Date:         Sat, 4 Sep 2004 21:28:39 -0600
Reply-To:     Richard A Jones <jones@COLORADO.EDU>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Richard A Jones <jones@COLORADO.EDU>
Subject:      alt output vs rpm
Content-Type: text/plain; charset=us-ascii; format=flowed

>> Please note that the alternator charges at very low rate
>>when the engine is idling. It will only provide decent charging current
>>at above 2500 RPM.

Is this true?  I thought we gave up generators for
alternators so they would charge at idle--so I spent
some time with Google trying to understand this.

Chart of RPM vs amps:
http://www.fordmuscle.com/archives/2004/07/6GAlternator/index.shtml
Look like a pretty flat "curve" on this.

Balmar (marine alternators) with very good tables:
http://www.balmar.net/PDF/Alternator-Regulator%20Guide.pdf

A table that seems to contradict these:
http://www.uuhome.de/william.darden/carfaq5.htm

What I think I understand is that where generators were
run at 1/2 engine speed (so they wouldn't self-destruct),
alternators are typically run a 2x engine speed.
The higher the output (amps) of the alternator, the
more "curve" to the curve.  The lower the output, the
flatter the "curve."

Here's Balmar data for an 80 amp alternator (vs the
90 amp used in most Vanagons, except early ones--which
were 45 or 65 amp!):
rpm             amps
2000            56
3000            74
4000            82
5000            85
(These are alt rpms, so about 2x engine rpms.)

So in this case, reving the engine to 2000 rpm
(4000 rpm alt) gives ~45% more amps than idle.  To me,
that means idle isn't so bad after all.

Anyone with certain knowledge of this stuff wanna
comment?

                        Richard A Jones
                        Boulder, Colorado