# car battery as an oil-filled capacitor

Discussion in 'General Electronics' started by Chris, Apr 30, 2004.

1. ### ChrisGuest

If I took the acid out of a car battery
and filled it instead with a dielectric fluid,
maybe transformer oil or mineral oil, would
it then be useful as a capacitor?

2. ### Some GuyGuest

Disclaimer: This response is off the top of my head. I did NOT do the math
and figure out what the capacitance would be. I guesstimated it. Take it
for what it's worth.

A very small value one. The storage capacity of a capacitor is determined
by the surface area of the plates and the distance between them. The
farther apart the plates are, the smaller the capacity. The smaller the
plates, the smaller the capacity. The plates in a car battery have about as
much surface area as a medium to small electrolytic. That is, only a few
feet square. They are also a LOT farther apart. We're talking a quarter
inch or more, where the plates in an electrolytic are separated by the
thickness of a piece of rice paper. A few thousandths of an inch. This
distance would seriously reduce capacity. You might get a pF or two out of
it... if that.

Why do you want to do this?

3. ### Jerry GreenbergGuest

Since it was not designed as such, the results would most likely not
be very dependable. Anything that has plates that are sandwitched
together between an insulator type material will have some type of
capacitive reaction.

If you were to make your own capacitor, the size of the surface plate
area, the mass of the plates, the distance between the plates,
Thickness of the dialectric material, and the dialectric constant of
the insulation material would be the main factors of determining the
total capacitance. The maximum safe voltage would basicaly be
dependend on the dialectric material's electrical breakdown
characteristics for its total area and mass. The methods to calculate
all of this would be involved, but it would make a nice project for
physics class.

I would not want to use a car battery as a capacitor for anything
serious. For an experiment, this cannot hurt anything. I would not
apply too high a voltage or high current to it. This may turn out to
be a hazardous experiment if things go wrong! You would have a fair
volume of oil, or whatever liquid you use as a dialectric!

Jerry G.
http://www.zoom-one.com

4. ### Ken TaylorGuest

'Useful'? No. It's already a very useful 'capacitor'.

Ken

5. ### Robert BaerGuest

Most definitely *not* that small; a typical gimmick is in that
capacitance region, and the surface area is a *lot* smaller.
My guesstimate would be in the order of hundreds of picofarads.

6. ### Bill VajkGuest

1) Modern automotive batteries consist of 6 cells in *series*.
To maximize the capacitance you'd have to find a way to
rewire them in parallel.

2) Auto batteries usually fail because at least one cell
develops a short circuit.

7. ### David KnaackGuest

The only purpose I can see for such a thing would be a very high
voltage, low capacity device. You'd probably be better of using
parallel strings of a number of high voltage commercial devices. These
will be much smaller and safer.

For an example of such a thing, search for 'Tesla coil MMC'.
Here is an example: http://www.deepfriedneon.com/tesla_f_mmc.html

DK

8. ### ChrisGuest

Yes, I was originally thinking about building
a high voltage DC power supply. Filter caps needed
would be 20-40 uf rated for 4-5kv. Since an oil-filled
capacitor has a similar configuration to a battery,
I was wondering if anyone had ever tried it.

I understand that the chemical makeup of the plates
in a battery are much different than those in
a regular capacitor. So yes, I am skeptical
that it would work very well. But the concept
is an interesting one.

If I had a capacitance meter I might try it.

I have no idea how to determine the voltage
rating of the resulting device. If I went
too high, I guess there would be arcing.
Would that cause an explosion or other
dangerous situation?

9. ### Robert BaerGuest

I strongly recommend that you DO *NOT* try such experiments!
It is far safer to pound a stick of dynamite with a 50 pound
sledgehammer.

10. ### ExternetGuest

If you leave the electrolyte instead of replacing it with oil, it is
the equivalent of a ~170 Farads capacitor.
Miguel

LOL

12. ### ArtGuest

I can just envision the dynamic spreading of sulpheric acid coated high lead
content chunks of debris being projected over a fairly large area. With
accompanying damage to anything or anyone in the immediate vicinity. IMHO Do
Not attempt to do this. If the manufacturers of the batteries considered
this an appropriate application they certainly would indicate it in their
safety brochures presented to the consumer at point of purchase.

13. ### Robert BaerGuest

Sort-of; but the rating is 12 volts...

14. ### ChrisGuest

You think it will explode?
Why?

15. ### ExternetGuest

The formula is :

C = I x t ÷ V

For 100 Amperes during 20 seconds , the battery C = ~170 Farads
For 100 Amperes cranking during 60 seconds , the battery C = 500 Farads

Miguel