Charging System Description and Operation
Electrical Power Management Overview
The electrical power management 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 electrical power management 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 electrical power management system performs 3
||It monitors the battery
voltage and estimates the battery condition.
||It takes corrective actions by
boosting idle speeds, and adjusting the regulated voltage.
||It performs diagnostics and
The battery condition is estimated during ignition-off and
during ignition-on. During ignition-off the state-of-charge of the
battery is determined by measuring the open-circuit voltage. The
state-of-charge 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
The state-of-charge 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 state-of-charge
based on adjusted net amp hours, battery capacity, initial
state-of-charge, 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 electrical power management function is
designed to perform regulated voltage control to improve battery
state-of-charge, battery life, and fuel economy. This is
accomplished by using knowledge of the battery state-of-charge 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 electrical power
management. 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
Charging System Components
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 V.
Body Control Module (BCM)
The body control module (BCM) is a GMLAN device. It communicates
with the engine control module (ECM) and the instrument panel
cluster for electrical power management (electrical power
management) operation. The BCM determines the output of the
generator and sends the information to the ECM for control of the
generator turn on signal 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. The BCM performs idle boost.
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 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)
When the engine is running, the generator turn-on signal is sent to
the generator from the ECM, turning on the regulator. The
generator's voltage regulator controls current to the rotor,
thereby controlling the output voltage. The rotor current is
proportional to the electrical pulse width supplied by the
regulator. When the engine is started, the regulator senses
generator rotation by detecting AC voltage at the stator through an
internal wire. Once the engine is running, the regulator varies the
field current by controlling the pulse width. This regulates the
generator output voltage for proper battery charging and electrical
system operation. The generator field duty terminal is connected
internally to the voltage regulator and externally to the ECM. When
the voltage regulator detects a charging system problem, it grounds
this circuit to signal the ECM that a problem exists. The ECM
monitors the generator field duty cycle signal circuit, and
receives control decisions based on information from the BCM.
Instrument Panel Cluster
The instrument panel cluster provides the customer notification in
case a concern with the charging system. There are 2 means of
notification, a charge indicator and a driver information center
message of SERVICE BATTERY CHARGING SYSTEM if equipped.
Charging System Operation
The purpose of the charging system is to maintain the battery
charge and vehicle loads. There are 6 modes of operation and they
The engine control module (ECM) controls the generator through
the generator turn ON signal circuit. The ECM monitors the
generator performance though the generator field duty cycle signal
circuit. The signal is a 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
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 body control
module (BCM). The signal is 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
Battery Sulfation Mode
The BCM will enter this mode when the interpreted generator
output voltage is less than 13.2 V for 45 minutes. When this
condition exists the BCM will enter Charge Mode for 2-3 minutes.
The BCM will then determine which mode to enter depending on
The BCM will enter Charge Mode when ever one of the following
conditions are met.
||The wipers are ON for than 3
||GMLAN (Climate Control Voltage
Boost Mode Request) is true, as sensed by the HVAC control head.
High speed cooling fan, rear defogger and HVAC high speed blower
operation can cause the BCM to enter the Charge Mode.
||The estimated battery
temperature is less than 0°C (32°F).
||Battery State of Charge is
less than 80 percent.
||Vehicle speed is greater than
145 km/h (90 mph)
||Current sensor fault
||System voltage was determined
to be below 12.56 V
When any one of these conditions is met, the system will set
targeted generator output voltage to a charging voltage between
13.9-15.5 V, depending on the battery state of charge and estimated
Fuel Economy Mode
The BCM will enter Fuel Economy Mode when the estimated battery
temperature is at least 0°C (32°F) but less than or equal
to 80°C (176°F), the calculated battery current is less
than 15 amperes and greater than -8 amperes, and the battery
state-of-charge is greater than or equal to 80 percent. Its
targeted generator output voltage is the open circuit voltage of
the battery and can be between 12.5-13.1 V. The BCM will exit this
mode and enter Charge Mode when any of the conditions described
above are present.
The BCM will enter Headlamp Mode when ever the headlamps are ON
(high or low beams). Voltage will be regulated between 13.9-14.5
Start Up Mode
When the engine is started the BCM sets a targeted generator
output voltage of 14.5 V for 30 seconds.
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 1 ampere and greater than -7 amperes,
and the generator field duty cycle is less than 99 percent. Its
targeted generator output voltage is 12.9 V. The BCM will exit this
mode once the criteria are met for Charge Mode.
Instrument Panel Cluster Operation
Charge Indicator Operation
The instrument panel cluster illuminates the charge indicator and
displays a warning message in the driver information center if
equipped, when the one or more of the following occurs:
||The engine control module
(ECM) detects that the generator output is less than 11 V or
greater than 16 V. The instrument panel cluster receives a GMLAN
message from the ECM requesting illumination.
||The instrument panel cluster
determines that the system voltage is less than 11 V or greater
than 16 V for more than 30 seconds. The instrument panel cluster
receives a GMLAN message from the body control module (BCM)
indicating there is a system voltage range concern.
||The instrument panel cluster
performs the displays test at the start of each ignition cycle. The
indicator illuminates for approximately 3 seconds.
Display Message: BATTERY NOT CHARGING SERVICE CHARGING SYSTEM
or SERVICE BATTERY CHARGING SYSTEM
The BCM and the ECM will send a serial data message to the driver
information center for the BATTERY NOT CHARGING SERVICE CHARGING
SYSTEM or SERVICE BATTERY CHARGING SYSTEM 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