Charging Systems

Charging System Basics:
The electrical system in an automobile is said to be a 12 volt system, but this is slightly misleading. The charging system in most cars will generally produce a voltage between 13.5 and 14.4 volts while the engine is running. It has to generate more voltage than the battery's rated voltage to overcome the internal resistance of the battery. This may seem strange, but the current needed to recharge the battery would not flow at all if the charging system's output voltage was the same as the battery voltage. A greater difference of potential (voltage) between the battery's voltage and the alternator's output voltage will provide a faster charging rate.

As long as the engine is running, all of the power for the accessories is delivered by the alternator. The battery is actually a load on the charging system. The only time that the battery would supply power with the engine running is when the current capacity of the alternator is exceeded or when engine is at a very low idle.

lternator Basics

Overview:
A basic alternator has 2 main electrical components. The rotor and the stator. The rotor is the part of the alternator that is spun by the drive belt. There are a group of electrical field coils mounted on the rotor. The stator is the group of stationary coils that line the perimeter of the inside of the alternator case. When current (supplied by the voltage regulator - to be explained later) is flowing in the rotor's coils, they induce current flow in the stationary coils. The induced current (and voltage) is an AC current. To convert this to DC, the current is passed through a bridge rectifier.

Stator and Rotor in Action:
In the following diagram, you can see three crudely drawn sets of rotors and stators. In the leftmost diagram (marked 'A'), you can see the rotor's coil approaching the stator coil. As the rotor coil approaches the stator coil, it induces current flow in the stator coils. This causes an increase in output voltage. As it approaches the position where the coils' centers are aligned ('B'), there is no induced current. When the coils move away from each other ('C') the induced current flows in the opposite direction and the generated voltage is negative.

Rectification:
You should have noticed that the generated voltage was AC. You already know that a vehicle's charging system needs to produce DC to recharge the battery. This is done with diodes. The following diagram shows a simple transformer and a bridge rectifier. The transformer is driven with a sine wave (similar to that produced in each stator coil). Since the transformer is driven with a sine wave, the output of the transformer is a sine wave (similar to the one shown). The sine wave is driven into the bridge rectifier and the output is a pulsed DC waveform.

Bridge rectifier:
You should also realize that there are 3 different groups of stator coils in an alternator (not shown in diagrams). The rectification is much like the simple transformer shown above but in the place of a single transformer winding there are 3 windings. It also uses 6 diodes instead of 4.

3 Phase:
The following diagram shows the 3 different phases from the 3 groups of stator windings. The three phases of AC are shown in three different colors. The next set of lines shows the rectified waveforms overlapped. The bottom waveform (white line) is what the rectified voltage would actually look like if viewed on an oscilloscope. Connecting the battery to the alternator will smooth the white line even more.

Alternator Schematic:
The following is a generic schematic showing the stator windings and the bridge rectifier. You also see a diode trio. the diode trio takes part of the output and sends it to the voltage regulator. The output diodes are the rectifiers that rectify the AC and supply power to your electrical accessories.


Brushes and Slip Rings:
For an alternator to produce electrical current, there needs to be some excitation current flowing in the rotor windings. Since the rotor is spinning, you can't just connect a couple of wires to it (cause they'd just be twisted off :-). To make the electrical connection, slip rings and brushes are used. The slip rings are fixed to the shaft of the rotor. The brushes are fixed to the stationary part of the alternator. The brushes, which are generally made of carbon, are spring loaded to keep constant pressure on the slip rings as the brushes wear down. The following diagram shows the general location of the rotor and the associated parts.
Voltage Reguation:
As you already know from the 'wire' page, all wire has resistance. You also know that when you have current flow through a resistive element (wire), there will be a voltage loss. If the current draw from the charging system was constant, there would be no need for a voltage regulator. If there was no loss, the design engineer would simply design the alternator to produce a given voltage. This won't work with a car audio system because the current draw is anything but constant. This means that the alternator needs a compensating voltage regulator. The voltage regulator controls the flow of current in the rotor's windings. The voltage regulator's output current will typically be between 0 amps (with little or no current draw) and 5 amps (at maximum current draw). The regulator can vary the current flow infinitely to keep the voltage precisely at the target voltage. Generally the regulator is built into the alternator. There are some high current/special use alternators which may have external regulators. Some of the external regulators are adjustable via a potentiometer.

Current demand and flow:
If you have an alternator that can produce 120 amps of current (max) and the the total current demand from the electrical accessories (including the battery) is only 20 amps, the alternator will only produce the necessary current (20 amps) to maintain the target voltage (which is determined by the alternator's internal voltage regulator). Remember that the alternator monitors the electrical system's voltage. If the voltage starts to fall below the target voltage (approximately 13.8 volts depending on the alternator's design), the alternator produces more current to keep the voltage up. When the demand for current is low, the full current capacity of the alternator is not used/produced (a 120 amp alternator does not continuously produce 120 amps unless there is a sufficient current draw).

Dimming lights:
When you play your system at very high volumes and the lights on your vehicle dim slightly, it generally means that your alternator can not supply enough current for all of your electrical accessories (including your amplifiers). If you play a long bass note/tone and the lights get dim and stay dim until the note is over, your alternator clearly can not keep up with the current demand. If, on a long bass note, the lights dim just for a fraction of a second but return to their original brightness while the note/tone is still playing, the alternator's regulator may just be a little slow in reacting to the voltage drop. Since the lights return to their original brightness during the bass note, the alternator is able to supply the current needed by your power your amplifiers and other electrical accessories.