Purpose of the Charging System
The purpose of the charging system is to convert the mechanical energy of the engine into electrical energy that is used to recharge the battery and power the electrical accessories. When the engine is first started, the battery(s) supplies all the current required by the starting and ignition systems.
As the battery drain continues and engine speed increases the charging system is able to produce more voltage than the battery can deliver. When this occurs, the electrons from the charging device are able to flow in a reverse direction through the battery's positive terminal. The charging device now supplies the electrical system's load requirements and recharges the battery.
The charging system consists of
- Battery
- Generator
- Drive Belt
- Rectifier Assembly
- Voltage Regulator
- Charge Indicator
- Ignition Switch
- Cables and Wiring Harness
Scheme 14
Battery
The Battery is the primary EMF source in the automobile. The automotive battery is an electro-mechanical device that provides the potential difference (voltage). The battery does not store electrical energy. It stores chemical energy that is converted to electrical energy as it discharges.
Generator
The Generator produces free electrons necessary to charge the battery. The electron flow is produced through inductance, a magnetized rotor spinning inside a stator. The generator produces AC voltage which is converted to DC voltage or rectified.
The MINI uses an air cooled Brush Type generator.
Note. In an attempt to standardize terminology in the industry, the term alternator is being replaced with generator. Often an alternator is referred to as an AC generator.
The Brush Type generator consists of the following main components
- Generator Housing
- Stator Assembly
- Rotor Assembly
Generator Housing
The Housing is made of two pieces of die-cast aluminum. Aluminum is used because it is non-magnetic, light weight and provides good heat dissipation.
Bearings for support of the rotor assembly are mounted in the front and rear housings.
Drive Belt
It is the function of the Drive Belt to transfer rotating energy from the engine to the generator. The drive belt rotates the rotor, spinning the magnetic field. A loose belt can inhibit charging system efficiency and a belt that too tight causes early bearing failure.
Voltage Regulator - Multifunction Controller
The electronic regulator prevents excessively high voltage output of the generator. Excessive voltage would cause damage to the battery (through overcharging), light bulbs, motors, and particularly sensitive electronic components.
The voltage regulators is mounted internally towards the back of the generator assembly. It prevents damage to components by varying the amount of time the field coil is energized. The duty cycle of the field coil ground side is varied based on the demand placed on the electrical system.
The electronic voltage regulator compares field current supply voltage (from the stator windings through the diode trio) against a set voltage level (using a zener diode).
As the field current supply voltage surpasses the zener diode's breakdown voltage, the field current to the rotor is switched off. When the field current voltage to the rotor is off, the generator is not producing voltage. A rapid switching of the field current allows a fixed voltage output to be maintained. Additional diodes in the regulator, prevent current flow when the ignition is off preventing battery drain.
Generator Load Signal
This "sensor" is a special output of the generator that determines the level of electrical load. The alternator must generate more current when electrical loads such as headlights, rear-window defroster are activated. In doing so, the alternator creates a greater load on the engine.
The signal from the generator is pulse width modulated (PWM). The duty cycle is variable between 0% and 100% and is proportional to the generator load. However, the signal is too low to compensate for the disturbances that occur at idle speed. The signal enables adjustment of the idle speed.
Charge Indicator
The purpose of the Charge Indicator is to advise the driver that the vehicle's electrical system is not operating at peak efficiency and service should be performed.
The charge indicator operates on the basis of opposing voltages. If there is no output through the diode trio, then the lamp circuit is completed to ground through the rotor field.
Diode output applies voltage to the previously grounded side of the bulb, turning the bulb off (No current flow with equal voltage on both sides of the bulb).
Scheme 15
Ignition Switch
The Ignition Switch provides initial power for the field circuit of the alternator, reducing the time required for the field to develop the magnetic field. Depending on which type of regulator is employed the ignition switch supplies power to the charge indicator to check bulb integrity.
Voltage Rectification
The battery and the electrical system cannot store or use the 3-phase AC voltage produced by a generator, it must be rectified or converted to DC voltage.
A diode rectifier bridge is used to make the conversion.
The diode is similar to a non-return or one way valve which permits the passage of a fluid or gas in only one direction.
In a simple conversion the rectifier diode suppresses the negative half waves and allows only positive half waves to pass.
To make use of the negative value half waves full rectification is applied.
Full rectification of the negative half waves invert them into positive half waves.
The result is a rectified pulsating direct current.
Scheme 16
Scheme 17
Three-Phase Voltage Rectification
Six diodes are used to achieve three-phase AC voltage rectification. Three diodes are positive biased and three are negative biased.
The positive half-waves pass through the positive biased diodes and the negative half-waves through the negative biased diodes.
Diode rectification of the negative half-waves invert them into positive half-waves.
With full rectification DC voltage supplied to vehicle by generator is not ideally smooth, but exhibits a slight ripple. This ripple is further smoothed by the battery which is connected in parallel with the generator.
The rectifier diodes in the generator not only convert the current but also prevent battery discharging through the 3 phase windings of the stator. Current flow can only take place from the generator to the battery.
Scheme 18
Scheme 19
Scheme 20
Voltage Regulation
The Electronic Regulator uses a zener diode that blocks current flow until a specified voltage is obtained.
Sensing current from terminal 2 passes through a thermistor to the zener diode (D2). As the system voltage exceeds the breakdown voltage of the zener diode, current flows through the zener diode turning transistor 2 (TR2) on.
With TR2 on transistor 1 (TR1) is shut off.
Transistor 1 controls field current to the rotor.
With TR1 off no current flows to the field coil and the generator has no output.
A voltage drop below the breakdown voltage of the zener diode stops the current flow to TR2 which turns on TR1.
Voltage is again applied to the field allowing the generator to produce voltage.
Scheme 21
Scheme 22
Charge Indicator System
The Charging System Indicator light operates on the principle of opposing voltage. Battery voltage is supplied to one side of the light bulb, the other side of the bulb is connected to the voltage regulator.
With the key turned on, power is sent to the light bulb, through to the regulator. No voltage is being produced by the stator, so there is no voltage from the diode trio. This lack of voltage from the diode trio, allows the voltage from the ignition switch to flow through the regulator to ground. This completes the circuit allowing current to allow the charge indicator bulb to illuminate.
As the generator begins to produce voltage, the output of the diode trio equals battery voltage. This equal voltage is supplied to the light bulb. With equal voltage on each side of the light bulb, no current can flow and the light is turned off.
Note. For charging system wiring diagram, see CHARGING CIRCUIT in WIRING DIAGRAMS.
Workshop Hints
- Harmonic Content - This is an expression of AC voltage contained in DC voltage. The higher the percentage of harmonic content the larger the amount of residual AC voltage in the DC voltage. High residual AC voltage is caused by weak or failing diodes in the generator. Generators which have harmonic content readings higher than specified (In TIS) should be replaced.
- Before beginning diagnostic procedures on the charging system do the following: Run engine at idle speed for about 5 minutes Switch off all electrical loads This is done to ensure the battery is charged to such a level that the generator will not be fully utilized and that loads required during starting (e.g. starter, secondary air pump) will be already switched off.