williams@kirk.DEC (John Williams 223-3402) (12/06/84)
A battery is an extremely nonlinear capacitor. This is due to
the fact that the charge changes the characteristics of the dielectric.
Period. They optimize difference aspects of similar phenomenon. You will
find that all capacitors are somewhat nonlinear. Vendors tend to stuff
this characteristic into the absolute tolerance. Polystyrene and
polypropalene ( spelling? ) tend to be the most linear. Batteries are
the most nonlinear. You don't find many that are inbetween until you
get to solid state capacitors processed in silicon, where the depletion
layer varies with charge. The development of linear capacitors in IC's
was a major breakthrough for systolic arrays and switched capacitor
networks. It should be noted the the nonlinearity in silicon is inverse
to the nonlinearity you find in batteries.
----{ john williams }----darrelj@sdcrdcf.UUCP (Darrel VanBuer) (12/07/84)
John williams said batteries are nonlinear capacitors because charging
changes the dielectric???
I've never seen a battery in which any dielectrics played an important role
(relegated to such roles as hard rubber spacers to prevent contact between
poles). All batteries contain an electrolyte, but it's a conductor, not an
insulator. Not all batteries display any significant changes in the
electrolyte with charging either (lead batteries as in cars do change,
nickel cadmium and Edison batteries do not change the electrolyte).
The mechanism of storing the charge is a capacitor is forcing either a
surplus or deficit of electrons on the electrodes.
The charge stored in a battery involves transport of (charged) ions between
the electrodes balancing the transport of electrons into the electrical
circuit, and the energy stored in the different strengths of chemical bonds.
Batteries have a discharge curve which looks much more like
\
\
\_______________________
-----------------------_______________
\
\
The tails of the curve are the result of exhaustion of active material at
the extremes of the chemical cycle (and battery switches to other reactions).
The flatness of the main curve depends on discharge rate (high rates swamp
ion transport and active surface area) and kinds of changes in chemistry in
discharge (e.g. lead battery electrolyte loses sulfuric acid, while NiCd
electrolyte remains constant 25% KOH--that's why NiCd have such constant output
till almost dead).
--
Darrel J. Van Buer, PhD
System Development Corp.
2500 Colorado Ave
Santa Monica, CA 90406
(213)820-4111 x5449
...{allegra,burdvax,cbosgd,hplabs,ihnp4,orstcs,sdcsvax,ucla-cs,akgua}
!sdcrdcf!darrelj
VANBUER@USC-ECL.ARPAjj@alice.UUCP (12/09/84)
While I do agree with the author at sdcrdcf about the chemical differences between batteries and capacitors, I can't buy the comments about the dielectric "not playing a large part", since the point of almost all dielectrics is EXACTLY to increace both the insulation capability and capacitance of tthe battery. Most strong dielectrics (electrolytics, etc) do this by a physical rearrangement of polar molecules, or electron shells, etc, within the dielectric. Frankly, I would define capacitors as devices that store energy in an E field, and which may use physical MOTION to store energy (in a microscopic sense, one must realize), and batteries as devices that store energy via chemical change. One must realize that all capacitors undergo a bit of chemical change, and all batteries have a significant capacitance.
tonjon@fluke.UUCP (Tony Johnson) (12/11/84)
> > A battery is an extremely nonlinear capacitor. This is due to > the fact that the charge changes the characteristics of the dielectric. > Period. They optimize difference aspects of similar phenomenon. You will > find that all capacitors are somewhat nonlinear. Vendors tend to stuff > this characteristic into the absolute tolerance. Polystyrene and > polypropalene ( spelling? ) tend to be the most linear. Batteries are > the most nonlinear. You don't find many that are inbetween until you > get to solid state capacitors processed in silicon, where the depletion > layer varies with charge. The development of linear capacitors in IC's > was a major breakthrough for systolic arrays and switched capacitor > networks. It should be noted the the nonlinearity in silicon is inverse > to the nonlinearity you find in batteries. > > ----{ john williams }---- I have a question as to what we're talking about when we say 'non- linear'...nonlinear as a function to what; frequency, applied voltage, time? Polystyrene and 'polypros' have great performance with respect to a phenonmenon called dielectric absortion i.e., they don't 'soak' as much as say mylar or (yeech) electrolytic. That's why they're great for use in integrating A/D converters. But if one is concerned about nonlinear performance with respect frequency i.e. 'hook', NPO ceramics perform very well. A few references: Horowitz, Paul and Hill, Winfield; 'The Art of Electronics'; pg 18-19; Cambridge University Press; Cambridge, 1980. Pease, Robert A.; 'Understand capacitor soakage to optimize analog systems'; EDN, 13 October, 1982. Johnson, F.L.; 'Technical Bullitens 1-10'; Electrocube Corp.. You can probably get this stuff from the local Electrocube rep. Tony Johnson John Fluke Mfg. Co. Inc. Everett, WA