How much amperage can an alternator support? no really...

[LL - NY]

The alternator will only deliver as much current as is drawn, basically,
through the control of the voltage regulator, the alternator will produce
enough current as requried by the voltage regulator following the equation,
I = V/R.  R is essentially constant, but the lower the R (by adding more and
more parallel circuits, suchs as multiple auxiliary lamps) the greater the
demand of I to support the required voltage. Since most cars don't draw as
much as the alternator can deliver, the alternators are essentially
underutilized with respect to their rating. As you start getting towards
their rated max, more current does flow through them, causing greater heat
and therefore faster breakdown of the heat effected components, including
insulation, connections and bearings. Greater current at the brushes will
cause greater amounts of arc damage (it always does occur, it's just not
usual for it be severe) which will prematurely wear out the brushes.

So, IOW, yes, greater demands will cause earlier failure of the alternator.
How much earlier, IDK.

Seeing that you run the cooling fan manually as a point of "reliablity" (you
don't trust the temp switch?) and you have rather large lighting demands,
are you using the car for racing/rally? From a reliability standpoint, you
may wish then to either deal with a higher rated alternator, or a second
battery (as a back-up for alternator failure, kinda stops an event when the
fuel pump quits) or both.

HTH,

LL - NY

On 2/4/08, Vittorio Bares <Vittorio.Bares at nuance.com> wrote:
>
> Thanks - this is good info, and encouraging.
>
> My cooling fan is on a manual switch and I typically run it always-on
> (eliminates one more point of failure in the temp-switch). Any idea what
> amperage that draws?
>
> Looks like I'm pretty close to max w/90amp - I can reduce the draw by
> converting my lights to HID, where instead of using 100w per light,
> they'll only draw 35w - and give better light.
>
> I've heard people say the alternator has to 'work harder' given the load
> - I don't understand how a device on a pully system that varies in speed
> based on engine speed 'works harder' given a greater draw ? Doesn't it
> continuously do the same thing - but its output is regulated according
> to need? Will the alternator wear out sooner the more power it needs to
> produce?
>
> Vittorio -
>
> -----Original Message-----
> From: Huw Powell [mailto:audi at humanspeakers.com]
> Sent: Monday, February 04, 2008 1:12 PM
> To: Vittorio Bares
> Cc: quattro at audifans.com
> Subject: Re: How much amperage can an alternator support? no really...
>
>
> > I've got 580 watts of lights on the rally car, this will require
> > 48amps to run. Typically the 4000 has a 90amp alternator, I know I
> > could go with the heftier 110amp alt from a 200 or what not.
> >
> > But my question really is, how much amperage does the car need to run?
> > The only other acc I have is the heater fan, radiator fan, a few
> > gauges and lighting for those, brake lights, fuel pump, intercom
> > system - potentially the wipers. How much amperage does an alternator
> > (typically 20 years old) really produce?
> >
> > My assumption is that the big current draw is going to come from the
> > radiator fan, fuel pump and possibly the heater blower fan - but also
> > the ignition (spark, etc)...
>
> My coupe has an essentially stock alternator, and on high beams I'm
> running 400w of lights.  Car still runs and charges the battery fine
> even if I turn up the 480w of stereo up real loud.
>
> I think the FP runs about 10 A, the CIS another 10 or so, and the coil a
> few.  The radiator fan might not come on much as long as you are in
> motion.
>
> I would make sure to do a few things - install a good, accurate
> voltmeter (like the coupe/90Q ones), make sure your alternator/voltage
> regulator is in good shape, use a good battery, and make sure all your
> primary wiring and connections are in good shape.
>
> The car will work down to about 9.5 volts or so, so if you are
> overloading the alt you'll have plenty of time to do something about it
> if you see the voltage running below 12.6.
>
> --
> Huw Powell
>
> http://www.humanspeakers.com/audi
>
> http://www.humanthoughts.org/
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