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 (SOC) and life, and minimize the system's impact on fuel economy. The EPM system performs 3 basic functions:
| • | It monitors the battery voltage and estimates the battery condition. |
| • | It takes corrective actions by adjusting the regulated voltage. |
| • | It performs diagnostics and driver notification. |
The battery's condition is estimated during key-off and during key-on. During key-off the SOC of the battery is determined by measuring the open-circuit voltage. The SOC is a percentage number, with 100 percent being fully charged and 0 percent being fully discharged. A normal SOC value is typically greater than 80 percent.
The SOC can be used as a diagnostic tool to tell the customer or the dealer the condition of the battery. Throughout key-on operation, the EPM software (algorithm) continuously estimates SOC based on adjusted net amp hours, battery capacity, initial SOC, and temperature.
While running, the battery's 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's 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 operation of the charge indicator, driver information center 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 generator's output voltage by controlling the amount of current provided to the rotor. The generator on this vehicle has RVC, which means that the EPM software determines what the regulated voltage should be, not the generator. The output voltage command comes to the generator from the engine control module (ECM) / powertrain control module (PCM) by way of the "L" terminal. If that signal is lost, the generator defaults to an output voltage of 13.8 volts.
Body Control Module (BCM)
The body control module (BCM) is a class 2 device utilizing digital serial data links with other control modules. The BCM contains the EPM software and communicates with the ECM/PCM and the instrument panel cluster for EPM operation. The BCM determines the desired output voltage of the generator and sends the voltage command to the ECM/PCM for transmittal via the "L" terminal line to the regulator. The BCM also monitors the generator field duty cycle signal (F terminal) sent via the ECM/PCM. The BCM monitors the battery current sensor, the battery positive voltage, and estimated battery temperature to determine SOC. The BCM commands idle boost and load management operations, whenever battery discharge warrants it.
Battery Current Sensor
The battery current sensor is a serviceable component that is assembled on 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 BCM. It creates a 5-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) / Powertrain Control Module (PCM)
The ECM/PCM directly controls the generator field control circuit input to the generator. It monitors the generators generator field duty cycle signal circuit and sends the information to the BCM.
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 message of SERVICE CHARGING SYSTEM and CHARGING SYSTEM FAULT.
Charging System Operation
The purpose of the charging system is to maintain the battery charge and vehicle loads. There are 5 modes of operation and they include:
| • | Charge Mode |
| • | Fuel Economy Mode |
| • | Voltage Reduction Mode |
| • | Start Up Mode |
| • | Battery Sulfation Mode |
The ECM/PCM controls the generator through the generator field control circuit. It monitors the generator performance through the generator field duty cycle signal circuit. The signal is a 5-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/PCM, this information is sent to the BCM. 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 BCM will enter Charge Mode when ever one of the following conditions are met:
| • | The headlamps are ON, low or high beam. |
| • | The wipers are ON for more than 3 seconds. |
| • | The electric cooling fans are on high speed for more than 1 minute and either fluid head pressure is greater than 1600 kpa (232 PSI) or engine coolant temperature is greater than 105°C (221°F). |
| • | The rear defogger is ON for more than 1 minute. |
| • | The HVAC front blowers are on high speed for more than 1 minute and estimated battery temperature is greater than 32°C (89°F). |
| • | The estimated battery temperature is less than 0°C (32°F). |
| • | Battery SOC is less than 80 percent. |
Once one of these conditions is met, the generator battery control module will set the targeted generator output voltage to a charging voltage between 13.9 volts and 15.5 volts, depending on battery SOC and estimated battery temperature.
Fuel Economy Mode
The BCM will enter Fuel Economy Mode when the estimated battery temperature is above 0°C (32°F), the calculated battery current is less than 15 amperes and greater than -8 amperes, and the battery SOC is greater than 80 percent. Its targeted generator output voltage is the open circuit voltage of the battery and can be between 12.56 volts and 13.2 volts. The BCM will exit this mode once the criteria are met for Charge Mode.
Voltage Reduction Mode
The BCM will enter Voltage Reduction Mode when the estimated battery temperature is above 0°C (32°F), the calculated battery current is less than 1 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 BCM will exit this mode once the criteria are met for Charge Mode. If the conditions for both Fuel Economy Mode and Voltage Reduction Mode are met at the same time, Fuel Economy Mode has priority.
Start Up Mode
After the engine has started the BCM sets a targeted generator output voltage of 14.5 volts for 30 seconds.
Battery Sulfation Mode
The BCM will enter this mode when the interpreted generator output voltage is less than 13.2 volts for 45 minutes. Once in this mode the BCM will enter Charge Mode. The BCM will then determine which mode to enter depending on voltage requirements.
Instrument Panel Cluster (IPC) Operation
Charge Indicator Operation
The instrument panel cluster (IPC) illuminates the charge indicator in the message center when the one or more of the following occurs:
| • | The ECM/PCM detects that the generator output is less than 11 volts or greater than 16 volts. The IPC receives a class 2 message from the ECM/PCM requesting illumination. |
| • | The IPC determines that the system voltage is less than 11.3 volts or greater than 16.2 volts for more than 30 seconds. The IPC receives a class 2 message from the body control module (BCM) 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. |
Charging System Failure
The ECM/PCM will send a class 2 message to the IPC for the CHARGING SYSTEM FAILURE 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
The IPC displays the system voltage as received from the BCM over the class 2 serial data circuit. If there is no communication with the BCM, then the display will read all dashes until communication is restored.
Battery Saver Active
The BATTERY SAVER ACTIVE message will display on the DIC when the vehicle enters a load shed 2 event. Refer to Load Shed System Description and Operation for load shed 2 criteria.
