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.
Battery Relay
The battery relay is a disconnector that disconnects the vehicle's battery and starter motor circuit from the rest of the electrical system during the time that the engine is restarted after an autostop. This is to prevent the low voltage, which briefly occurs when the starter motor is run, from propagating to the rest of the vehicle's electrical system. The secondary battery relay is also activated very shortly before the battery relay is activated. Then the secondary battery is connected and supplies the vehicle's other power users during the start phase.
See also: SECONDARY BATTERY RELAY and DESIGN
HINT: The Central Electronic Module (CEM) can, depending on the actual charge status of both batteries, check 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 battery relay is connected to the Engine Control Module (ECM) and is normally closed.
The battery relay is diagnosed by Engine Control Module (ECM). The output from Engine Control Module (ECM) to the relay, the inner inductance in the relay coil as well as supply to the relay are diagnosed by Engine Control Module (ECM).
The relay's secondary side, that is, the part of the circuit that is connected to the battery, is not diagnosed.
The battery relay is located in the main fuse box.
For more information about the battery relay's function, see Design and Function, Automatic start/stop of engine.
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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.
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