Starting and Charging: Description and Operation
LOAD SHED SYSTEM DESCRIPTION AND OPERATIONELECTRICAL 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 dash integration module (DIM) is awake, which includes engine running, key on, accessory mode, and retained accessory power (RAP). During this time the DIM 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 modules (ECM) air intake temperature or HVACs outside air temperature.
When the EPM algorithm is running, the DIM 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 DIM 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, 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 DIM sends a serial data request to the PCM/ECM to increase the idle speed. The PCM/ECM 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 PCM/ECM responds to changes in the inputs from the sensors used to control the powertrain. In order to maintain Idle quality the ECM 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 DIM also has responsibility for requesting up to 3 levels of idle boost from the ECM 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:
During each load management function, the DIM checks the battery temperature, battery voltage and amp-hour calculations and determines if the DIM should implement a different power management function.
The highest loads on the electrical system are the resistance load of heating elements. The DIM 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 DIM 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 load-shed actions shown in the table.