Starting and Charging: Description and Operation
Load Shed System Description and Operation
Electrical Load Management
Electrical load management is designed to maintain battery voltage and covers more than load-shed. The body control module (BCM) will request increases in idle speed from the engine control module (ECM), when the vehicle is in park or neutral, as well as turn off loads in order to manage the electrical system and preserve the vehicle electrical power availability. The BCM calculates the battery temperature, voltage and charging rate at all times while the engine is running.
The BCM calculates the battery temperature by factoring in:
* The current intake manifold air temperature compared to the last temperature recorded when the ignition switch was turned OFF
* The current battery voltage compared to the last battery voltage recorded when the ignition switch was turned OFF
* The length of time since the last battery temperature calculation
If the calculated battery temperature is below -15°C (+5°F), the PMM institutes steps to control the load.
The BCM calculates the voltage of the battery by making constant voltage measurements and using the measurements to calculate the true battery voltage. If the BCM detects a low voltage, the BCM institutes steps to control the load.
The BCM calculates the net charge rate on the electrical system by making constant voltage measurements and using 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, the PMM institutes steps to control the load.
The BCM will either request an increase in the engine idle speed from the ECM or the BCM will turn off loads, called the load-shed function, in order to preserve the vehicle electrical system operation. The criteria used by the BCM to regulate this electrical load management are outlined below:
Each load management function, either idle boost or load-shed, is discrete. No two functions are implemented at the same time.
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.
Idle Boost Functions
The BCM sends a serial data request to the ECM to increase the idle speed. The 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 ECM responds to changes in the inputs from the sensors used to control the powertrain.
