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capacitors as batteries?

Discussion in 'Power Electronics' started by donkey, Feb 19, 2013.

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  1. donkey


    Feb 26, 2011
    ok I will make the first question very simple but there are heaps more. if i was to use a cap as a battery do I need a special circuit?
    second is how the discharge is calculated. I know in a battery you use the Mah rating but how do you get that from a cap?
    3rd if I use a 1.5volt power source to charge a 16volt cap would the cap output 16volt or 1.5volt?
    4th if I leave the cap charged how long til it self discharges... if it does?

    thats the start hope I can get some answers
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    There is one noteable diffference between capacitors and batteries:
    A capacitor's voltage will fall continuously when current is drained from the capacitor.
    A battery's voltage will stay at a (more or less) fixed level for some time until the capacity is drained. Then the voltage will fall rapidly. So yes, you may need a special circuit (Google SEPIC converter).

    The capacitor equation: C=Q/V
    Therefore Q=V*C which will give you a value usually in fractions of AmpereSeconds (As).
    Since 1h=3600s, 3600As=1Ah Or calculate backwards to get the required C for a given As and Volts rating.

    Honestly: as stated, that's a silly question. Consider any electronic circuit that has almost certainly at least one capacitor across the supply voltage rails. If that circuit is operated by a battery, would the voltage rise above the battery's voltage? Why should it?
    You can charge a capacitor to more than Vbat using a step-up regulator or charge pump, but that is way more than just connecting a battery to a capacitor.

    It does. How fast depends on the type and quality of the capacitor. An electrolytic capacitor will discharge faster than a foil capacitor.
  3. donkey


    Feb 26, 2011
    ok cool all except one point Q=V*C
    could you be so kind as to define eachletter for me. c is amp seconds i guess v=volts I guess and q is how long it will take for the item to discharge?
    the question I have is how to calculate farad to ah... unless q is farad...
  4. Laplace


    Apr 4, 2010
    Q - coulombs (1 coulomb is the electrical charge of approx 6 quintillion electrons)
    V - volts
    C - capacitance in farads

    I - current in amperes (one ampere is the flow of one coulomb per second)

    So the more useful equation is I*T=V*C or

    I*(delta T)=C*(delta V)

    The rate of capacitor charge or discharge (volts/sec) = I/C and is applicable over any interval where the current is relatively constant or can be averaged to a constant.
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