The alternator charges the battery and powers the vehicle's electrical system when the engine is running. It provides electrical energy to keep the battery fully charged and to power vehicle systems in use (e.g. the starter motor, lights, and stereo).
The alternator produces an alternating current, which is changed to usable direct current by the rectifier diodes within the alternator. The voltage regulator regulates the output voltage of the alternator. On most modern vehicles, the regulator is housed inside the alternator, but it can be externally-mounted. In either case, the voltage regulator can usually be replaced apart from the alternator.
To replace the alternator, the technician must first disconnect the battery and remove the alternator drive belt. Then the alternator is removed from the engine compartment. After installing the new alternator, the technician tightens the alternator drive belt and reconnects the battery.
To prevent damage to the alternator, proper procedures must be followed when jump-starting the vehicle or when using a battery charger to charge the battery. Never disconnect the battery from the vehicle when the engine is running. The battery absorbs excess voltage from the alternator and without it, the higher voltage may destroy the vehicle's electrical and electronic components.
Important Note: Since the alternator is designed to maintain a charged battery, charging a dead battery by driving the vehicle places a strain on the alternator. Charging a dead battery requires a sustained and substantial flow of current, which is not what the alternator is designed for. Repeatedly jump-starting and then driving a vehicle to charge the battery can lead to alternator failure.
Mechanics' Corner: More Technical Detail
The alternator is an electrical-generating device with a belt-driven pulley, which is turned or driven by the engine. Once the engine is started, the alternator supplies the electrical energy the vehicle needs.
The alternator sends electrical energy back to the battery in order to keep it fully charged. When the vehicle is running, the battery stores a large reserve of electrical energy to be used during moments of peak electrical demand (like on a cold, dark, and rainy morning when the lights and heater are on, the windshield wipers are working hard, the seat warmers are on, and the electric windshield defrosters are operating). When the vehicle is stopped and the engine is idling, the alternator cannot keep up with all of this electrical load, so the electrical system will draw energy from the reserve in the battery to meet the demand. When the vehicle is moving, the engine turns the alternator much faster, therefore generating enough electricity to meet (and exceed) demand—this is when the battery is recharged.
In modern vehicles, the amount of voltage supplied by the alternator is regulated by the engine computer where previously, a voltage regulator was built in to the alternator itself. Due to the growing number of onboard computer systems and the fact that newer vehicles need to maintain an ever-tightening envelope of voltage, the voltage regulation was transferred to the more powerful and sophisticated engine (or powertrain) computer.
How an Alternator Works
An alternator is a device that uses a rotating iron shaft that is wound with very thin copper wires into what is called a field winding. The ends of the windings are connected to a regulated voltage source. As the vehicle needs more voltage, the electrical system provides more electrical current, which is applied to the field windings on the rotating iron core (the "rotor"). This creates a magnetic field that induces AC voltage into a surrounding set of copper windings (called the "stator" because it is stationary). The AC voltage from the stator flows through a set of one-way electrical gates called diodes. The diodes only allow DC voltage back into the electrical system by way of the B+ or battery output terminal on the back of the alternator that is directly cabled to the positive terminal on the battery.
Things to Be Aware of When Replacing the Alternator
Before replacing the alternator, always make sure that the key is out of the ignition, that all doors are closed, and all interior and exterior lights are off. Disconnect the negative battery cable first. This will shut down all the computers on the vehicle without sending a harmful electrical surge throughout the system (which can damage the sensitive electronic devices). These electronics are designed with specific polarity in mind and utilize diodes (one-way electrical gates) to protect them. Disconnect the negative (ground) side first; then disconnect the positive battery cable. During the mechanical removal of the alternator, while the alternator fasteners are removed and the drive belt is released, the electrical system releases any reserve capacitance (the electrical energy stored in the various systems and their components). When the new alternator is installed, the positive battery cable must be connected first and the negative battery cable connected last.