# Re: Big Capacitor?

```Maxwell (I think) makes a 2.5kF cap rated around 87V for backup purposes.

I distinctly remember from my physics 2 course (one of the only things I
remember from that class, actually), is that a standard parallel plate cap
1F in
value will have an approximate physical size of 10,000 sq meters spaced .001 m
apart.  The properties of electolytics make a large value possible in a small
container, so a 1F cap at 50V is actually much smaller.

Mark

Tesla list wrote:

> Original poster: "Phillip Heslin" <pheslin-at-home-dot-com>
>
> Hi folks,
>
>   I think the key here is something that hasn't been mentioned yet. The
> size of the cap is a function of the total energy storage of the the cap
> as measured in Joules and the energy density of the particular type of
> cap. The total energy stored in a cap is calculated with the equation
> E=(F/2)*(V^2) where E is the total energy in Joules, F is the
> capacitance in farads and V is the voltage. ( I hope I remembered that
> correctly) Energy densities vary with the type of construction I.E.
> electrolytics have a different energy density than a foil cap. I believe
> that the highest energy density record is held by the relatively new
> Maxwell powercache capacitors. I have seen a rectangular cap about the
> size of a 12 oz coke can with a rating of 2500 farads, yes farads! and
> rated at 2.5V
>
> B.T.W if I mis-remembered that formula, someone please correct me.
>
>            Safe Coiling!
>
>            Phil Heslin
>
> --snip--
> > I have a question or two for any one out there. How big would a capacitor
> > have to be, (physical dimensions), to have a value of 1 Farad? I know that
> > voltage rating would be a determining factor, but just a ballpark figure.
> > Has anybody ever seen one?
> > Just curious.

```