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Question: capacitors as special use batteries?

Discussion in 'Electronic Basics' started by [email protected], Jul 9, 2008.

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  1. Guest

    I recall from my hobbyist days that high-voltage capacitors can keep
    a
    charge for a significant amount of time, especially if they are cut-
    off from the circuit subsequent to charging.

    I'd like to know whether it might be feasible, from a technical and
    cost-effectiveness standpoint, to use a large array of specially
    designed capacitors to hold excess electricity from solar plants for,
    say, periods of a week or more. This would be helpful to bridge gaps
    caused by overcast weather, and for other purposes.

    I know that there are potentially problems with dielectric breakdown
    and leakage. On the plus side, such "batteries" could be charged and
    discharged very quickly, as needed, and without the complications
    involved in conventional batteries using chemical electrolytes.
     
  2. The fact that something can keep a charge for a significant amount of
    time is irrelevant. The fact that those capacitors are isolated from
    the circuit so there is virtually no load is the giveaway. If you
    keep a load on the capacitor, it will discharge. A really high impedance
    load will not affect the voltage on the capacitor for a long time, but
    the moment you try to draw any real current and the voltage on the
    capacitor goes away immediately.

    Capacitors don't have the capacity to do what you want. They are
    good at storing voltage, they are lousy at storing power, and you need
    power (ie voltage times current) if you want to do anything with this.

    You will very easily learn that you would need massive capacitors (in
    terms of capacitance and thus in terms of physical size) before you
    could even come close to doing anything useful this way.

    Spend your efforts on lowering current draw, which then requires less
    power. I have one of those "atomic clocks" that is at least four years
    old and still running on the two AA batteries that I put in when I bought
    it. That's low current, considering there is radio circuitry that has
    to come alive each night to get the update from the time station. No need
    to fuss with large capacitors, or rechargeable batteries, it just does
    its job well with very little current draw.

    In a lot of cases, you can't do that, since the device is required
    to supply power of some sort (like that stereo amplifier over there).
    Radios are another place where low current isn't necessarily the best
    route; I just got one of those crank radios and the crank function is
    great, the radio is horrible, likely the result of low current being
    seen as the primary design factor, rather than good reception.

    Michael
     
  3. Modern capacitors are starting to have enough capacitance to be used
    in this sort of application.

    Here is a research starting point:
    http://en.wikipedia.org/wiki/Supercapacitor

    Alan Nishioka
     
  4. Eeyore

    Eeyore Guest

    Basically no. The energy stoage capacity per volume / weight is absurdly
    low when compared with any other method.

    So-called 'super caps' have niche applications but will never be used
    like that.

    Graham
     
  5. John

    John Guest

    The capacities aren't yet at that level, but one company is now
    marketing a capacitor powered cordless drill with a recharge time of
    less than 2 minutes.

    ..
     
  6. BobG

    BobG Guest

    Tecate sells the Maxwell Boostcaps that are the size of a D-cell,
    2.7V, 350 Farads. Its takes several minutes to charge one up on the
    lab supply supplying 2.5V 3A. Several 1000 watt-sec.
     
  7. BobG

    BobG Guest

    On Jul 11, 6:03 pm, John Larkin
    =====================================
    I bet you could build form-fit-and-function battery packs for a drill
    using boostcaps and a wide input range switcher like you said. You
    drill for a couple minutes then change the pack out. The charger could
    hump 50 or 100 amps into the pack being charged and charge the thing
    up 10x faster than it discharged under use. Better than the charge for
    several hours arrangement they have now.
     
  8. Jasen Betts

    Jasen Betts Guest

    Someting like that would be dynamite!

    Infact, better than dynamite.
    Dynamite only manages an energy density of 4300J/g at about 0.002g each,
    mass for mass your (dream) caps would pack 1000 times the punch when fully
    charged.

    :)

    Bye.
    Jasen
     
  9. Ross Herbert

    Ross Herbert Guest

    On Wed, 9 Jul 2008 10:50:31 -0700 (PDT), ""

    :I recall from my hobbyist days that high-voltage capacitors can keep
    :a
    :charge for a significant amount of time, especially if they are cut-
    :eek:ff from the circuit subsequent to charging.
    :
    :I'd like to know whether it might be feasible, from a technical and
    :cost-effectiveness standpoint, to use a large array of specially
    :designed capacitors to hold excess electricity from solar plants for,
    :say, periods of a week or more. This would be helpful to bridge gaps
    :caused by overcast weather, and for other purposes.
    :
    :I know that there are potentially problems with dielectric breakdown
    :and leakage. On the plus side, such "batteries" could be charged and
    :discharged very quickly, as needed, and without the complications
    :involved in conventional batteries using chemical electrolytes.


    At this stage supercapacitors are only useful and cost effective for supplying
    short term, high current delivery.

    In 2000,CSIRO and other Australian auto companies developed a hybrid electric
    demonstration vehicle called aXcessaustralia LEV
    http://www.csiro.au/solutions/aXessaustralia.html

    CSIRO developed a lead-acid battery with combined supercapacitors to produce a
    battery pack which was about 20% the weight of battery packs normally used in
    electric vehicles. The Supercaps were charged by regenerarive braking and
    supplied the short term boost current for rapid acceleration. When running on
    electric only, the vehicle was capable of around 20 minutes use in urban
    environment.
     
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