Electrical Power Management (EPM) Overview
The electrical power management (EPM) system is designed to monitor and control the charging system and send diagnostic messages to alert the driver of possible problems with the battery and generator. This EPM system primarily utilizes existing on-board computer capability to maximize the effectiveness of the generator, to manage the load, improve battery state-of-charge and life, and minimize the system's impact on fuel economy. The EPM system performs 3 functions:
| • | It monitors the battery voltage and estimates the battery condition. |
| • | It takes corrective actions by boosting idle speeds, managing the loads, and adjusting the regulated voltage. |
| • | It performs diagnostics and driver notification. |
The battery condition is estimated during ignition-off and during ignition-on. During ignition-off the state-of-charge (SOC) of the battery is determined by measuring the open-circuit voltage. The SOC is a function of the acid concentration and the internal resistance of the battery, and is estimated by reading the battery open circuit voltage when the battery has been at rest for several hours.
The SOC can be used as a diagnostic tool to tell the customer or the dealer the condition of the battery. Throughout ignition-on, the algorithm continuously estimates SOC based on adjusted net amp hours, battery capacity, initial SOC, and temperature.
While running, the battery degree of discharge is primarily determined by a battery current sensor, which is integrated to obtain net amp hours.
In addition, the EPM function is designed to perform regulated voltage control (RVC) to improve battery SOC, battery life, and fuel economy. This is accomplished by using knowledge of the battery SOC and temperature to set the charging voltage to an optimum battery voltage level for recharging without detriment to battery life.
The Charging System Description and Operation is divided into 3 sections. The first section describes the charging system components and their integration into the EPM. The second section describes charging system operation. The third section describes the instrument panel cluster (IPC) operation of the charge indicator, driver information center (DIC) messages, and voltmeter operation.
Charging System Components
Generator
The generator is a serviceable component. If there is a diagnosed failure of the generator it must be replaced as an assembly. The engine drive belt drives the generator. When the rotor is spun it induces an alternating current (AC) into the stator windings. The AC voltage is then sent through a series of diodes for rectification. The rectified voltage has been converted into a direct current (DC) for use by the vehicles electrical system to maintain electrical loads and the battery charge. The voltage regulator integral to the generator controls the output of the generator. It is not serviceable. The voltage regulator controls the amount of current provided to the rotor. If the generator has field control circuit failure, the generator defaults to an output voltage of 13.8 volts.
Instrument Panel Module (IPM)
The instrument panel module (IPM) is a GMLAN device. It communicates with the engine control module (ECM) and the instrument panel cluster (IPC) for EPM operation. The IPM determines the output of the generator and sends the information to the ECM for control of the generator field control circuit. It monitors the generator field duty cycle signal circuit information sent from the ECM for control of the generator. It monitors a battery current sensor, the battery positive voltage circuit, and estimated battery temperature to determine battery state of charge (SOC). The IPM performs idle boost and load management operations.
Battery Current Sensor
The battery current sensor is a serviceable component that is connected to the negative battery cable at the battery. The battery current sensor is a 3-wire hall effect current sensor. The battery current sensor monitors the battery current. It directly inputs to the IPM. It creates a 10-volt pulse width modulation (PWM) signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-95 percent. Between 0-5 percent and 95-100 percent are for diagnostic purposes.
Engine Control Module (ECM)
The ECM directly controls the generator field control circuit input to the generator. The ECM receives control decisions based on messages from the IPM. It monitors the generators generator field duty cycle signal circuit and sends the information to the IPM.
Instrument Panel Cluster (IPC)
The IPC provides a means of customer notification in case of a failure and a voltmeter. There are 2 means of notification, a charge indicator and a driver information center (DIC) message of BATTERY VOLTAGE , BATTERY VOLTAGE HIGH, and BATTERY VOLTAGE LOW.
Charging System Operation
The purpose of the charging system is to maintain the battery charge and vehicle loads. There are 2 modes of operation and they include:
| • | Charge Mode |
| • | Voltage Reduction Mode |
The engine control module (ECM) controls the generator through the generator field control circuit. It monitors the generator performance though the generator field duty cycle signal circuit. The ECM controls the generator through the generator field control circuit. The signal is a 10-volt pulse width modulation (PWM) signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-95 percent. Between 0-5 percent and 95-100 percent are for diagnostic purposes. The following table shows the commanded duty cycle and output voltage of the generator:
Commanded Duty Cycle | Generator Output Voltage |
|---|---|
10% | 11 V |
20% | 11.56 V |
30% | 12.12 V |
40% | 12.68 V |
50% | 13.25 V |
60% | 13.81 V |
70% | 14.37 V |
80% | 14.94 V |
90% | 15.5 V |
The generator provides a feedback signal of the generator voltage output through the generator field duty cycle signal circuit to the ECM. This information is sent to the instrument panel module (IPM). The signal is a 5-volt PWM signal of 128 Hz with a duty cycle of 0-100 percent. Normal duty cycle is between 5-99 percent. Between 0-5 percent and 100 percent are for diagnostic purposes.
Charge Mode
The IPM will enter Charge Mode when the headlamps are ON, low or high beams. The voltage is controlled to 13.4 volts if the system is below 13.3 volts or to 14.5 volts if the system voltage is above 14.6 volts.
Voltage Reduction Mode
The IPM will enter Voltage Reduction Mode when the calculated ambient air temperature is above 0°C (32°F). The calculated battery current is less than 2 amperes and greater than -7 amperes, and the generator field duty cycle is less than 99 percent. Its targeted generator output voltage is 12.9 volts. The IPM will exit this mode once the criteria are met for Charge Mode.
Instrument Panel Cluster (IPC) Operation
Charge Indicator Operation
The instrument panel cluster (IPC) illuminates the charge indicator and displays a warning message in the driver information center (DIC) when the one or more of the following occurs:
| • | The engine control module (ECM) detects that the generator output is less than 11 volts or greater than 16 volts. The IPC receives a GMLAN message from the ECM requesting illumination. |
| • | The IPC determines that the system voltage is less than 11 volts or greater than 16 volts for more than 30 seconds. The IPC receives a GMLAN message from the instrument panel module (IPM) indicating there is a system voltage range concern. |
| • | The IPC performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds. |
| • | The ignition is ON, with the engine OFF. |
Battery Voltage
The IPC displays the system voltage as received from the IPM over the GMLAN serial data circuit. If there is no communication with the IPM then the display will read all dashes until communication is restored.
Battery Voltage High
The IPM and the ECM will send a GMLAN message to the DIC for the BATTERY VOLTAGE HIGH message to be displayed. It is commanded ON when a charging system DTC is a current DTC. The message is turned OFF when the conditions for clearing the DTC have been met.
Battery Voltage Low
The IPM and the ECM will send a GMLAN message to the DIC for the BATTERY VOLTAGE LOW message to be displayed. It is commanded ON when a charging system DTC is a current DTC or when a low voltage input to the IPM is present. The message is turned OFF when the conditions for clearing the DTC have been met or the low voltage condition for the IPM has been repaired.
Battery Saver Active
The BATTERY SAVER ACTIVE message will display on the DIC when the vehicle enters a load shed 2 event or a load shed 3 event. Refer to Load Shed System Description and Operation for load shed 2 or 3 setting criteria.
