Most (all?) hybrids and BEVs have a 12V (or 16V) battery. Why?

[VectorBear]

For standby (car powered down), why not have a low quiescent power traction voltage to 12V converter to power standby peripherals such as RFID key fob detector, security system, door locks, computer when the car initially powers up, etc. Once the car does power up, immediately the traction contactor closes and turns on the larger (100-200W?) converter to not only power all of the running 12V system but charge that 12V battery.
Usual arguments are silly: no isolation of traction voltages (converter can be before contactor, with its own protection), drain on traction pack (50-100kWh can handle a few W for a very long time), safety if traction pack fails (but what if 12V system fails in a collision? in the same boat), status quo AKA legacy (time for a new status quo).
So why not replace the 12V battery with a low power, low quiescent power traction voltage to 12V converter to run standby systems?

 

[VectorBear]

Dimming lights is just poor design, regardless of voltage. Here are the currents, here are acceptable losses, do it. The mechanical equivalent might be too close to yield.

 

[VectorBear]

Ah, the sample of one. Look at the other vehicles and see if there is any sign of corrosion on any of the cars around you.

Anticorrosion in the past 20-30 years has become so good that very few vehicles in our road salt belt have visible corrosion. It really isn't a problem anymore. However, body damage will corrode quite well, most often visible corrosion on vehicles is at a visible dent or deep scratch.

 

[3DDave]

Right. Sure. In New York state a typical car lasts less than 12 years before the frame/underside rots out; that is if eating through the electronics modules doesn't finish it off first. The reason you may not see cars with visible corrosion is that the ones that have corroded are scrapped.

Still, why are car makers passing on this great idea to save so much money on their electric vehicles?

Is the real reason that they aren't smart enough?

So far no reason is good enough, apparently. It must be a conspiracy.

 

[VectorBear]

A couple of searches for average lfespan of cars in Canada says about 12 years but due to mileage of >300k km (why don't we say 300Mm?). None of the results mention corrosion, there is mention of longer times. Search "what is the average lifespan of a car in canada" or salty region of choice.
Please, drop any secret plans, stick to verifiable info.

 

[3DDave]

Please, get hired by a car maker and show them how wrong they are.

 

[cathysam]

Yeah, pretty much all hybrids and BEVs still use a 12V battery (sometimes 16V in newer setups), and it’s not just legacy holdover—it actually makes a lot of sense.

Even though the main propulsion system runs at 400V or even 800V, the 12V battery handles all the low-voltage stuff: lights, power windows, locks, infotainment, sensors, ECUs, etc. Reengineering all that for high voltage would be overkill and way more complex (not to mention unsafe in a lot of cases).

Another big reason is safety and startup logic. The high-voltage battery is typically isolated when the vehicle is off—so you need the 12V system to wake up the car, close the contactors, and power essential electronics. Think of it like the BIOS battery in a PC—it gets everything else going.

And if there’s ever a fault or shutdown in the HV system, the 12V keeps basic things alive—hazards, door unlock, emergency comms, etc.

Some newer BEVs are shifting to 16V lithium batteries for weight savings or higher efficiency DC-DC conversion, but the role’s basically the same.