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 causing damage to the electrical system itself. Alternator charging voltage can be regulated using an electro-mechanical, electronic, or computer controlled regulator.
Most late model vehicles use computer controlled regulation. This is when the function is incorporated into the ECM engine control module. Computerized voltage regulators use PWM “pulse width modulation” to control charging rates. These rates are typically adjusted by output voltages and battery temperature. A battery is more willing to accept a charge at warmer temperatures like 70F or 80F than in frigid conditions.
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 alternators field circuit the more voltage output the alternator will produce. The increase in field current increases the magnetic field created inside the alternator increasing voltage output.
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 alternators setting (lights on) the system will increase the current to field circuit increasing output. If the vehicle were idling and no accessories were on the battery voltage is high and the sensing voltage is high. This would result in low current supplied to the field circuit and low alternator output.