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