Batteries provide power to start your car, run the engine, computer, radio, headlights, etc. Anything that requires power is run off your battery. Since batteries have a limited supply of energy they continually recharged by the alternator. Alternators work by rotating a coil of wires (rotor) inside a magnetic field. The amount of power produced is related to the rotor size, the strength of the magnetic field, and the speed at which rotor turns.
The engine turns the alternator (rotor) by a belt. Years ago the alternator had a dedicated belt. Newer cars have a ‘master’ belt, referred to as a serpentine belt, which turns the alternator, a/c compressor, power steering pump etc.
The amount of power produced by the alternator is controlled by a voltage regulator which is located inside the alternator. Since the voltage regulator cannot change how fast the rotor turns, it changes the strength of the magnetic field to vary the amount of power the alternator produces. Based on the speed of the engine, and how much power is needed, the voltage regulator increases or decreases the strength of the magnetic field.
Starting your car requires a great deal of power from your battery. Therefore, once your car starts the alternator goes to maximum to recharge your battery. As the battery ‘charges-up’ the voltage regulator scales back the quantity of electricity produced, until the power into the battery equals the power out of the battery.
There are two reasons alternators need to be changed. One, the alternator doesn’t produce enough electricity (measured in amps and in volts) to keep your battery fully charged. Two, the alternator only produces maximum power. If the alternator only produces maximum power, it will destroy the battery. Also, a bad battery may cause the alternator go produce maximum power. Since alternators are not designed to produce maximum power for a long period of time, a bad battery may destroy an alternator.
When I check a charging system I must check both the battery and the alternator. If I have a bad battery I must replace it before I can be certain the alternator is good.
I have seen cars towed here and the customer tells me, “They put in a new battery because they thought it was the reason the car would not start. With a new battery the car started. A few weeks later they had the same problem, the car would not start, so they replaced the alternator. A few weeks later the car does not start.” At this point they bring the car to my repair shop. The problem…. The alternator was bad and destroyed the battery. When the customer replaced the battery it lasted until the alternator ‘fried’ the new battery (by continually producing maximum power). The customer then replaced the alternator not knowing the alternator and the battery are bad. The fried battery tells the new alternator that is needs “maximum” power, and the production of maximum power for a long time makes the alternator go bad.
To prevent this from happening the battery and the alternator must be replaced at the same time.