Electric vehicles do not have an auxiliary motor (a.k.a. a starter motor) of any shape or form. Electric motors start spinning by, essentially, flipping a switch that supplies electricity to the motor. For devices like household fans, instead you simply turn them on and the motor spins. Notice how the motor simply turns on, and there is no auxiliary motor required to start the motor. The device simply starts, and stops, repeatedly, as soon as you turn the switch on and off.
The starter motor in a gas engine itself proves the point. Every time we start a gas engine we worry whether it will actually start, but it's not the starter motor we worry about. Compared to the starter motor, a gasoline engine is an unreliable finicky machine that might or might not start on any given day.
Internal combustion engines are feedback systems, which, once started, rely on the inertia from each cycle to initiate the next cycle. Engines must continue running for the engine to be "on" and available to drive the vehicle. Electric motors do not operate the same way, and can be turned on and off with no delay or extra energy cost.
As we'll see a 12 volt battery is required to turn on an electric car. Because there is no starter motor for an electric car, the 12 volt battery has a different purpose than it does on a gasoline car. In a gasoline car those purposes are to aid starting the engine, to keep the engine running, and to operate exterior lights and stuff in the passenger cabin. On an electric car the sole purpose is powering the exterior lights and stuff in the passenger cabin. Please bear with the oversimplification.
Starting a gasoline or diesel engine requires an electric starter motor
A gasoline or diesel engine is started by mechanically rotating the engine while supplying electricity to the spark plugs. With luck, and the driver praying honestly enough for the engine to turn over, explosions begin happening in the cylinders. From there the mechanical system of the engine takes over, and the explosions keep the engine rotating so long as the starter battery has electricity to keep the spark plugs sparking away. For that purpose an electric generator is attached to the engine to keep the starter battery charged up.
Originally gasoline motors did not have a starter motor, and instead the driver had to manually crank the motor. The story is that often the engine would kick while starting up, and this could break the arm of the person cranking the engine. The starter motor replaced the hand-crank on the gas engine, and was a godsend which also spelled the doom of early electric cars since an easily-started gasoline engine erased one of the advantages electric cars had over gasoline.
Namely getting an electric car to move is as simple as flicking a switch, compared to the complicated rigamarole of getting a gas engine started.
The key to the functioning of a gasoline engine is that the engine must keep running even when the vehicle is stopped. In other words, the engine must idle in order to keep running. This of course wastes fuel because the car is not serving any useful purpose but the engine has to keep burning gasoline.
Electric motors simply spin
By contrast electric motors operate on the principle of opposing magnetic fields. To start the motor spinning you simply power up the magnetic fields, and the motor can't help but start spinning. No auxiliary anything is required, just electricity flowing through copper coils to generate the magnetic fields.
The Rotor (on the left) is the rotating part of an electric motor, while the Stator (right) is the stationary part. In some electric motors one of these will have magnets, while the other has copper coils. In other electric motors both parts have copper coils.
Energizing a coil forms a magnetic field around the coil. This magnetic field interacts with the other magnetic field, and that interaction causes the Rotor to spin. The spinning Rotor is then connected to a drive shaft that ultimately causes the wheels to turn.
That it. Getting an electric motor to spin does not require the sort of complex system required to start gasoline engines. You simply supply power to the coils and the electric motor spins on its own.
Electric motors do not need to idle, unlike gasoline engines. When an electric car is stopped like at a stop light, its motor is stopped and is not spinning. Therefore an electric car is not wasting energy while stopped, and when it's time to get going again electricity is simply supplied to the motor and it starts spinning.
And before someone complains - this explanation is an extreme oversimplification. Please recognize that it is enough for this topic.
Why do electric cars have a 12 volt battery?
On a gasoline or diesel car the 12 volt battery is an essential to starting the engine. This battery has the power to crank the starter motor and fire the spark plugs, kicking the gas engine into motion. That battery then keeps powering the spark plugs while being charged by the alternator.
Why is there a 12 volt battery since an electric car does not do any of that?
There is another function of the 12 volt battery - powering the exterior lights, the interior lights, and accessories like the sound system or heating/cooling system or powered seats or powered doors. These systems are extremely similar between gasoline and electric cars.
One key system is the set of switches and dashboard indicator lights involved with turning the electric car on and off. These components run on the 12 volt power system. Ensuring an electric car can always be turned requires a 12 volt battery.