The 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 three functions:

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 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 key-on the 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.

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.

Body Control Module (BCM)

The BCM is a class 2 device. It communicates with the engine control module (ECM) / powertrain control module (PCM) and the instrument panel cluster for EPM operation. The BCM determines the output of the generator and sends the information to the ECM/PCM for control of the generator field control circuit. It monitors the generator field duty cycle signal circuit information sent from the ECM/PCM for control of the generator. It monitors a battery current sensor, the battery positive voltage circuit, and estimated battery temperature to determine battery SOC. The BCM 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 BCM. It creates a 5 volt PWM signal of 128 Hz with a duty cycle of 0-100%. Normal duty cycle is between 5-95%. Between 0-5% and 95-100% 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. The ECM/PCM will over ride the BCM control of the generator when one of the following conditions are met:

Instrument Panel Cluster (IPC)

The IPC provides a means of customer notification in case of a failure and a voltmeter. There is two means of notification, a charge indicator and a driver information center message of SERVICE CHARGING SYSTEM and CHARGING SYSTEM FAULT.

The purpose of the charging system is to maintain the battery charge and vehicle loads. There are 6 modes of operation and they include:

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 PWM signal of 128 Hz with a duty cycle of 0-100%. Normal duty cycle is between 5-95%. Between 0-5% and 95-100% are for diagnostic purposes. The following table shows the commanded duty cycle and output voltage of the generator:

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%. Normal duty cycle is between 5-99%. Between 0-5% and 100% are for diagnostic purposes.

Charge Mode

The BCM will enter Charge Mode when ever one of the following conditions are met:

Once one of these conditions are met, the generator battery control module will set the targeted generator output voltage to 13.4 volts and then ramp that voltage up to 14.5 volts at a rate of 50 mV per second.

Fuel Economy Mode

The BCM will enter Fuel Economy Mode when the calculated ambient air 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%. Its targeted generator output voltage is 13.0 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 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%. Its targeted generator output voltage is 12.9 volts. The BCM will exit this mode once the criteria are met for Charge Mode.

Start Up Mode

After the engine has started the BCM sets a targeted generator output voltage of 14.5 volts for 20 seconds.

Windshield Deice Mode

After the engine has run for more than 10 seconds the BCM sets a targeted generator output voltage of 13.8 volts if the calculated ambient air temperature is less that 0°C (32°F). The BCM will stay in this mode until the engine coolant temperature reaches 75°C (167°F) for 10 minutes.

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 set a targeted output voltage of 13.8 volts for 5 minutes. The BCM will then determine which mode to enter depending on voltage requirements.

Charge Indicator Operation

The IPC illuminates the charge indicator in the message center when the one or more of the following occurs:

Charging System Failure

The BCM and 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 for load shed 2 setting criteria.