Charging Systems

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As mentioned earlier, the battery is the heart of the electrical system in the car. Something has to keep feeding the battery, or it will lose it’s voltage and fail to work. This job is left to the charging system. The principal part of the charging system is the alternator. It generates and delivers electrical power to the battery and the rest of the vehicles electrical system. This current must be maintained at between 13 and 15 volts, this is the job of the voltage regulator. Amperage output of the alternator is dependent on the state of charge of the battery. The charging system consists of the battery, which provides field current to the alternator, so that it can function. The battery also controls the amperage output of the alternator. The voltage regulator, which controls the voltage output of the alternator. An ammeter or voltage indicator lamp, to inform the driver of the condition of the system.

As the principal part of the charging system, the alternator’s major roles include generating and delivering electrical power to the battery and the rest of the vehicles electrical system.


The alternator uses the principle of electromagnetism induction to produce voltage and current. A magnet, called a rotor, is rotated inside a stationary looped conductor, called a stator, to generate a voltage. The strength and polarity of the voltage is dependent on the direction of rotation, the strength of the magnetic field, the number of conductors and the speed of the rotor inside the stator. This type of generator puts out AC voltage, hence an alternator instead of a generator, while an automobile is operated on DC current. To convert the AC voltage into DC voltage a series of diodes are used. A diode is an electrical one way check valve that allows current to flow in one direction only. By allowing current to flow in only one direction, we are able to convert alternating current to direct current. Another big difference between an AC alternator and a DC generator, is that an alternator will put out a large current flow at low speed, while a generator needs high speed to put out high current flow. As mentioned earlier voltage output is dependent on the strength of the magnetic field. The magnetic field is created by applying current to the stator from the voltage regulator.

Voltage Regulator

The voltage regulator controls the field current applied to the alternator. When there is no current applied to the field, there is no voltage produced from the alternator. When voltage drops below 13.5 the regulator will apply current to the field and the alternator will start charging. When the voltage exceeds 14.5 the regulator will stop supplying voltage to the field and the alternator will stop charging. This is how voltage output from the alternator is regulated. Amperage or current is regulated by the state of charge of the battery. When the battery is weak, the electromotive force (voltage) is not strong enough to hold back the current from the alternator trying to recharge the battery. As the battery reaches a state of full charge, the electromotive force becomes strong enough to oppose the current flow from the alternator, the amperage output from the alternator will drop to 0, while the voltage will remain at 13.5 to 14.5. When more electrical power is used, the electromotive force will reduce and alternator amperage will increase. It is extremely important that when alternator efficiency is checked, both voltage and amperage outputs are checked. Each alternator has a rated amperage output depending on the electrical requirements of the vehicle.

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Fred Bordoff

Automotive Expert

Fred Bordoff has been a specialist in automotive diagnostics and repairs for over 30 years. He is currently the owner and president of NYCAT, the New York Center for Automotive Technology, a state-of-the art facility specializing in automotive diagnostics and repair, located in Long Island City.

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