Electrical Load Management

The power management function is designed to monitor the vehicle electrical load and determine when the battery is potentially in a high discharge condition. This is accomplished by using a high accuracy battery voltage reading as an indicator of battery discharge rate. The following six levels of load management will execute in the load management control algorithm when there is a high discharge condition:

1. The first action requests a vehicle idle speed increase to the powertrain control module (PCM) in order to raise alternator output.
2. The second action requests a greater vehicle idle speed increase to the PCM in order to raise alternator output.
3. The third action begins to shed vehicle loads in an attempt to remedy the heavy discharge condition.
4. The fourth action requests another vehicle idle speed increase to the PCM in order to raise further the alternator output.
5. The fifth action begins to shed further vehicle loads in an attempt to remedy the heavy discharge condition.
6. If the above five corrective actions fail, the sixth action of power management further sheds loads in a final attempt to remedy the high discharge condition.

Loads subject to reduction include the following:

• The A/C clutch
• The heated mirrors
• The heated seats
• The rear defog
• The HVAC blowers

The power mode master (PMM) calculates the battery temperature, voltage and charging rate at all times while the engine is running. The PMM 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 battery temperature is below set limits, the PMM institutes steps to control the load.

The PMM calculates the voltage of the battery by making constant measurements and using the measurements to calculate the true battery voltage. If the PMM detects a low voltage, the PMM institutes steps to control the load.

The PMM calculates the discharge rate, or draw, on the battery by making constant measurements and using the measurements to calculate the discharge rate in amp/hours. If the PMM detects a high current draw from the battery, the PMM institutes steps to control the load.

The PMM will either request an increase in the engine idle speed to the PCM or the PMM will cycle or turn off loads, called the load-shed function, in order to preserve the vehicle electrical system operation. The criteria used by the PMM to regulate this electrical load management are outlined below:

Fig 1: Electrical Load Management Table (1 Of 2)

Courtesy of GENERAL MOTORS CORP.

Fig 2: Electrical Load Management Table (2 Of 2)

Courtesy of GENERAL MOTORS CORP.

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 PMM checks the battery temperature, battery voltage and amp-hour calculations and determines if the PMM should implement a different power management function.