[DeltaB]

So how does an alternator know when to produce more (or less) current?

Let me try to help clear some of the confusion here. Alternators are controlled by voltage regulators. These are not current regulators. They sense voltage. An alternators output will continue to rise to load until the specified voltage is achieved. As more load is applied, the more voltage drop you will have, hence, the voltage regulator will apply more voltage signal to the commutator, until such point it reaches it's maximum output. When an alternator reaches it's intended set point for voltage, it decreases voltage to the commutator, thus decreasing output. If you have overvoltage issues, then you have either lost voltage sense, or the regulator has failed.

One point of suggestion to all who read this, the body of the vehicle itself, provides the lowest resistance for all grounding. Run your very large gauge cable, as short as it is possible from the negative post to the body itself, and at your amps ground with shortest large gauge cable directly to the body. Also, do the same from a place on the block to the body. Positive feeds must home-run to its intended feed. Just a word to the wise.

 

[DeltaB]

Exactly what I was struggling to convey, current will be based on the demand (the capacity of the battery or the load of the car). An alternaor cannot react to instantaneous demands (especially at idle) therefore the battery will supply the load and the Alt. will replenish the battery as needed.

And your advice about large gauge grounds being as short as possible is spot on!

A car does not run soley on one or the other, they work together. I can pull 120 amps with the system turned up and the car demands high, if my car was running soley on the alternator it would stall

Since most batteries can provide hundreds of amps (current) instantaneously, then your understanding that the battery is the actual device providing the current for you amplifier is certainly correct. The role of the alternator is to replenish the current used. If your current demands for a fully loaded system, (i.e. lights, accessories, fans, engine electronics) exceed the current capacity of the alternator, then you would seek a larger alternator output. However, voltage regulators are not instantaneous to load. (this takes seconds or longer) If your lights dim on heavy instantaneous peaks, then you haven't got your grounding setup properly.

In another hobby of mine (amateur radio) I have other operators who come to me all the time with this very same issue, utilizing linear amps that can instantaneously demand hundreds of amps of current during voice peaks, and a simple explanation of the importance of how they implemented their installation, (mostly never providing large gauge connect from the negative terminal to the body, and engine to body) find their lights almost always flashing at night due to the fact all accessories are running through a mostly 10 gauge or less connection from the negative terminal to the body, which cannot support a 500w or 1kW linear and all the accessories. Same holds true for audio amps.