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2222Caps vs Bats: Power storage device.

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  • gjc0@aol.com
    Oct 18, 2006
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      Greetings all:

      For future reference here is a little physics review:

      The energy stored in a capacitor is 1/2 C * (V^2) or one half Capacitance times voltage squared.
      Voltage is measured in volts and capacitance is measured in farads in honor of Michael Faraday.

      Take a 10 farad capacitor charged to 150 volts.
      we get 0.5 * 10 * (150 * 150) = 112,500 joules (a joule is 1 watt time one second).
      That sounds like a lot.

      Take a 80 amp hour gel cell at 12.5 volts.
      amps times volts = watts. We get 1,000 watt-hours. An hour is 3,600 seconds.
      So the battery holds 3,600 * 1,000 = 3,600,000 watt-seconds or 3.6 megajoules.
      This is 36 times the energy of the capacitor.

      To make matters worse, the 1/2 C*V^2 formula assumes that you discharge the capacitor to zero volts.
      We EVers know that the cars get sluggish long before the batteries get to zero volts.

      If we were to assume that we charge our 10 farad cap to 150 volts and then discharge it to 120 volts, we get:
      0.5 * 10 * (150 * 150) - 0.5 * 10 * (120 * 120) = 5 * (150*150 - 120*120) = 35,620 joules.
      This figure is about 1/100 of the battery energy. And the battery does not need to discharge to zero volts to get the energy out of it.

      Hope you find this useful,
      Gary Carlson
      1992 Solectria Force @ 28,000 miles
      ________________________________________________________________________
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