On Mon, 04 Feb 2008 10:17:12 -0600, Steve <no@[EMAIL PROTECTED]
> wrote:
>John Bailey wrote:
>
>> Is that a fact and reason based answer or just a guess? A battery is
>> not as efficient as a capacitor and there is a theorem from sophomore
>> EE that "proves" no more than 1/2 the energy stored in a capacitor can
>> be recovered.
>
>Maybe at some EE correspondence school in outer Elbonia, but not
>anywhere creditable.
http://www.iop.org/EJ/abstract/0031-9120/40/4/008
Two theorems on dissipative energy losses in capacitor systems
Ronald Newburgh 2005 Phys. Educ. 40 370-372
doi:10.1088/0031-9120/40/4/008
"Abstract. This article examines energy losses in charge motion in two
capacitor systems. In the first charge is transferred from a charged
capacitor to an uncharged one through a resistor. In the second a
battery charges an originally uncharged capacitor through a
resistance. Analysis leads to two surprising general theorems. In
the first case the fraction of energy dissipated in the resistor
depends solely on the ratio of the two capacitances. The values of the
original charge and the resistance play no role. In the second case
half of the energy supplied by the battery is dissipated and half is
stored in the capacitor. The values of the battery emf and the
resistance play no role."
If you don't have an account, I will try to sketch the proof. In
simple RC charging, the power/ current relation****p is invariant up to
the limit 0/0 of no resistance and down to the 0/0 instance of
infinite resistance/conductance. The proof is a matter of
generalizing this result along the lines of the Norton and Thevenin
theorems.
>Most capacitors are nearly 100% efficient at low
>frequency and moderate charge/discharge rates, I have no idea where you
>came up with some "proof" that no more than half can be recovered. Even
>in very stressing conditions, such as advanced dielectric capacitors
>under high discharge rates (pulsed power systems, for example) you can
>get well over 90% of the energy back out of a capacitor. If you're
>losing energy in a capacitor, then the capacitor was poorly selected for
>the application- such as using an electrolytic cap at radio frequencies
>or higher.
By the way, the problem is getting the power IN to the capacitor as
well as getting it out! Of course you can get nearly all power out of
the capacitor, its how to avoid putting half of it into a
resistor--sooner or later.
>
>Batteries are indeed a different story, though. I agree that current
>hybrids are probably achieving well below 50%.
http://www.sandia.gov/pv/docs/PDF/batpapsteve.pdf
A Study of Lead-Acid Battery Efficiency Near Top-of-Charge
and the Impact on PV System Design by Stevens and Corey
"Notice also that the overall efficiency shows high values, with full
charge represented by approximately 85% efficiency, a commonly used
value for battery charge efficiency. More im****tantly, notice the
dramatically lower efficiencies for the increments above
about 80% state of charge, where most values are below
60% efficiency, and full charge is represented by less
than 50% efficiency."
>
>Maybe at some EE correspondence school in outer Elbonia, but not
>anywhere creditable.
If I didn't eschew personal attacks, I would invite you to compare
degrees. Only one of mine is an Ivy League school.
>> Is that a fact and reason based answer or just a guess?
Bluster, puffery, and obfuscation prevail.
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