Installed 120 amp Audi Alternator

[amalventano at sc.rr.com (Allyn)]

> Larry
> PS  No matter what the amp output of an alternator..70 or 140...the 
> voltage
> is the same if the alternator is functioning properly.

alternator regulators work on the 'voltage droop' principle. the regulator 
maintains some high value (~13.2) at 0 current draw, and this drifts down to 
some low value (~12) at max current draw. this is done to prevent battery 
damage due to excessive charge rate. imagine installing a 600 amp (!) 
alternator in your rocco. if your battery was low on charge, and you didnt 
have a drooping regulator, the alternator would give the battery the full 
600 amps to do everything possible to get to 13.2 volts, which would not be 
very good for the battery (or the cabling, or the following explosion from 
excessive battery gassing that would follow shortly thereafter).

the regulator is usually suited to the particular application, aiming for 
ideal battery voltage with the typical load seen during normal driving. 
system load as well as battery capacity / specs are taken into account. this 
will cause a higher amperage rated alternator to put out a higher voltage 
when installed on a system which places less load on that alternator (i.e. 
beefy audi alternator installed in a rocco). this is also why there is such 
a thing as a 'recommended battery' for a particular car, and this 
recommendation is not based solely on minimum cranking amps required. you 
can install higher capacity batteries, and there is usually no issue, but 
realize this will result in longer battery charge times for a given 
alternator.

there is also a battery temperature compensation in some advanced 
alternators, which is intended to correct for the fact that battery cell 
voltage varies with temperature.

hth
Al