Torque Shop: Why is there a 12v battery in an EV?

Can you spot the 12-volt battery in the Opel Zafira-e? It is on the right, with the red cap and black cables. ST PHOTO: CHRISTOPHER TAN

If electric vehicles (EVs) use high-voltage lithium-ion batteries, why do they still carry a 12-volt lead acid battery?

The various electrically powered components in a conventional vehicle – including lights, wipers, power windows and electronic control units – operate on a 12-volt direct current supplied by a lead acid battery.

This is the traditional electrical network that has been in production for decades and has proven to be safe and reliable, and with a battery that is easily replaceable and inexpensive.

Component manufacturers base all their designs on the 12-volt system. Hence, all vehicles, including EVs, are fitted with a 12-volt system and battery. On cars with internal combustion engines, the lead acid battery is charged by an alternator when the engine is running. In an EV, it is charged via a DC-DC converter by the vehicle’s high-voltage battery.

The primary function of the high-voltage battery is to power the motors and the air-conditioning compressor. All other electrical equipment and control systems rely on the lead acid battery.

Although it is possible to step down the high-voltage battery output to 12 volts for these functions, the current technology and design codes specify that the networks be totally separate. This is to ensure maximum safety and safe isolation of the high-voltage system when the vehicle is shut down during servicing or maintenance and in the event of a road accident. 

Whenever an EV is switched on, it is the 12-volt system that energises a contactor – essentially a switch which transmits high electrical power – to allow high-voltage supply to be transmitted to the motors. This is a crucial feature of all EVs. Whenever the control system signals the high-voltage system to be disconnected, it is the 12-volt switch gear that activates the isolation.

This will cause the EV drivetrain to be immobilised. But the car’s power steering, brakes, power windows and central locking will continue to operate, thanks to the independent 12-volt battery and its low-voltage network.

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