Secondary Battery (2010-, Only On Vehicles With The Function Automatic Start/Stop Of Engine).
The secondary battery is used in the function automatic start/stop of engine.
The secondary battery is only connected to the vehicle's electrical system when the engine is restarted after autostop and when the secondary battery needs to be charged. Connecting the secondary battery to the vehicle's electrical system is controlled by Engine control module (ECM) via a relay.
Central electronic module (CEM) is monitors the secondary battery's voltage and controls the secondary battery's charging.
When the secondary battery needs charging the Central electronic module (CEM) sends a request to Engine control module (ECM) to connect the secondary battery to the vehicle's electrical system. When the alternator is charging and secondary battery is connected, the battery is charged.
When restarting the engine, it is only Engine control module (ECM) that controls connection of the secondary battery.
When the engine is on (alternator charging) and the secondary battery is connected to the vehicle's electrical system, the secondary battery's voltage will be the same as the rest of the electrical system's voltage. If the secondary battery is disconnected, the voltage will be lower.
The secondary battery's voltage can be diagnosed.