Secondary Battery Relay
The secondary battery relay connects the secondary battery to the vehicle's electrical system during the time that the starter motor rotates, until the engine has been restarted after having been in autostop. Then the secondary battery supplies the engine's consumers except the battery and starter motor circuit, which are separated from the vehicle's electrical system via the battery relay. The secondary battery relay is activated shortly before the battery relay is activated.
HINT: The Central Electronic Module (CEM) can, depending on the actual charge status of both batteries, determine which of the batteries is to be used to supply the electrical system while the starter motor is running. The control module can, under extraordinary circumstances, select to allow both the batteries to work together to ensure a start without any interference in the electrical system. In some versions normal start (start with start button) can also mean that the electrical system is supplied by the secondary battery if the Central Electronic Module (CEM) calculates what is optimal.
The secondary battery's voltage is monitored by Central Electronic Module (CEM). When the battery needs charging Central Electronic Module (CEM) sends a request to Engine Control Module (ECM) to connect the secondary battery to the vehicle's electrical system. The battery is charged when the alternator charges and the secondary battery is connected. The time that the relay is activated depends on how much the secondary battery needs to be charged. When the engine is running (alternator charging) and the battery is connected to the vehicle's electrical system, the battery's voltage will be the same as the rest of the electrical system's voltage. If the battery is disconnected the voltage will be lower.
The secondary battery relay is connected to the Engine Control Module (ECM) and is normally open.
The secondary battery relay is diagnosed by the Engine Control Module (ECM). The output from the Engine Control Module (ECM) to the relay, the inner inductance in the relay's coil and the supply to the relay is diagnosed by the Engine Control Module (ECM). The relay's secondary side, i.e. the part of the circuit that is connected to the secondary battery, is not diagnosed.
The secondary battery relay is located in the main fuse box.
For more information about the secondary battery relay's function, see Design and Function, Automatic start/stop of engine.