What is the auxiliary battery of an electric car and what function does it have?

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Modern cars featuring CO2-reducing technologies, high levels of specification and new electronic driver aids may include a auxiliary battery along with the main starting battery. Electric cars also have a conventional 12-volt auxiliary battery in addition to the high-voltage traction battery.

In this article you will learn what is auxiliary battery and what it is used for in electric cars, in addition to other related aspects.

What is the auxiliary battery?

Just like gasoline and diesel internal combustion cars, electric cars have an auxiliary battery. This is not used in EVs by the traction motor, but it is used to provide the power needed by accessories to function.

In most cases, auxiliary batteries in electric cars are like standard 12V lead-acid batteries. The same ones used in internal combustion cars.

This is not something that electric car owners like very much, because for them it is a somewhat frustrating solution. Tesla electric car owners need to replace their lead acid batteries every 2 to 4 years.

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What auxiliary battery is Tesla putting in the Model S and Model X?

On the Tesla Model S and Model X, Tesla switched to a 12-volt lithium-ion auxiliary battery. The auxiliary lithium-ion battery is lighter and smaller, withstands up to 2,000 recharge cycles while retaining 80% health.

In theory, this new auxiliary battery used by Tesla can last as long as the car, or at least match the cycle life of the main battery. Tesla is actively considering moving to a 48-volt auxiliary battery in future models.

A 48 volt battery would provide improvementsallowing the use of thinner cables throughout the car, which would make it lighter and cheaper.

How are HEV and EV batteries recharged?

The big difference between the PHEV and EV auxiliary battery systems from the recharging system of conventional cars is the method used to recharge the battery. Instead of using an alternator to charge the auxiliary battery, PHEVs and EVs are recharged by the HV battery using the inverter/converter.

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Types of 12 Volt Auxiliary Batteries

Most 12-volt auxiliary systems use a Absorbent glass fiber (AGM) battery or one valve regulated lead acid (VRLA) battery. The auxiliary battery can be placed almost anywhere in the car:

  • In the trunk.
  • Under the floor of the rear door.
  • under a seat.
  • In the engine compartment.
  • Under the base of the car.

The auxiliary battery provides 12 volts to run electronics and accessories, and its power capacity is often very low. Due to this low capacity, the battery will discharge rapidly if there is a fault associated with the recharge function of the battery.

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