A vehicle's automotive voltage regulator is the device responsible for controlling the voltage used to recharge the car's battery. It keeps the voltage coming from the vehicle's alternator between 13.5 and 14.5 volts, recharging the battery while the engine is running.
Whenever a vehicle's ignition is turned, power from the battery turns the engine, beginning the combustion process and starting the car. Once the engine is running on its own power, however, it produces power and directs it to the battery instead of using battery power. This process makes use of an alternator and an automotive voltage regulator.
When the engine is running, its drive belt spins a rotor that turns inside the vehicle's alternator. This causes the alternator to operate like a generator and produce power. The electric current flows to the voltage regulator, where the device turns the alternator on or off depending on the voltage produced. If the voltage falls below 13.5 volts or rises beyond 14.5 volts, the regulator closes the circuit. Maintaining a voltage in this range prevents the vehicle battery from draining or dangerously overcharging.
Older automotive voltage regulators make use of electromechanical components to physically interrupt the circuit, cutting the power flow. Modern automotive voltage regulators typically use electronic components that start and stop the flow of voltage.