Output current is important, I reckon, when a battery needs a lot of
amps for charging. Me, I'd want to know if the alternator had enough
current /and/ voltage to charge my battery at idle.
--

Mike "Rocket J Squirrel" Elliott
71 VW Type 2 / 84 Westy: A poor but proud race.
KG6RCR

Richard A Jones wrote:

>>> 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
>