Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site brl-tgr.ARPA Path: utzoo!watmath!clyde!bonnie!akgua!mcnc!philabs!cmcl2!seismo!brl-tgr!ron From: ron@brl-tgr.ARPA (Ron Natalie) Newsgroups: net.analog Subject: Re: Big Capacitors Message-ID: <6429@brl-tgr.ARPA> Date: Thu, 6-Dec-84 23:09:08 EST Article-I.D.: brl-tgr.6429 Posted: Thu Dec 6 23:09:08 1984 Date-Received: Sun, 9-Dec-84 06:32:40 EST References: <1812@sun.uucp> <1215@hou4b.UUCP> <6171@brl-tgr.ARPA> <10102@watmath.UUCP> Organization: Ballistic Research Lab Lines: 32 > > 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). Ok, lets see, I take two 1.5 volt batteries and place them in series I get 3 volts. I take two capacitors in series, doesn't quite do the same thing. Batteries will flow DC through them much better than capacitors. You can do it continuosly. Part of my definition of capacitor is one that passes AC but not DC. More practical for designing electrical circuits that do more than blow up screwdrivers and inquisitive cats. Lets leave internal resistance out of the discussion for a moments and discuss only capacitance. -Ron