Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why not use high potential capacitors?

  1. Feb 11, 2010 #1
    I mean like...20KV or above...I've seen their energy density and specific energy is pretty high compared to batteries (I hope I'm right)...so why not use them for battery replacement in vehicles?
  2. jcsd
  3. Feb 11, 2010 #2


    User Avatar
    Science Advisor

    Why not you post a link to a currently available 20kV+ capacitor that has energy storage capacity anything close to current battery technology.

    Here's a link to some available high voltage capacitors. http://www.hivoltcapacitors.com/page1.html [Broken]

    Let's look at the largest in their 25kV range, part number PMR 250-504. It's volume is over 3L and its rating is 0.5uF 25kV DC working voltage.

    Max energy storage is 1/2 C V^2 = 156 Joules.

    For comparison LiPo batteries currently have volume energy densities around 300 W-h / L, so you'd store over 3000000 Joules in the same volume as the above capacitor!

    Sure lower voltage supercapacitors will have higher energy density than the 25kv capacitor I linked, but even those currently have only about one tenth the energy density of what's available in batteries. The higher charge/discharge rate of the supercapacitors does give them some advantages which makes then useful in conjunction with a battery (as in a battery/supercapacitor hybrid).

    Any dE_logics, please post a link to the currently available 20kV+ capacitor that you claim to have such good storage capacity compared to batteries. You've made the claim so please substantiate it. Personally I'll believe it when I see it.
    Last edited by a moderator: May 4, 2017
  4. Feb 11, 2010 #3
    Perhaps you were thinking of high power density? http://www.nytimes.com/2008/01/13/a...ed=1&ref=automobiles&oref=slogin&oref=slogin" describes a hybrid electrical car that combines lithium ion batteries with ultracapacitors:
    Last edited by a moderator: Apr 24, 2017
  5. Feb 11, 2010 #4


    User Avatar
    Science Advisor
    Homework Helper

    The dielectric used for capacitors, especially supercaps, breaks down at high voltages.
    So unfortunately you can't increase the capacity (by CV^2) by simply upping the voltage
  6. Feb 11, 2010 #5
    Ok...we dont have such capacitors...I was wrong.

    Thanks for clarifying.
  7. Feb 11, 2010 #6


    User Avatar
    Science Advisor
    Homework Helper

    Worse than that - it's pretty much a fundemental feature of dielectrics that as the dielectric constant goes up the breakdown voltage goes down.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook