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Difference between a capacitor and a battery

  1. Jul 3, 2013 #1
    What is the difference between a capacitor and a battery?

    As far as I have browsed, the most common answer found is capacitor discharges faster than a battery. Would any body elaborate it in a clear way.
     
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  3. Jul 3, 2013 #2

    Vanadium 50

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    A capacitor stores energy in an electric field. A battery stores its energy chemically.
     
  4. Jul 3, 2013 #3

    wukunlin

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    Most batteries should have better voltage regulation as well
     
  5. Jul 3, 2013 #4
    Batteries work by inducing a difference in electrical potential which causes electrons to flow. Capacitors store an electric charge supplied by an external source. The capacity of capacitors is pretty low compared to batteries; in addition most of them do not retain their charge for very long (the exception being aluminum and tantalum capacitors, as well as ultracapacitors.) Ultracapacitors apparently store hundreds of Farads as opposed to standard electrolytic capacitors, which I've never seen exceed 2200 microfarads (or 2.2 millifarads.)

    But yeah, capacitors are pretty cool. I have a bunch of 200V ones lying around that I've pulled out of power supplies. You can just charge them up to like, 150V and lick them. No, don't do that. That's probably a very bad idea.
     
  6. Jul 4, 2013 #5

    BruceW

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    Yeah, although I think it is bad terminology to say that a battery 'discharges'. In most electronics problems, we assume that the chemicals in the battery take a long time to get used up. While on the other hand, the charge in a capacitor usually takes a much shorter time to get used up. One other difference is that battery often has some internal resistance that can't be ignored, while in capacitors I think you can usually assume they have zero internal resistance.
     
  7. Jul 4, 2013 #6
    I will have a better view of the answer, if the answer is expressed numerically.
    Let us say, We charge a battery as well as a capacitor to the same voltage (say 1.5 V).
    The battery (if a commercial AA battery) when used in a Flash light or MP3 player will supply voltage for atleast 10 to 12 hours.
    In this scenario, how long will a capacitor supply voltage?
     
  8. Jul 4, 2013 #7

    Vanadium 50

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    It depends on the capacitor. It also is not fundamental to the difference: the difference is in how the energy is stored, not how much energy is stored.
     
  9. Jul 4, 2013 #8

    BruceW

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    It is important to keep in mind that they are two completely different mechanisms. The battery is essentially 'having its chemicals replenished' while the capacitor is physically gaining charge. The battery is made to work within a certain range of voltage and current drawn. And the time it can supply voltage for is complicated, in that it depends on the chemicals.

    The capacitor is relatively simple, in that it has a certain charge on it, so it will be able to supply voltage until that charge is used up. And therefore, the time it will supply voltage for depends on the current drawn from it.

    Another important thing is that the battery gives a roughly steady d.c. voltage, while the capacitor must give a changing voltage. So if you put a capacitor in a d.c. circuit, it will not give a steady voltage then zero. It will give a voltage that slowly decreases when the charge leaves the capacitor.
     
  10. Jul 4, 2013 #9

    Nugatory

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    It depends on the size of the capacitor. You're talking about delivering roughly 100 mA at 1.5 volts for ten hours, which is about 5000 total Joules (about right for an alkaline AA battery). A 3500 Farad capacitor (absurd for a traditional electrolytic capacitor, plausible for some supercapacitor technologies) would be needed.
     
  11. Jul 4, 2013 #10

    OmCheeto

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    Thank you for doing the math. (I'm in a bit of a hurry this morning)

    Some more differences:

    A 3000 Farad, 2.7 volt capacitor(3 watt hour) will cost $65
    A 3.24 watt hour lithium ion rechargeable AA costs $5

    The capacitor is much bigger: 60.7 mm Dia. x 138 mm L
    AA batteries measure: 14 mm Dia. x 50 mm L

    cycles:
    capacitor: 1,000,000
    battery: 400 to 1200

    The capacitor would require additional boost-buck circuitry to yield a useable flat voltage.
    The battery maintains a fairly flat voltage.

    Dag nabit, I can't figure out how to share images from within PF.

    Here, just go to this link, and click on the image to see what I'm talking about.
     
  12. Jul 4, 2013 #11
    What happens when a conductor is placed in between the plates?

    does the conductance decreases?
     
  13. Jul 4, 2013 #12

    Nugatory

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    Put a conductor between the plates of a charging capacitor and you've just created a dead short. There will be a current flow through the conductor, limited only by Ohm's law and the near-zero resistance of the conductor. If a fuse or other current-limiting device doesn't kick in, something will melt, burn, or explode.

    Put a conductor between the plates of a charged capacitor and the capacitor will discharge, again at a rate determined by Ohm's law and near-zero resistance of the conductor. The current involved will be fairly impressive while it flows, although it won't flow for long. For example, if you connect 10cm of copper wire across a traditional capacitor charged to one volt, the initial current flow could be in the thousands of amperes. Of course it will fall quickly to zero as the capacitor discharges; how long that takes will depend on the capacitance.

    Digression: Once, long ago when I was young and unwise and easily amused by things going "boom", I was working in a lab that had a one farad capacitor (damn thing looked like an oil barrel) and a 15 kilovolt power supply. My plan was to use the one to charge the other, then stand across the room and throw a screwdriver at the terminals of the capacitor, just to see what would happen.

    The professor who owned the lab wouldn't let me.
     
    Last edited: Jul 4, 2013
  14. Jul 4, 2013 #13

    davenn

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    have a look in any parts list from an electronics supplier and you will see lots available that are bigger than this :)

    just for starters 4700uF, 6800uF, 10,000uF,


    cheers
    Dave
     
  15. Jul 11, 2013 #14
    I was speaking in terms of circuits that I've seen. I disassemble TV's and PSU's often.
     
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