Electrical Power Management
Electrical power management (EPM) is used to monitor and control the charging system and alert the driver of possible problems within the charging system. The EPM system makes the most efficient use of the generator output, improves the battery state-of-charge (SOC), refer to Battery Description and Operation , extends battery life, and manages system electrical loads. EPM has also been know as "load management" or "load-shed" in the past.
EPM performs the following 3 functions:
| • | Monitors battery voltage and estimates battery SOC |
| • | Takes corrective actions by boosting idle speed, reducing system electrical loads, and/or adjusting the generator regulated voltage control (RVC). |
| • | Perform diagnostics and informs the driver of charging system malfunctions |
The EPM algorithm is run any time the body control module (BCM) is awake, which includes engine running, key on, accessory mode, and retained accessory power (RAP). During this time the BCM continuously estimates the battery SOC based on battery voltage, estimated net amp hours, refer to Battery Description and Operation , battery capacity, initial SOC, and estimated battery temperature. Estimated battery temperature is a calculation based on key off time, engine run time, and the temperature reading provided by the engine control module (ECM)/powertrain control module (PCM) air intake temperature or HVACs outside air temperature.
When the EPM algorithm is running, the BCM calculates the net charge rate on the electrical system by making constant voltage measurements. It uses the measurements to calculate the charge rate in amp/hours. If the BCM detects a negative charge rate, equal to a discharge from the battery, EPM can request up to 3 levels of idle boost from the ECM/PCM, when the vehicle is in park or neutral, as well as up to 2 levels of load shedding for a total of 5 levels of corrective action to maintain proper vehicle electrical system operation.
The BCM sends a serial data request to the ECM/PCM to increase the idle speed. The ECM/PCM then adjusts the idle speed by using a special program and idle speed ramp calculations in order to prevent driveability and safety concerns. The idle speed boost and cancel function will vary from vehicle to vehicle and from one moment to another on the same vehicle. This happens because the ECM/PCM responds to changes in the inputs from the sensors used to control the powertrain. In order to maintain Idle quality the ECM/PCM may not enter idle boost for up to 120 seconds, unless there is a manual change in throttle position. There is no set time limit for the idle decrease when the criteria has been met to exit idle boost, unless there is a manual change in throttle position.
Important: The BCM also has responsibility for requesting up to 3 levels of idle boost from the ECM/PCM for the HVAC system based upon head pressure, vehicle speed, and engine running. EPM has priority over the HVAC system when requesting idle boost. However, idle boost will be maintained at the level the HVAC system requires, even though the exit criteria has been met for EPM.
Each EPM function, either idle boost or load-shed, is discrete. No 2 functions are active at the same time. However, the set flags may be set at the same time. Idle boost is activated in incremental steps, idle boost 1 must be active before idle boost 2 can be active. The criteria used by the DIM to regulate EPM are outlined below:
Function | Battery Temperature Calculation | Battery Voltage Calculation | Amp-hour Calculation | Action Taken |
|---|---|---|---|---|
Idle Boost 1 Start | Less Than -15°C (5°F) | Less Than 13 V | -- | First level Idle boost requested |
Idle Boost 1 Start | -- | -- | Battery has a net loss greater than 0.6 AH | First level Idle boost requested |
Idle Boost 1 Start | -- | Less Than 10.9 V | -- | First level Idle boost requested |
Idle Boost 1 End | Greater Than -15°C (5°F) | Greater Than 12.0 V | Battery has a net loss less than 0.2 AH | First level Idle boost request cancelled |
Load Shed 1 Start | -- | -- | Battery has a net loss of 1.6 AH | Controlled outputs cycled OFF for 20% of their cycle |
Load Shed 1 Start | -- | Less Than 10.9 V | -- | Controlled outputs cycled OFF for 20% of their cycle |
Load Shed 1 End | -- | Greater Than 12.0 V | Battery has a net loss of less than 0.8 AH | Clear Load Shed 1 |
Idle Boost 2 Start | -- | -- | Battery has a net loss greater than 5.0 AH | Second level Idle boost requested |
Idle Boost 2 Start | -- | Less Than 10.9 V | -- | Second level Idle boost requested |
Idle Boost 2 End | -- | Greater Than 12.0 V | Battery has a net loss less than 2.0 AH | Second level Idle boost request cancelled |
Idle Boost 3 Start | -- | -- | Battery has a net loss of 10.0 AH | Third level Idle boost requested |
Idle Boost 3 Start | -- | Less Than 10.9 V | -- | Third level Idle boost requested |
Idle Boost 3 End | -- | Greater Than 12.0 V | Battery has a net loss of less than 6.0 AH | Third level Idle boost request cancelled |
Load Shed 2 Start | -- | Less Than 11.9 V | Battery has a net loss greater than 20.0 AH -- | Controlled outputs cycled OFF for 100% of their cycle, Battery Indicator or Battery Saver Indicator ON request sent |
Load Shed 2 Start | -- | Less Than 10.9 V | -- | Controlled outputs cycled OFF for 100% of their cycle, Battery Indicator or Battery Saver Indicator ON request sent |
Load Shed 2 End | -- | Greater Than 12.6 V | Battery has a net loss of less than 15.0 AH | Clear Load Shed 2 |
During each load management function, the BCM checks the battery temperature, battery voltage and amp-hour calculations and determines if the BCM should implement a different power management function.
The highest loads on the electrical system are the resistance load of heating elements. The BCM controls the heating elements in the outside rear view mirrors, the rear window and the heated seats, either directly or by sending messages to any module controlling power to these devices.
The second highest load on the electrical system are the blowers used in the HVAC system. The BCM will send messages to the HVAC system controller that will result in reducing the blower operation on vehicles equipped with automatic HVAC systems.
Load-Shed
This vehicle uses the following load-shed actions:
Load Shed Level | Affected Systems | Action Taken |
|---|---|---|
Load-Shed Level 0 | No systems affected | Normal operation |
Load-Shed Level 1 | Heated Outside Rear View Mirrors, Heated Rear Window / Rear Window Defrost, Heated Seats | Cycled at 80% duty cycle, OFF for 4 of every 20 second cycle. Indicator and timer not affected. |
Message Center, Instrument Cluster | No messages or indicators are displayed. Data (DPID) indicating that the Load-Shed 1 was entered is stored and may be accessed with a scan tool. DPID will reset after 40 ignition switch cycles with no repeated load-shed 1 action or with a battery disconnection. | |
Load-Shed Level 2 | Heated Outside Rear View Mirrors, Heated Rear Window / Rear Window Defrost, Heated Seats | Turned OFF. Indicator and timer are not affected. The indicators and timers are controlled by the RIM. The operator must turn ON system when load-shed level is exited. System will not respond to operator input until current load-shed level is exited. This system will respond to only one Load-Shed Level 2 command per ignition switch cycle. |
Message Center, Instrument Cluster | "Battery Saver Action" message is displayed. Charge indicator is illuminated. Data (DPID) indicating that the Load-Shed Level 2 was entered is stored and may be accessed with a scan tool. DPID will reset after 40 ignition switch cycles with no repeated Load-Shed 2 actions or with a battery disconnection. |
