Why do electric vehicles (e.g., Tesla) have a 12v battery in addition to the much higher-capacity main battery?

Quora

Because it turns out to be enormously impractical to get 480V DC motors for the power windows, when everybody else uses 12V DC.

Therefore, to reduce costs, you put in a 12V DC system so you can buy off the shelf parts for everything else on the car, except the main drive system.* Also, much safer because all the secondary systems on the car will be running off 12V DC, which is considerably safer for emergency responders to deal with. Much scarier if the light fixtures in the interior, and even the door locks all had 480V DC wires running to them. So, for the safety of everyone involved, as well as the considerable economic benefits, you run 12V DC wiring from your 12V DC accessory battery to all the off-the-shelf 12V DC accessories in the car, and you leave the high-voltage stuff to just the bits that need it.

*And, apparently, the heater, because running the heater does increase drain on the main battery pack.

The automotive industry is standardised on 12v. The motor control ECU will run on 12v, along with ABS, windscreen wipers, lights and all the other anciliary systems. It makes no sense to break with the standard system. When the car is shut down, the main traction battery will be completely isolated and it’s the 12v system which will operate the isolation relays.

In use, there will be a DC/DC converter which provides 12v power from the main battery instead of the alternator of a combustion-powered vehicle. The 12v battery acts as an energy buffer to handle current spikes. As an off the shelf solution, something like a motorcycle battery would do the job.

The automotive industry is standardised on 12v. The motor control ECU will run on 12v, along with ABS, windscreen wipers, lights and all the other anciliary systems. It makes no sense to break with the standard system. When the car is shut down, the main traction battery will be completely isolated and it’s the 12v system which will operate the isolation relays.

In use, there will be a DC/DC converter which provides 12v power from the main battery instead of the alternator of a combustion-powered vehicle. The 12v battery acts as an energy buffer to handle current spikes. As an off the shelf solution, something like a motorcycle battery would do the job.