The more I looked at the schematic the more confused I got because the regulator voltage has to come through the 3 diodes and voltage is only flowing if the battery is feeding the rgulator. The BMW diagram helped because the lamp can be seen as feeding the regulator when the switch is on. But I was stuck on the amount of current I thought the lamp would have to carry. I went to Wkipedia and found a really good explanation, Basically the batter does supply the alternator through the lamp just enough based on the resistance of the lamp (which is why an LED lamp in that role will not turn off until the motor revs. Once the alternator is "excited" (which means it is generating it's own voltage in a loop from the 3 little diodes through the regulator tto the rotor then to ground. And this rotor current is a small percent of the output current so it doesn't burn out the lamp during exitation.) the 3 little diodes then feed the regulator to what ever few amps of current it needs to recharge the battery and keep the electronics going.
From WIKI: Automotive alternators require a voltage regulator which operates by modulating the small field current to produce a constant voltage at the battery terminals. Early designs (c.1960s-1970s) used a discrete device mounted elsewhere in the vehicle. Intermediate designs (c.1970s-1990s) incorporated the voltage regulator into the alternator housing. Modern designs do away with the voltage regulator altogether; voltage regulation is now a function of the electronic control unit (ECU).
The field current is much smaller than the output current of the alternator; for example, a 70 A alternator may need only 7 A of field current. The field current is supplied to the rotor windings by slip rings. The low current and relatively smooth slip rings ensure greater reliability and longer life than that obtained by a DC generator with its commutator and higher current being passed through its brushes.
The field windings are supplied power from the battery via the ignition switch, [charge warning indicator], and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator. (which is why the indicator is on when the ignition is on but the engine is not running [enough current flows from the battery through the ignition switch, lamp, regulator and rotor (field windings) to ground to light the lamp, 1/3 AMP for a 4W bulb). Once the engine is running and the alternator is generating power [excited], a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire.
The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power. Some warning indicator circuits are equipped with a resistor in parallel with the lamp that permit excitation current to flow if the warning lamp burns out. The driver should check that the warning indicator is on when the engine is stopped; otherwise, there might not be any indication of a failure of the belt which may also drive the cooling water pump. Some alternators will self-excite when the engine reaches a certain speed.
Older automobiles with minimal lighting may have had an alternator capable of producing only 30 A. Typical passenger car and light truck alternators are rated around 50-70 A, though higher ratings are becoming more common, especially as there is more load on the vehicle's electrical system with air conditioning, electric power steering and other electrical systems. Very large alternators used on buses, heavy equipment or emergency vehicles may produce 300 A. Semi-trucks usually have alternators which output 140 A. Very large alternators may be water-cooled or oil-cooled.
In recent years, alternator regulators are linked to the vehicle's computer system and various factors including air temperature obtained from the intake air temperature sensor, battery temperature sensor and engine load are evaluated in adjusting the voltage supplied by the alternator.
