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