Functional Description - Power Control Relay

The Engine Stop/Start (ESS) system  uses a Power Control Relay (PCR)  and two Batteries to eliminate the voltage drop in the system voltage during an Auto-start event. This allows the consumer electronics and vehicle modules subsystems to continue to function in a normal manner. The Power Control Relay is connected in-line between the Main (Cranking) Battery and the Auxiliary (Vehicle) Battery. The Power Control Relay is normally closed when de-energized, connecting the two Batteries. When the Power Control Relay is energized, the relay opens and breaks the connection between the Batteries.

The engine starting system  is connected directly to the Main Battery. The engine charging system  is connected to the Auxiliary Battery side of the system. When the Power Control Relay is closed (de-energized), the Main Battery is charged through the Power Control Relay. Therefore, if the Power Control Relay is stuck on (open), the Main Battery will not charge. If the fuse in the Front PDC fuse array is open the Main Battery will no charge as well.

When ESS is active,  and an Auto-start crank event is initiated  , the Power Control Relay  is temporarily energized when 12.0 volts is supplied from the output of the PCR Control Relay.  When the Power Control Relay is energized it breaks the connection between the Batteries and allowing them to work independently. When this occurs, the Main Battery is used to crank the engine. The Auxiliary Battery, which is now solely connected to the PDC and is not affected by cranking, can maintain full Battery voltage to the rest of the vehicle modules and subsystems. This only occurs during the first 20-40 ms of initial starter engagement.