Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site watmath.UUCP 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).