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Date:         Wed, 12 Nov 2008 22:09:42 -0800
Reply-To:     David Kao <dtkao0205@YAHOO.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Kao <dtkao0205@YAHOO.COM>
Subject:      Re: alternator--voltage vs amps
In-Reply-To:  <c4e7c5f90811122101n167aa141w2bcf7df8bff585aa@mail.gmail.com>
Content-Type: text/plain; charset=us-ascii

I don't think an alternator can crank out rated amperage at engine's idle speed. But it can be still a very significant amperage. I once watch a tech checking my 73 beetle's charging system. He measured the charging current with all lights turned on, including wipers, signals radios and whatever he could turn on with the engine running at idle. The beetle had a DC generator, not an alternator. The tech adjusted the regulator to make sure that there was still a small charghing current to the battery. The generator at idle still cranked out something like 25 - 30 amps to power everything that were turned on. The regulator was adjustable to make sure the battery would not run out of juice when driving in a raining night over a long distance. Even at idle it was still able to keep the battery from draining out.

Alternators should be no different. They can crank out a lot of amperes even at idle.

David

--- On Wed, 11/12/08, neil N <musomuso@GMAIL.COM> wrote:

> From: neil N <musomuso@GMAIL.COM> > Subject: Re: alternator--voltage vs amps > To: vanagon@GERRY.VANAGON.COM > Date: Wednesday, November 12, 2008, 9:01 PM > Darn it! Now you made me study! > > (kidding :^) > > I was curious after reading your post as I had never really > thought > about the in/outs of an automotive alternator. > > An excerpt from > http://www.familycar.com/Classroom/charging.htm : > > "...... Amperage or current is regulated by the state > of charge of the > battery. When the battery is weak, the electromotive force > (voltage) > is not strong enough to hold back the current from the > alternator > trying to recharge the battery. As the battery reaches a > state of full > charge, the electromotive force becomes strong enough to > oppose the > current flow from the alternator, the amperage output from > the > alternator will drop to close to zero, while the voltage > will remain > at 13.5 to 14.5. When more electrical power is used, the > electromotive > force will reduce and alternator amperage will increase. It > is > extremely important that when alternator efficiency is > checked, both > voltage and amperage outputs are checked. Each alternator > has a rated > amperage output depending on the electrical requirements of > the > vehicle." > > For me, it made it clear that the state of the battery > determines (in > part) how much current is used. It doesn't explain (in > laymens terms) > if the current output is dependent on the RPM of the > alternator. i.e. > Does a 70 amp alternator put out 70 amps at idle? > > The good news for me is that I'm pretty sure the 90 am > alternator I > have on my VanaJetta is not going to somehow be too > powerful for my > Westy. Something I was thinking about recently. > > Neil. > > On Wed, Nov 12, 2008 at 7:15 PM, Richard A Jones > <jones@colorado.edu> wrote: > > I'm trying to understand alternators now. I'm > retired, > > so have nothing better to do.... > > > > There are lots of web pages that explain how/why the > > voltage is adjusted with the alternator. What I > cannot > > find is an explanation of amperage. Amps is what > charges > > the battery--at the correct voltage. I do not > understand > > how the amperage output of the alternator is varied. > > > > It seems to me that a discharged battery would need > lots > > of amps. A full battery, car with no lights on, etc, > > would need very little. What does the voltage have to > > do with this, except to correct for the temp of the > battery? > > > > You can see how I was a math major, not an engineer. > Help > > me out. > > > > Thanks, > > Richard > > > > To unsubscribe from the Vanagon List send an e-mail to > listserv@gerry.vanagon.com with SIGNOFF VANAGON in the body > of the message. > > > > > > -- > Neil Nicholson '81 JettaWesty "Jaco > http://groups.google.com/group/vanagons-with-vw-inline-4-cylinder-gas-engines > http://web.mac.com/tubaneil > http://tubaneil.googlepages.com/ > > ------------------------------------------------------------ > + To unsubscribe from the Vanagon List send an e-mail to > + listserv@gerry.vanagon.com with SIGNOFF VANAGON > + in the body of the message. > ------------------------------------------------------------

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