The purpose of the charging system is to maintain the charge in the vehicle's battery, and to provide the main source of electrical energy while the engine is running.

If the charging system stopped working, the battery's charge would soon be depleted, leaving the car with a "dead battery."   If the battery is weak and the alternator is not working, the engine may not have enough electrical current to fire the spark plugs, so the engine will stop running.

If the battery is "dead", it does not necessarily mean that there is anything wrong with it.  It is just depleted of its charge.  It can be brought back to life by recharging it with a battery charger, or by running the engine so that the alternator can charge it.

The main component in the charging system is the ALTERNATOR.  The alternator is a generator  that produces Alternating Current (AC), similar to the electrical current in your home.  This current is immediately converted to Direct Current (DC) inside the alternator.  This is because all modern automobiles have a 12 volt, DC electrical system.

A VOLTAGE REGULATOR regulates the charging voltage that the alternator produces, keeping it between 13.5 and 14.5 volts to protect the electrical components throughout the vehicle.

There is also a system to warn the driver if something is not right with the charging system.  This could be a dash mounted voltmeter, an ammeter, or more commonly, a warning lamp.  This lamp is variously labeled "Gen" Bat" and "Alt.".  If this warning lamp lights up while the engine is running, it means that there is a problem in the charging system, usually an alternator that has stopped working.  The most common cause is a broken alternator drive belt.

The alternator is driven by a belt that is powered by the rotation of the engine.  This belt goes around a pulley connected to the front of the engine's crankshaft and is usually responsible for driving a number of other components including the water pump, power steering pump and air conditioning compressor.  On some engines, there is more than one belt and the task of driving these components is divided between them.  These belts are usually referred to as: Fan Belt, Alternator Belt, Drive Belt, Power Steering Belt, A/C Belt, etc.  More common on late model engines, one belt, called a Serpentine Belt will snake around the front of the engine and drive all the components by itself.

On engines with separate belts for each component, the belts will require periodic adjustments to maintain the proper belt tension.  On engines that use a serpentine belt, there is usually a spring loaded belt tensioner that maintains the tension of the belt, so no periodic adjustments are required. A serpentine belt is designed to last around 30,000 miles.  Check your owner's manual to see how often yours should be replaced.

Alternator output is measured in both voltage and amperage.  To understand voltage and amperage, you must also know about resistance, which is measured in ohms.  An easy way to picture this is to compare the movement of electricity to that of running water.  Water flows through a pipe with a certain amount of pressure.  The size (diameter) of the pipe dictates how much resistance there will be to the flowing water.  The smaller the pipe, the more resistance.  You can increase the pressure to get more water to flow through, or you can increase the size of the pipe to allow more water to flow using less pressure.  Since too much pressure can burst the pipe, we should probably restrict the amount of pressure being used.  You get the idea, but how is this related to the flow of electricity?

Well, voltage is the same as water pressure.  Amperage is like the amount or volume of water flowing through, while resistance is the size of the wire transmitting the current.  Since too much voltage will damage the electrical components such as light bulbs and computer circuits, we must limit  the amount of voltage.  This is the job of the voltage regulator.  Too much water pressure and things could start breaking.  Too much voltage and things could start burning out.

Let's get technical

Now, let's go a little deeper and see how these charging system components actually work to produce the electrical power that a modern automobile requires.

The Alternator

The alternator uses the principle of electromagnetism to produce current.  The way this works is simple.  If you take a strong magnet and pass it across a wire, that wire will generate a small voltage.  Take that same wire and loop it many times, than if you pass the same magnet across the bundle of loops, you create a more sizable voltage in that wire.

There are two main components that make up an alternator.  They are the rotor and the stator.  The rotor is connected directly to the alternator pulley.  The drive belt spins the pulley, which in turn spins the rotor.  The stator is mounted to the body of the alternator and remains stationary.  There is just enough room in the center of the stator for the rotor to fit and be able to spin without making any contact.

The stator contains 3 sets of wires that have many loops each and are evenly distributed to form a three phase system.  On some systems, the wires are connected to each other at one end and are connected to a rectifier assembly on the other end.  On other systems, the wires are connected to each other end to end, and at each of the three connection points, there is also a connection to the rectifier.  More on what a rectifier is later.

The rotor contains the powerful magnet that passes close to the many wire loops that make up the stator.  The magnets in the rotor are actually electro magnets, not a permanent magnets.  This is done so that we can control how much voltage the alternator produces by regulating the amount of current that creates the magnetic field in the rotor.  In this way, we can control the output of the alternator to suit our needs, and protect the circuits in the automobile from excessive voltage. 

Now we know that every magnet has a north and a south pole and electro magnets are no exception.  Our rotor has two interlocking sections of electro magnets that are arranged so that there are fingers of alternating north and south poles. that are evenly distributed on the outside of the rotor.

When we spin the rotor inside the stator and apply current to the rotor through a pair of brushes that make constant contact with two slip rings on the rotor shaft.  This causes the rotor to become magnetized.  The alternating north and south pole magnets spin past the three sets of wire loops in the stator and produce a constantly reversing voltage in the three wires.  In other words, we are producing alternating current in the stator.

Now, we have to convert this alternating current to direct current.  This is done by using a series of 6 diodes that are mounted in a rectifier assembly.   A diode allows current to flow only in one direction.   If voltage tries to flow in the other direction, it is blocked.  The six diodes are arranged so that all the voltage coming from the alternator is aligned in one direction thereby converting AC current into DC current.

There are 2 diodes for each of the three sets of windings in the stator.  The two diodes are facing in opposite directions, one with its north pole facing the windings and the other with its south pole facing the windings.  This arrangement causes the AC current coming out of the windings to be converted to DC current before it leaves the alternator through the B terminal.  Connected to the B terminal of the alternator is a fairly heavy wire that runs straight to the battery.

Current to generate the magnetic field in the rotor comes from the ignition switch and passes through the voltage regulator.  Since the rotor is spinning, we need a way to connect this current from the regulator to the spinning rotor.  This is accomplished by wires connected to two spring loaded brushes that rub against two slip rings on the rotor's shaft.  The voltage regulator monitors the voltage coming out of the alternator and, when it reaches a threshold of about 14.5 volts, the regulator reduces the current in the rotor to weaken the magnetic field.  When the voltage drops below this threshold, the current to the rotor is increased.

There is another circuit in the alternator to control the charging system warning lamp that is on the dash.  Part of that circuit is another set of diodes mounted inside the alternator called the diode trio.  The diode trio takes current coming from the three stator windings and passes a small amount through three diodes so that only the positive voltage comes through.  After the diodes, the wires are joined into one wire and sent out of the alternator at the L connection.  It then goes to one side of the dash warning lamp that is used to tell you when there is a problem with the charging system.  The other side of the lamp is connected to the run side of the ignition switch. If both sides of the warning lamp have equal positive voltage, the lamp will not light. Remove voltage from one side and the lamp comes on to let you know there is a problem.

This system is not very efficient.  There are many types of malfunctions of the charging system that it cannot detect, so just because the lamp is not lit does not mean everything is ok.  A volt meter is probably the best method of determining whether the charging system is working properly.

The Voltage Regulator

The voltage regulator can be mounted inside or outside of the alternator housing.  If the regulator is mounted outside (common on some Ford products) there will be a wiring harness connecting it to the alternator.

The voltage regulator controls the field current applied to the spinning rotor inside the alternator. When there is no current applied to the field, there is no voltage produced from the alternator. When voltage drops below 13.5 volts, the regulator will apply current to the field and the alternator will start charging. When the voltage exceeds 14.5 volts, the regulator will stop supplying voltage to the field and the alternator will stop charging. This is how voltage output from the alternator is regulated. Amperage or current is regulated by the state of charge of the battery. When the battery is weak, the electromotive force (voltage) is not strong enough to hold back the current from the alternator trying to recharge the battery. As the battery reaches a state of full charge, the electromotive force becomes strong enough to oppose the current flow from the alternator, the amperage output from the alternator will drop to close to zero, while the voltage will remain at 13.5 to 14.5. When more electrical power is used, the electromotive force will reduce and alternator amperage will increase. It is extremely important that when alternator efficiency is checked, both voltage and amperage outputs are checked. Each alternator has a rated amperage output depending on the electrical requirements of the vehicle.

Charging system gauge or warning lamp

The charging system gauge or warning lamp monitors the health of the charging system so that you have a warning of a problem before you get stuck.

When a charging problem is indicated, you can still drive a short distance to find help unlike an oil pressure or coolant temperature problem which can cause serious engine damage if you continue to drive. The worst that can happen with a charging system problem is that you get stuck in a bad location.

A charging system warning lamp is a poor indicator of problems in that there are many charging problems that it will not recognize. If it does light while you are driving, it usually means the charging system is not working at all. The most common cause of this is a broken alternator belt.

There are two types of gauges used to monitor charging systems on some vehicles: a voltmeter which measures system voltage and an ammeter which measures amperage. Most modern cars that have gauges use a voltmeter because it is a much better indicator of charging system health. A mechanic's voltmeter is usually the first tool a technician uses when checking out a charging system.

A modern automobile has a 12 volt electrical system. A fully charged battery will read about 12.5 volts when the engine is not running. When the engine is running, the charging system takes over so that the voltmeter will read 14 to 14.5 volts and should stay there unless there is a heavy load on the electrical system such as wipers, lights, heater and rear defogger all operating together while the engine is idling at which time the voltage may drop. If the voltage drops below 12.5, it means that the battery is providing some of the current. You may notice that your dash lights dim at this point. If this happens for an extended period, the battery will run down and may not have enough of a charge to start the car after shutting it off.  This should never happen with a healthy charging system because as soon as you step on the gas, the charging system will recharge the battery.  If the voltage is constantly below 14 volts, you should have the system checked. If the voltage ever goes above 15 volts, there is a problem with the voltage regulator. Have the system checked as soon as possible as this "overcharging" condition can cause damage to your electrical system.

If you think of electricity as water, voltage is like water pressure, whereas amperage is like the volume of water.  If you increase pressure, then more water will flow through a given size pipe, but if you increase the size of the pipe, more water will flow at a lower pressure.  An ammeter will read from a negative amperage when the battery is providing most of the current thereby depleting itself, to a positive amperage if most of the current is coming from the charging system.  If the battery is fully charged and there is minimal electrical demand, then the ammeter should read close to zero, but should always be on the positive side of zero. It is normal for the ammeter to read a high positive amperage in order to recharge the battery after starting, but it should taper off in a few minutes.  If it continues to read more than 10 or 20 amps even though the lights, wipers and other electrical devices are turned off, you may have a weak battery and should have it checked.

What can go wrong?

There are a number of things that can go wrong with a charging system: 

Insufficient Charging Output
If one of the three stator windings failed, the alternator would still charge, but only at two thirds of its normal output.  Since an alternator is designed to handle all the power that is needed under heavy load conditions, you may never know that there is a problem with the unit.  It might only become apparent on a dark, cold rainy night when the lights, heater, windshield wipers and possible the seat heaters and rear defroster are all on at once that you may notice the lights start to dim as you slow down.  If two sets of windings failed, you will probably notice it a lot sooner.

It is more common for one or more of the six diodes in the rectifier to fail.  If a diode burns out and opens one of the circuits, you would see the same problem as if one of the windings had failed.  The alternator will run at a reduced output.  However, if one of the diodes were to short out and allow current to pass in either direction, other problems will occur.  A shorted diode will allow AC current to pass through to the automobile's electrical system which can cause problems with the computerized sensors and processors.  This condition can cause the car to act unpredictably and cause all kinds of problems.

Too much voltage
A voltage regulator is designed to limit the voltage output of an alternator to 14.5 volts or less to protect the vehicle's electrical system.  If the regulator malfunctions and allows uncontrolled voltage to be released, you will see bulbs and other electrical components begin to fail.  This is a dangerous and potentially costly problem.  Fortunately, this type of failure is very rare.  Most failures cause a reduction of voltage or amperage.

Noise
Since the rotor is always spinning while the engine is running, there needs to be bearings to support the shaft and allow it to spin freely.  If one of those bearings were to fail, you will hear a grinding noise coming from the alternator.  A mechanic's stethoscope can be used to confirm which of the spinning components driven by the serpentine belt is making the noise.

Repairing Charging System Problems

The most common repair is the replacement of the alternator with a new or rebuilt one.  A properly rebuilt alternator is as good as a new alternator and can cost hundreds less than purchasing a brand new one.

Labor time to replace an alternator is typically under an hour unless your alternator is in a hard to access location.  Most alternators are easily accessible and visible on the top of the engine.

Replacing an alternator is usually an easy task for a backyard mechanic and rebuilt alternators are readily available for most vehicles at the local auto parts store.  The most important task for the do-it-yourselfer is to be careful not to short anything out.  ALWAYS DISCOnnECT THE BATTERY BEFORE REPLACING AN ALTERNATOR.

Alternators can be repaired by a knowledgeable technician, but in most cases, it is not economical to do this.  Also, since the rest of the alternator is not touched, a repair job is usually not guaranteed.

In some cases, if the problem is diagnosed as a bad voltage regulator, the regulator can be replaced without springing for a complete rebuild.  The problem with this is that there will be an extra labor charge for disassembling the alternator in order to get to the internal regulator.  That extra cost, along with the cost of the replacement regulator, will bring the total cost close to the cost of a complete (and guaranteed) rebuilt.

This is not the case when the regulator is not inside the alternator.  In those cases, the usual practice is to just replace the part that is bad.