RI951

I know basically that the alternator uses engine power to turn and create a charge. I'm thinking that the regulator tells the the alternator when to draw power from the engine in order to do the work.
Is this true?

The basics:

Voltage: similar to electrical pressure. Think of a full tank of water, with the valve at the bottom that's closed. The "pressure" felt at the spigot it is voltage.

Current: The "flow" of electrons when a path is created from a higher "pressure" (voltage!) to a lower pressure. If one were to connect a hose to the spigot, and open it, you'd have flow. The larger the diameter of the hose, the less "resistance", and hence, the higher flow. Or, if you open the valve more, the higher the flow. Simple, right?

Think of your battery as the tank full of water. If nothing were charging it, eventually, the tank would run dry. So something has to fill it. That's the alternator, which can be thought of as a pump sitting on the ground outside the tank. And this pump develops a certain amount of pressure. In fact, it has to pump hard enough to push water into the tank, or the tank (battery) will never get refilled.

Since this pump is engine-driven, its pressure (voltage) varies with engine speed. We don't want to overpressurize the tank, so we regulate the pumps pressure (voltage), to no more than just a smidge more than the tank's normal pressure. Voila! A voltage regulator!

I'm not gonna go into how a voltage regulator works. It's not super difficult, but it ain't easy to explain without pictures.

So, you have a 12 volt battery (tank), being filled by a 14v alternator (pump). If the tank is filled at a sufficient pressure (voltage), when you turn on a load, like lights, current begins to flow.
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Q:What determines this to happen? (important question) Is the regulator a sensor of some sort?

Not so much a sensor as it is a limiter. It will only allow so much pressure, er, voltage to be generated by the alternator to fill the battery. As one lister stated, too much pressure can cook a battery.
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Q:Therefore with a strong/new battery the regulator should tell the alternator to work less often or less hard?

A new battery has the maximum capacity to accept and store voltage. Back to the tank analogy, as the tank ages, maybe it can't filled as much. If it can't store as much, it must be filled more often, or at a higher rate.
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Q:Is it totally an on/off scenario, or can there be various degrees of work drawn from the engine?
If its the on/off scenario and the alternator is in its "non working" mode shouldn't the in car voltmeter read only about 12 volts because there is no charge being produced on top of a 12 V battery? It almost always reads more than 12 V.

It's not on/off. The alternator is probably always producing some power, but once the battery is topped off, and if there are few electrical loads, then the alternator doesn't do much "work". (I know you physics majors are thinking that there is no work done of there was no movement... but this is basic electricity).
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Sometimes when I start a cold engine the voltmeter will read almost 14 V, and sometimes it will read barely 13 V. on a cold engine. In both cases it's the same ambient temp and no accessories on.

Batteries are interesting animals. A lead-acid battery's capacity is affected by cold, and this is shown by it's dropping pressure on the voltmeter.

Conversely, current flow is affected by heat. Back to the hose analogy, which is akin to wire in our example, the higher the heat, the smaller the diameter the hose becomes. It's resistance becomes higher, and it can't flow as much current. Therefore, your lights dim, or the output of the alternator drops. Depends on where in the circuit the heat is being felt.
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I have a new battery and new aternator. I think the regulator comes with the alternator? My voltmeter reads correctly (had it checked).

As far as what the voltmeter is indicating, I think it indicates alternator output, downstream of the voltage regulator. On my old Jeep, you could disconnect the battery once the truck was running, and the truck kept running, and the voltmeter indicated normal. Wouldn't do in this car though. Too much electronic stuff. And it's normal for the reading to fluctuate a tad; these regulators are a pretty simple device, and can't entirely make up for changes in engine speed.
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If anybody is bored and feels like answering these questions; that would be great.

Don't know if this helps or hurts, but the basics are good to know. Not that anyone cares, but since others have tossed around their credentials, I'll throw mine out there too: 9 years Naval Nuclear Electrician, including a few teaching electricity at Nuclear Power School. Now a reactor operator at a electrical power station.

Having said that, watch me be totally wrong about some facet of this... It's been a long time since I had to think about this stuff.

Pat

TurboTommy

Thanks all you guys for the effort in answering my questions. There are some contradictory statements but I'll have to sort them out. I will definitely be filing this thread away for future reference.