When the ignition is switched on, terminal 15 is energized. But the generator is not running yet, so the de-energized D+ contact acts as «minus». The control lamp lights up, because there is a difference between the voltage of the on-board network, which is powered by the battery, and the voltage of the generator still not running.
When the engine starts and the alternator reaches the charging speed, then the voltage regulator connects the alternator to the onboard electrical equipment. The positive current now flows from terminal 15 and additionally from terminal D+. Thanks to this, there is no longer a difference in voltage, the charging indicator lamp goes out.
The control light is no longer needed for the so-called «pre-excitation» alternator, in the Audi A4 this already occurs during engine start via ignition switch terminal 50a.
Doesn't always charge
The fact that the battery is being charged by the alternator does not prove that the indicator light goes out. This only says that there is no longer a difference in voltage between the battery and the alternator. If, for example, when the engine is idling, all current consumers are switched on, the control lamp does not light up, although more battery current is consumed than less powerful alternators can provide: nevertheless, there is no difference in voltage compared to the battery.
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