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No need to take my word for it, copied from a publication on alternatorsTHE COMPARISON (explanation of system functions)

The battery is an electrical storage reservoir, similar in function to the air tank for the compressed air system. (Actually, the battery does not store electricity, it would be more correct to say; “the battery stores ingredients that can produce electricity.”) Both the battery and the air tank can store a source of energy in reserve, keeping energy available for the times we need it.

The alternator produces electrical power, which can operate devices that perform work for us. And the compressor produces the compressed air, which can be used as a source of power to operate tools or machinery.

The voltage regulator limits the maximum voltage in the electrical system. In the compressed air system the pressure regulator limits the maximum pressure. The voltage regulator will also cause the alternator to produce more output, when voltage (pressure) at the electrical system is low. And in the compressed air system, the pressure switch will turn on the compressor when system pressure gets low.

Lights, ignition, and accessories use power from the electrical system. Every time we switch an accessory ON, more power is drawn from the system. Voltage (electrical pressure) drops as power is drawn from the system, and then the voltage regulator causes the alternator to make more current. And in the compressed air system an impact wrench, blowgun, paint gun, or the fitting for filling a tire, can all use power (compressed air) from the system. When we use compressed air from the system, PSI (air pressure) drops, and the regulator turns the compressor ON. In the electrical system, the voltage regulator “turns the alternator ON,” or “turns OFF the alternator” as needed to maintain voltage at the proper level. And in the air compressor system the pressure regulator stops and starts the compressor as needed to maintain the proper level of pressure.

The useful electrical system will require an alternator that can produce an average of more output than we use, and the regulator will limit system voltage to the safe level we need. Like most machinery, the alternator cannot stand to work at maximum output for extended periods of time. Short bursts at maximum output are okay, but normal operation will require alternator operation at only a part of full output potential, most of the time. Alternators make heat as a by-product of making electrical power, and the more power they supply the more heat they make. Some models of alternators can stand to put out a much higher percentage of their gross output rating than others, during extended periods of operation.

Air compressors have “duty cycle” ratings. The compressor also produces heat as a by-product, and if it was called upon to run continuously while maintaining high pressure, the compressor will burn out. Some models of air compressors will have a greater duty cycle than others. Expect that a hobby shop model will not be intended to run for the long time periods that a professional workshop compressor is built for.

When the electrical system needs more power than the alternator can produce, for a short time, then the battery is already connected to the system and the battery will contribute the needed power. Entering into this picture is that the alternator must spin at sufficient RPM to produce power. And there is an alternator power output/RPM curve, where available output increases with RPM. There is also a minimum and maximum for practical alternator RPM operating range. Alternator RPM is somewhat adjustable by changing the ratio of the drive pulley at the crankshaft and alternator pulley diameters. But since the engine will run slowly at times, and rev very high at other times, there is no “perfect” pulley drive ratio for all applications. The pulley drive ratio is a compromise; and what’s acceptable at maximum RPM is the deciding point. (An alternator can be damaged with excessive RPM.) A pulley ratio that is good with 6,500 to 8,000 engine RPM on a circle track is far from ideal with the in-line six engine in “Grandma’s grocery getter.”

At low RPM, expect that early models of alternators often produced much less available output than more modern designs. And with many models of old alternators, electrical output at engine idle speed was not sufficient to support electrical demands. But when sitting at a stoplight, the battery could assist the alternator with support of the electrical system. And then when the light turned green we drove away with the engine spinning the alternator fast once again. The alternator soon replaced power used from the battery while sitting at the stop light, no harm done. System voltage will be low, when the alternator is not keeping up. (Voltage will be above 14 when the alternator is working, and about twelve and falling when supported by the battery.)

Drivers of old cars were accustomed to the lights dimming at idle, or the turn signals blinking slower–it was simply the result of low voltage when the alternator did not keep up. The older cars could get by with less than perfect performance. And with fewer electrical items to support, then the voltage did not drop off so quickly. The old cars also did not have electronics that would cease to operate at low voltage. With the duration of city traffic jams in modern times, the many accessories on a modern car, and electronics that are sensitive to low voltage, of course alternator output at engine idle speed had to get better. The newer designs of alternators can produce a lot more current at low RPM, even when the gross output rating is nearly the same with the old model.