Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.