electronics' started by Chris, Apr 30, 2004.

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  1. Chris

    Chris Guest

    If I took the acid out of a car battery
    and filled it instead with a dielectric fluid,
    maybe transformer oil or mineral oil, would
    it then be useful as a capacitor?
  2. Some Guy

    Some Guy Guest

    Disclaimer: This response is off the top of my head. I did NOT do the math
    and figure out what the capacitance would be. I guesstimated it. Take it
    for what it's worth.

    A very small value one. The storage capacity of a capacitor is determined
    by the surface area of the plates and the distance between them. The
    farther apart the plates are, the smaller the capacity. The smaller the
    plates, the smaller the capacity. The plates in a car battery have about as
    much surface area as a medium to small electrolytic. That is, only a few
    feet square. They are also a LOT farther apart. We're talking a quarter
    inch or more, where the plates in an electrolytic are separated by the
    thickness of a piece of rice paper. A few thousandths of an inch. This
    distance would seriously reduce capacity. You might get a pF or two out of
    it... if that.

    Why do you want to do this?
  3. Since it was not designed as such, the results would most likely not
    be very dependable. Anything that has plates that are sandwitched
    together between an insulator type material will have some type of
    capacitive reaction.

    If you were to make your own capacitor, the size of the surface plate
    area, the mass of the plates, the distance between the plates,
    Thickness of the dialectric material, and the dialectric constant of
    the insulation material would be the main factors of determining the
    total capacitance. The maximum safe voltage would basicaly be
    dependend on the dialectric material's electrical breakdown
    characteristics for its total area and mass. The methods to calculate
    all of this would be involved, but it would make a nice project for
    physics class.

    I would not want to use a car battery as a capacitor for anything
    serious. For an experiment, this cannot hurt anything. I would not
    apply too high a voltage or high current to it. This may turn out to
    be a hazardous experiment if things go wrong! You would have a fair
    volume of oil, or whatever liquid you use as a dialectric!

    Jerry G.
  4. Ken Taylor

    Ken Taylor Guest

    'Useful'? No. It's already a very useful 'capacitor'.

  5. Robert Baer

    Robert Baer Guest

    Most definitely *not* that small; a typical gimmick is in that
    capacitance region, and the surface area is a *lot* smaller.
    My guesstimate would be in the order of hundreds of picofarads.
  6. Bill Vajk

    Bill Vajk Guest

    1) Modern automotive batteries consist of 6 cells in *series*.
    To maximize the capacitance you'd have to find a way to
    rewire them in parallel.

    2) Auto batteries usually fail because at least one cell
    develops a short circuit.
  7. David Knaack

    David Knaack Guest

    The only purpose I can see for such a thing would be a very high
    voltage, low capacity device. You'd probably be better of using
    parallel strings of a number of high voltage commercial devices. These
    will be much smaller and safer.

    For an example of such a thing, search for 'Tesla coil MMC'.
    Here is an example:

  8. Chris

    Chris Guest

    Yes, I was originally thinking about building
    a high voltage DC power supply. Filter caps needed
    would be 20-40 uf rated for 4-5kv. Since an oil-filled
    capacitor has a similar configuration to a battery,
    I was wondering if anyone had ever tried it.

    I understand that the chemical makeup of the plates
    in a battery are much different than those in
    a regular capacitor. So yes, I am skeptical
    that it would work very well. But the concept
    is an interesting one.

    If I had a capacitance meter I might try it.

    I have no idea how to determine the voltage
    rating of the resulting device. If I went
    too high, I guess there would be arcing.
    Would that cause an explosion or other
    dangerous situation?
  9. Robert Baer

    Robert Baer Guest

    I strongly recommend that you DO *NOT* try such experiments!
    It is far safer to pound a stick of dynamite with a 50 pound
  10. Externet

    Externet Guest

    If you leave the electrolyte instead of replacing it with oil, it is
    the equivalent of a ~170 Farads capacitor.
  11. Bob Stephens

    Bob Stephens Guest

    LOL ;)
  12. Art

    Art Guest

    I can just envision the dynamic spreading of sulpheric acid coated high lead
    content chunks of debris being projected over a fairly large area. With
    accompanying damage to anything or anyone in the immediate vicinity. IMHO Do
    Not attempt to do this. If the manufacturers of the batteries considered
    this an appropriate application they certainly would indicate it in their
    safety brochures presented to the consumer at point of purchase.
  13. Robert Baer

    Robert Baer Guest

    Sort-of; but the rating is 12 volts...
  14. Chris

    Chris Guest

    You think it will explode?
  15. Externet

    Externet Guest

    The formula is :

    C = I x t ÷ V

    For 100 Amperes during 20 seconds , the battery C = ~170 Farads
    For 100 Amperes cranking during 60 seconds , the battery C = 500 Farads

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