The charging system uses the alternator output to keep the battery charged at a constant level under various electrical loads. Rotation of the excited field coil generates AC voltage in the stator. This alternating current is rectified through diodes to DC voltage having a waveform shown in the illustration at left. The average output voltage fluctuates slightly with the alternator load condition. The following schematic shows the Mitsubishi Space Wagon 4G9 Charging System Wiring Diagram.
When the ignition switch is turned on, current flows in the field coil and initial excitation of the field coil occurs. When the stator coil begins to generate power after the engine is started, the field coil is excited by the output current of the stator coil.
The alternator output voltage rises as the field current increases and it falls as the field current decreases. When the battery voltage (alternator S terminal voltage) reaches a regulated voltage of approx. 14.4 V, the field current is cut off. When the battery voltage drops below the regulated voltage, the voltage regulator regulates the output voltage to a constant level by controlling the field current. In addition, when the field current is constant, the alternator output voltage rises as the engine speed increases.