If electric vehicles (EVs) use high-voltage lithium-ion batteries, why do they still carry a 12-volt lead-acid battery?
The various electrical components of a conventional vehicle – including lights, windscreen wipers, electric windows and electronic control units – operate on 12 volts direct current supplied by a lead-acid battery.
This is the traditional power grid that has been in production for decades and has proven to be safe and reliable, with an easily replaceable and inexpensive battery.
Component manufacturers base all of their designs on the 12 volt system. Thus, all vehicles, including electric vehicles, are equipped 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 electric vehicle, 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 lower the high voltage battery output to 12 volts for these functions, current technology and design codes specify that the networks must be totally separate. This is to ensure maximum safety and safe isolation of the high voltage system when the vehicle is stopped during service or maintenance and in the event of a traffic accident.
Each time an electric vehicle is turned on, it is the 12-volt system that powers a contactor – essentially a switch that transmits high electrical power – to allow high voltage power to be transmitted to the motors. This is an essential feature of all electric vehicles. Whenever the control system signals that the high voltage system should be disconnected, the 12 volt switchgear activates the isolation.
This will cause the EV transmission to immobilize. But the car’s power steering, brakes, power windows and central locking will continue to work, thanks to the independent 12-volt battery and its low-voltage network.