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Path: utzoo!watmath!kpmartin
From: kpmartin@watmath.UUCP (Kevin Martin)
Newsgroups: net.analog
Subject: Re: Big Capacitors
Message-ID: <10102@watmath.UUCP>
Date: Sun, 2-Dec-84 09:03:10 EST
Article-I.D.: watmath.10102
Posted: Sun Dec  2 09:03:10 1984
Date-Received: Tue, 4-Dec-84 05:49:45 EST
References: <1812@sun.uucp> <1215@hou4b.UUCP> <6171@brl-tgr.ARPA>
Reply-To: kpmartin@watmath.UUCP (Kevin Martin)
Organization: U of Waterloo, Ontario
Lines: 34

In article <6171@brl-tgr.ARPA> ron@brl-tgr.ARPA (Ron Natalie ) writes:
>> Try a battery!  Your car probably has an electrolytic capacitor
>> consisting of a liquid electrolyte and lead plates...many Farads, I
>> think.  A small ni-cad is probably good for a few Farads, too.  Not
>> much bigger than some much smaller capacitors!
>
>Batteries are not capacitors, asshole.  Maybe they should limit submissions
>to this list to people who know something about electricity.  DC does not
>flow through capacitors.
>
>-Ron


Maybe we should limit submissions to this list to polite people.

Lets see. Start with a charged lead-acid battery, and a charged capacitor.
Measure the voltage. Wow. They can both power a voltmeter. Now, draw some
DC (!) current from each. Measure the voltage. Hmmm. It has dropped a bit.
I guess the battery/capacitor has discharged somewhat. Charge them up again.
Aha! back to the original voltage! Leave it sit for a year. Hmm. they
both discharge just sitting there.

Seeing as the vague definition of a capacitor is "something which can
store an electric charge", a rechargeable battery seems like a wonderful
huge capacitor. Their 'capacitance' varies with their charge state
(i.e. V is not proportional to Q), and they tend to have large series
resistance, but they also have graceful over-charge behaviour: Lead-acid
batteries just vent off H2 and O2. Electrolytics tend to explode.

Oh yes... Given a large enough capacitor, DC will "flow through" it for
just as long as it would through a battery.
                -Kevin
p.s. Your average car battery has roughly 36kF of capacitance averaged
over charge states from 10 to 12V (yes, I said kiloFarads).