The 12v system is to use standard 12v automotive electronics; every electric (and hybrid) car does this. A 12v SLA battery (these cars do tend to use sealed batteries / AGM with safety vent only) is the cheapest way to provide voltage regulation and power for accessories when the high voltage battery pack is "safed". (Generally, the "start" equivalent for both BEVs and hybrids is a process where the HV battery controller runs multiple safety tests on the high voltage system and then closes relays to connect the HV battery to the rest of the car. This bootstrap process is powered by the 12V SLA battery, since the HV battery is disconnected when "parked" - same reason that the lighting and horn need to run off the 12V battery.) Instead of an alternator, there's a DC-DC buck converter that recharges the SLA battery. SLA is a nice chemistry to use here because it's "self-managing" - you don't need a battery management system for the 12V battery.

Tesla has decided to not provide a 120v inverter from the factory. It's not the underlying tech, because other manufacturers do ship inverters and they're a straightforward thing to integrate (as the manufacturer) with the HV battery pack.

This is probably about (besides the normal Tesla-being-the-Apple-of-car-companies) battery pack degradation. Tesla currently warranties their battery packs based on mileage, and frequent use as a stationary power bank will put more cycles on the batteries.

There is precedent for this - ICE vehicle warranties can be voided for excessive stationary operation. Generally they don't bother for light duty, but medium and heavy duty trucks with PTO have maintenance requirements and warranty terms based on engine running hours as well as mileage.

In addition to the other reasons mentioned here, the 12v battery is important in emergency situations. The system is quick to disconnect the 400V battery in the case of a crash, and it relies on the 12V battery to run the air bags and hazard lights.

Also, any first responder approaching a crashed Tesla with metal tools doesn't want to face parts energized to 600v. A short in a 12v system isn't going to kill anyone.

Teslas, despite their hype, are not some kind of revolutionary car. They use the same technology as every other car, though they improve things where they can, such as their motor efficiency. Standard car parts still need to be powered by low voltage, and it wouldn't make sense to make everything proprietary just for the tesla.

Almost everything inside a car works on 12v (door locks, windows, lights, center screen etc). They would like to move to 48v and power over ethernet however they probably have a lot of suppliers and it takes a while to get them all to switch to a new standard.

Because they have a prewarm system and 12v lights etc.

It helps conserve your actual battery and works well.

They have a separate 12v system from their 400v+ system that they use to run the motor.

I would also assume that most of the cars electronics that aren't propulsion based are manufactured and designed by parts suppliers that base their experience and design around the 12v batteries. Giving them a similar power supply means not having to redesign the cars electronics specifically for Tesla.

needing to keep a step-down (plummet down, at those voltages!) voltage converter continuously running off of main batteries to power some background electronics would be pretty inefficient.

the batteries wouldn't like it. where-as alkaline & lead acid batteries are efficient at low-discharge, lithium ion batteries generally don't enjoy low-discharge . buck converters are also probably going to be more efficient when they have some modest load to deliver- otherwise the drive circuitry & mosfets are consuming some (little) juice, just to produce a trickle of power.

in general this is an automotive vehicle using automotive rated electronic parts. having a conventional 12v battery-powered architecture for most systems makes loads of sense, is is hella convenient, and based off my above reasoning i'd guess more efficient. yes with that low-drain the inefficiency feels like it probably won't matter but I rest easier thinking that the li-ions can truly rest when the car is off. it's better for them.

I don't own a Tesla, so I don't know.

I have dealt with these shenanigans with my ICE car and a dealer (they did not want to honor the warrenty for the car battery), this why I knew about bringing the battery to an Auto Parts store.

Most warranties treat Car Batteries/Rotors/Pads as a maintenance item and are excluded from warranties.

> are excluded from warranties.

If you are curious about my situation, essentially my fiancee went and got the battery replaced at the dealer (we brought in the car for a different issue, and the dealer wanted to replace the battery).

About 6 months later, the new battery failed, and this battery also had a very clear sticker saying "3 year warrenty" right on the battery (it was certainly not an OEM part).

An auto parts store which helped me confirm that the battery was dead (they were actually very helpful, they offered to charge the battery and diagnosed the issue free of charge).

After working with the auto parts store, I called the dealer asking about this. They wanted us to bring the car to them, charge us a $100 diagnostic fee (essentially, doing the same thing the auto parts store did for free), then they would decide if it needed replacing, and not commenting on whether they would honor a free replacement. The battery at the auto parts store was $100! I just decided to buy the battery and install it myself. That battery does have a five year warrenty.

I strongly suspect that the original battery was never replaced, based on how they treated my fiancee in both interactions.