At the heart of the automotive electrical system is the alternator. Without voltage regulation, the alternator would consistently produce too much voltage overcharging the battery and possibly causing damage to the electrical system. Full-fielding an alternator bypasses the voltage regulator and produces maximum voltage output. Charging voltage can be regulated using an electro-mechanical, electronic, or computer-controlled regulator.
The desired voltage output is achieved by changing the amount of current to the field circuit. Consider the field circuit as power into the alternator. The more current into the alternator's field circuit, the more output voltage the alternator will produce. The increase in field current increases the magnetic field created inside the alternator.
The system must have a sense of how much voltage the system already has before it can decide whether to increase or decrease current to the field circuit. If the sensing voltage is low and below the alternator's setting (lights on), the system will increase current to field circuit, increasing alternator output. When the vehicle is idling, and no accessories are on, the battery and the sensing voltage is high. This situation results in low current supplied to the field circuit and low alternator output.