Why can’t a battery-based car just use a step down to 12 volts instead of a separate 12 volt car battery?

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Why can’t a battery-based car just use a step down to 12 volts instead of a separate 12 volt car battery?

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You can’t step down DC like you can AC. AC you got a transformer which lets you go from 10kv down to 120v in a nearly lossless way. For DC, this does not exist. You are left with 2 options.

The first which dumps excess voltage to ground is super lossy and never used with batteries since it would just drain the batteries, this is often done in AC/DC things where you got a constant AC current coming it so no issue with energy.

The second is to Invert the DC to AC, use a transformer to get the new voltage then rectify the AC back into DC which is a lossy process and would require 3 pieces of equipment, an Inverter, a Transformer and a Rectifier or, you get 1 piece of equipment, a 12 volt battery.

I think the biggest reason is safety. The main battery is big. This means if it has a problem, it’s a big problem. Either big fire or big expense to replace. One of the worst problems is making the battery too empty. By using a separate smaller and cheaper battery to connect and disconnect the big battery, the big battery is better protected from getting too empty. That’s as ELI5 as I can explain it. A secondary reason is that most car accessories are designed to run off smaller 12v systems. Things like radio, power windows, power steering, etc. Having a 12v battery also allows buffering for the step down.

The 12V battery runs the boring things like the keys and the lights. It’s not a good idea to rely on the huge battery to do housekeeping tasks.

Aside from the obvious size and weight a system like that would incur, there are also safety concerns. Most, if not all, EV’s actually “turn off” their batteries when the car is off. EVERYTHING except the motors are powered by the 12V battery (for backward compatibility and safety reasons. You wouldn’t want an 800V turn signal bulb or airbag.) I believe Technology Connections showcased this in one of his many EV videos (the one about at-home AC chargers, I think) but an electric car battery has engagable contactors. Basically, these metal things that allow electricity to travel to and from the battery, and can be disconnected, killing the big battery. A car such as the Chevrolet Bolt EV has two contactors and three modes.

Both contactors are closed and connected: The car can be driven.

One contactor is closed and connected, the other is not: The car is off and cannot be driven, but it is able to be charged.

Both contactors are open and disconnected: The battery cannot be charged and the car cannot be driven. No electricity can enter or leave the battery.

It is this third position where a dedicated 12V battery is necessary. Anytime you turn the car off and don’t plug it in, this mode is engaged. The big battery cannot power it’s own contactors if it’s disconnected from everything. As a result, that job belongs to the 12V battery. And if you’re gonna have to have a 12V battery to turn on the big battery, you might as well just use it for everything else to and save some weight, space, and money.

Also, disconnecting the big battery is done for safety. Having high voltage pumping through a car while it’s sitting idle is a major safety risk. I’m not saying 12V is perfectly safe and you should go lick a car battery, but having 12V sitting idle is MUCH better than having 400-900V just sitting there.