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

[Mark R]

Since an academic "argument" was posed (indirectly), let's get an answer!!

OK, this one is for Larry, "Mr. Fizziks Teacher."

So, can we think of an automotive alternator like a DC motor... as the load
increases, it takes more current to maintain RPM?  In other words, as the
electrical load on the alternator increases, does it take more rotational
force (work, power, gasoline) to produce the higher electrical load?

Or, if the RPMs are fixed, is the amount of work (power, gasoline) fixed, no
matter what the electrical load?

Back to the methodology:
Personally, when I repowering a vehicle (car boat), I check the load.  If
available, make a loop at the fuse and measure with an inductive probe.  I
see they make commercial products now which do this.  I measure actual
loads... and add 20% for a "safety factor."  This gives my basis for sizing
everything (wiring, fuses, relays, alternators, batteries, etc.).  I've
found instances where 20A circuits are protected by 70A fusible links!
Needless to say, the fusible links are the first thing to go.  =)

Mark Rosenkrantz

On Feb 4, 2008 3:22 PM, LL - NY <larrycleung at gmail.com> wrote:

> 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