From Chrysler's
1960 Master Tech booklet.
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The wiring scheme changes slightly over the years but the overall strategy always used the ammeter in the same manner. It only shows battery discharge and charging. The 1976 a-bodies got a remote shunted ammeter. Instead of using a wide metal plate in the ammeter, most of the current flowed in parallel through a wire of known length in the engine bay.
This is a typical standard wiring scheme. Of course details vary with years and options.
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Power to run the car is supplied by either the battery or the alternator.
If the alternator can not provide sufficient energy at approximately 14 Volts, its output voltage will drop.
The battery begins to assist when the its voltage and the alternator's are equal.
It takes over when its voltage is higher.