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Why does Tesla have a 12v battery?

A 12V battery is mandatory in an internal combustion engine (ICE) vehicle. A “starter motor” starts the ICE engine with the help of this 12V battery. Later, an alternator connected to the engine generates electricity, powers electrical components on the car, and recharges the 12V battery.

On the contrary, we don’t need to start a Tesla. It’s an electric vehicle and has a massive Lithium-ion battery (more than 50kWh — varies depending on models). 

So, Why Tesla has a 12V battery? 

In 2012, an Original Tesla Roadstar owner parked his car unplugged for two months in a parking garage. When he returned to his car, he could not unlock his Roadstar as the battery discharged entirely. Though he failed to put the car into neutral, a tow truck company towed his vehicle to the nearest charging station. However, the Roadstar was unable to recharge.

He took his vehicle to the Tesla Service center, where they said the main battery is dead, and it would cost $40,000 to replace the onboard Lithium-ion battery. 

So, what was the problem here? Why could they not recharge the dead battery?

At that time, the Roadstar had only one main battery to drive the car and power onboard electronics. The battery was a sealed pack with necessary parts such as battery heating, cooling, recharge contactor, etc. To recharge the Roadstar, the onboard computer would close the recharge contactor. However, as the main battery died, the computer could not turn on, and the recharge contactor would not close. Thus, “bricking” the whole Lithium-ion battery pack.

Automotive experts1 said that Tesla could have easily avoided this by only including a 12V Lead Acid battery. This 12V battery could run the computers, the recharge contactors, and other vital electronics.

A Plugged-in Tesla is a happy Tesla.

After that controversy, Tesla changed its internal design and started including a 12V lead-acid battery. However, this is not the sole reason.

Let’s elaborate further.

Definition of “Traction Pack”:

The main Lithium-ion battery pack that drives a Tesla is called “Traction Battery” or “Traction Pack.” 

The traction pack is only engaged while a Tesla is in Drive mode or during a recharge session. While operating your Tesla vehicle, a 400V to 12V DC-DC converter provides 12V power from the main battery pack to run all the vehicles’ electronics, similar to an alternator on the ICE vehicle.

Separation of power:

The main battery — traction pack — is only for the drive motor and 12V battery to power auxiliary and vital electronics.

Standardized 12V parts:

The car industry has standardized 12V. Tesla uses numerous 3rd party parts in its vehicle, which runs on 12V. For example, lights, wiper motors, power window motors, power liftgate, ABS electronics, washer fluid pumps, mirror control, fans, power steering, power brakes, wipers motor, AC, heater, etc. Therefore, they need a 12V power supply. If Tesla includes a 12V battery, it solves two problems. One, your main battery would not brick as the original Tesla Roadstar. Two, operate all these ancillary systems.

Safety Reason:

There is a legal requirement to turn on the hazard lights after an accident. However, after a severe accident, Tesla automatically shuts off the main traction battery to prevent a possible fire. If there is no 12V battery, it will be impossible for Tesla drivers to turn on this hazard light.

It’s essential for safety reasons to have an auxiliary battery that can power headlights, ABS braking system, airbags, open door, and other onboard electronics. 

In the event of an accident, the Tesla computer shuts down the traction battery. If there is no 12V battery, how the airbags would get deployed?

Human safety:

Tesla’s main traction battery is 400V. It’s impractical to run every onboard electronics or electrical systems on 400V. A 12V battery is safe to touch. However, a 400V might kill you. Therefore, the wires running throughout our car can’t be of high voltage. 

It’s also safer for emergency responders as the traction battery is fully isolated.

So, why doesn’t Tesla use a portion of its main Lithium-ion battery to replace the 12V battery? Or, Use a separate 12V Lithium-ion battery instead of a 12V Lead-Acid battery?

We know that a Lead Acid battery is terrible for the environment. However, a Lead Acid battery is stable, can tolerate and operate at extreme temperatures, but a Lithium-ion battery can’t. 

A Lead Acid battery does not freeze until -90°F, can charge and discharge over a wide range of temperatures without damage. However, a Lithium-ion battery would get damaged — lithium plating — if we try to recharge it at cold temperatures. 

According to battery researchers2, though we can discharge a Lithium-ion battery from -4°F to 140°F, we can only recharge it if the temperature is more than 32°F. Moreover, a Lithium-ion battery performs best if the operating temperature is between 59°F to 95°F.

Therefore, if we use a Lithium-ion 12V battery, it would get damaged in extreme weather, such as in winter months, when the temperature drops below freezing. Therefore Lithium-ion 12V battery is not the right choice because Tesla ships cars all over the world. It’s why Tesla has to use a 12V Lead Acid battery.

Many suggest using a 12V lithium-ion battery with onboard cooling or heating system, similar to the traction battery. 

Tesla traction battery drains 1% per day, even if it’s parked. It’s called phantom drainage. A 12V lithium-ion battery with a cooling and heating system will make this drainage worse. So, it’s not a sensible solution for all weather conditions.

Conclusion:

In conclusion, Tesla has a 12V battery to power the onboard computer and electronics. Furthermore, In case of emergency, when the traction battery fails or shuts off, the 12V battery keeps all the vital car parts operable for safety reasons, for example, four-way flashers. Without a 12V battery, the whole car would shut down. Moreover, the industry standardized 3rd party parts such as airbags, the lights, wiper motors, power window motors, power liftgate, ABS electronics, washer fluid pumps, mirror control, fans, power steering, power brakes, wipers motor, AC, heater, etc. operate on 12V. 

It’s why Tesla uses a 12V battery on their vehicle.