Charging System Basics

As electrons start to flow through a circuit, a chemical reaction in the battery (explained on the electricity basics page) starts to drain the battery of electrical potential. The engines alternator is an electron pump which uses the engines rotational motion to create voltage. Many people describe the alternator as just recharging the battery. The alternator recharges the battery after the engine starts, when the battery is at its lowest. The starter takes a lot out of the battery, this needs to be made up for so the engine can start again next time. Many people don’t know that once the engine is running, the alternator becomes the primary source of electricity for the vehicle. Once the engine is running, you can remove the battery from the vehicle completely and the engine will continue to run, the radio will continue to play, but don’t expect the engine to start again once it is shut off. (note: feel free to try this but beware that the battery also absorbs voltage spikes and modern vehicles have many sensitive modules. If the alternator does spike for any reason, any one or all of the modules can be damaged.) The battery stores voltage to start the engine, but once the engine is running the alternator becomes “the source.” This is because the battery produces about 12.6V and the alternator produces 14+V. When the engine is running, 14+V is present at the battery because of the alternator. The alternator has a wire which runs from the alternator’s B+ terminal to the battery positive terminal. Since the alternator’s case will be bolted to the block, or in some way connected to the engine, that is how it is connected to ground (the battery negative terminal).


When the alternator is not charging, (eg. engine not running) all the electrical power for the vehicle comes from the battery. Electricity will flow (conventional theory) from the positive side of the battery, through the circuit and any switches/control devices to the load. From there it will travel to a frame ground which is connected to the battery negative terminal. Once electricity has flowed back into the battery, the battery’s job is to take that energy, and put it back into the positive side of the battery. This process consumes an amount of the battery’s electrical potential by changing the battery’s internal chemistry which means that this process cannot continue indefinitely.

When the alternator is charging, it is producing 14+V as opposed to the battery’s 12.6V. After electrical flow has passed through the load, it goes to the place that is “most negative,” which will be the alternators case and not the battery negative terminal. The alternator will then “pump” electrical current, directly to the positive battery terminal. Since the battery runs at 12.6V (it will be less after the starter has been activated) and the alternator produces 14+V, some current will flow from the 14+V alternator into the 12.6V battery, recharging it and converting the battery’s internal chemistry back to that of a fully charged battery. It also saves the battery the work of putting electrical potential back to the positive side of the battery. Since the battery positive terminal is also connected to the rest of the vehicles electrical system, the alternator becomes the primary source of electrial power for the vehicle.

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The Alternator

alternatorThe alternator consists of 3 main components, the stator with diodes, the rotor and the voltage regulator. Remember, to induce a voltage we need a magnetic field and a conductor moving through the field.







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The Rotor

alternator rotorThe rotor is used to create the rotating magnetic field necessary for charging. It consists of a coil of wire with an iron core, two pole pieces, a shaft and slip rings. The shaft transmits rotational movement from the drive pulley to the rotor, it also supports the slip rings. The slip rings allow voltage to be supplied to the coil of wire through the brushes of the voltage regulator. When current runs through the coil of wire an electromagnetic field is created. The pole pieces create a north-south-north-south magnetic field around the outside of the rotor assembly. This magnetic field becomes the “electron pump.”




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The Stator

alternator statorThe stator is the conductor that has voltage induced into it by the rotor. The stator is just coils of wire attached to eyelets at each end. The electrons in the stator get pushed around by the electromagnetic field created by the rotor. There are usually 3 separate coils of wire in a stator. Stators can be wound 2 different ways. A “wye” wound stator, if we took the stator apart and stretched out the wires, would look like a big “Y” or “wye” shape with a “neutral junction” in the middle. This type of winding is good for making high voltage at low engine RPM. The other type of winding is the “delta” wound stator, which if stretched out, would look like a triangle with two wires going to each eyelet. This type of stator is good for flowing higher amps at high engine speeds but has not as good of an output at low engine RPM. As the electromagnetic field passes through the stator, its electrons are pushed through one of the coils of wire to an eyelet.

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alternator diode operationThe current that comes out of the stator is AC current which flows back and forth instead of one direction like DC current. This is because, current can travel through any of the windings, in either direction depending on how the magnetic field interacts with a circuit in the stator, at any given time. AC current must be rectified to DC current because AC power cannot be stored, it needs to be used immediately or it is lost. To make this change happen, the alternator uses 6 diodes. A diode allows current to flow only one way. Current can be pulled in by the stator through its negative diode. It will then push current out another eyelet and through its positive diode and to the B+ terminal. This can happen in any direction, from any eyelet to any other eyelet depending on the magnetic field the rotor is exposing the stator to.

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Voltage Regulator

alternator voltage regulatorAs you may have guessed, the voltage regulator regulates system voltage. It does this by controlling the current flow through the rotor. The voltage regulator’s brushes ride on the rotors slip rings, this is how the voltage regulator supplies current to the rotor. The more current that flows through the rotor, the more powerful of an electromagnet is produced and the more voltage is induced into the stator. Without proper regulation, an alternator can produce 200+V, frying the electrical system. There are two main types of regulators; an A-type and a B-type. An A-type regulator controls current flow on the ground side of the rotor. A B-type regulator controls current flow on the power side. Modern regulators work with transistors which pulse current on and off many times per second to regulate alternator output. The alternator charges until the system reaches a set voltage, at this point current flow is to the rotor is cut off. When system voltage reaches a set low, current flow to the rotor is restored and the alternator charges. This cycle repeats many times per second resulting in a PWM (pulse width modulation) type signal, which results in a constant system voltage. Some newer systems may have voltage regulation controlled by the PCM.

When an alternator does not charge, the charge light will come on. Since the alternator needs a small amount of electrical power to make more power, many systems will run a circuit where current has to travel through the charge light to power the rotor. Once the alternator starts producing voltage (usually within a second), the alternator can use its own current to run itself. At this point, current stops flowing through the charge light because there is equal voltage on both sides of the light and the light goes out. If the alternator does not produce voltage, the light will remain illuminated. Many newer systems will simply have the PCM control the charge light.

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Alternators get hot and need to be cooled. Most light to medium duty alternators will have a fan, located inside the alternator, attached to the rotors shaft (see the rotor picture). Older alternators used to have the fan located on the outside of the case. These were a common place for people to get hurt and is why they were phased out. Heavy duty alternators can be liquid cooled, they have engine coolant running through them to provide cooling.

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