[Tuna-Fish]

And to explain why this hadn't been done before/how we got here:

Nothing in a car actually wants 12V DC. Most of the low voltage stuff will run better at 5V or below, while a lot of the higher voltage stuff would benefit from going as high as possible. 12V exists because DC-DC conversion used to be expensive, and you had to make a compromise about the voltage based on losses, wire thickness, and picking a low enough voltage that all the low-voltage stuff doesn't suffer too much.

What's changed is that you can get a single-device DC-DC converter for really cheap these days. Cheap enough that you might as well put it in the light bulbs, and everywhere else that wants a low voltage.

[myself248]

12v exists because 6v was too low; wires were impractically thick for even the early accessories being added in the 1950s. The 6v-12v transition happened in 1955/56 for many cars. Some stuff like lightbulbs could be reused by putting two 6v bulbs in series in a 12v car, so it was a very cheap and relatively straightforward transition.

If they'd just had some foresight and gone 48v in 1955, we would've saved 50 million tons of copper in the years since. It's no harder to make 48v motors or lightbulbs or relays or anything else (and in fact, the telephone network contains plenty of exactly those things, and has, in staggering numbers, for over a century), but the automotive industry isn't exactly known for being forward-thinking.

[robocat]

A 12-volt battery typically has six cells. A 48 Volt Lead-acid battery would have 24 cells - I'm not sure how that would change the constraints on charge balancing and starter-motor stress.

I can say that the 24 Volt deisel vehicles I have used makes buying two batteries expensive.

[15155]

> If they'd just had some foresight and gone 48v in 1955

How would they have done this without cheaply-available high-speed switch-mode power supplies with low-DCR inductors/MOSFETs?

[myself248]

Why would they need those?

All the stuff that's natively 12 volts now could simply have been made natively 48 volts. You can make a 48-volt lightbulb as easily as a 12-volt lightbulb. You can make a 48-volt motor as easily as a 12-volt motor. Actually, motors for higher voltage tend to be smaller and lighter, which is why industry tends to go straight for 4160-VAC motors whenever 480VAC is inadequate.

What applications cannot be made to work at 48? I'm not aware of any. As I said in the comment to which you're replying, the telegraph and later the telephone network had been running similar DC systems since the 1850s or so at various voltages depending on the length of the telegraph line, with the telephone network taking over and 48 volts firmly entrenched by the 1910s. There was a huge manufacturing base producing 48-volt equipment, including motors and generators, indicator lamps, and a mindboggling array of switches, relays, stepping selectors, and their ilk, and all that was before WWI.

Furthermore, Charles Kettering who invented the automotive starter motor (and made it work at 6 volts), was around the same time making Delco-Light plants for rural electrification, which mostly ran at 32 volts DC. These supported a whole line of 32VDC appliances -- lights, vacuum cleaners, kitchen gadgets, irons, motors that could be attached to other machines in the shop. There was also a less common 110VDC version of the system but I can't find any contemporary literature discussing the differences, although I'm sure they would've quickly discovered that the 32V system was pretty docile while the 110V encouraged extreme care around open contacts.

As for why cars didn't use the higher voltages already in use and superior in many ways, my only guess is that a lead-acid battery with a high number of small cells must've been difficult or expensive to manufacture, compared to one with a small number of large cells. The Delco-Light plant used a large rack of 2-volt cells, whereas the starter motor used a single 3-cell packaged battery that fit easily under the hood. If they'd just figured out how to package more smaller cells together....

[15155]

> Why would they need those?

To convert voltages to useful levels without suffering massive losses in efficiency.

> What applications cannot be made to work at 48?

Basically every logic-level transistor will not work at 48V. It's nice that these last-century analog devices could be made to operate at different voltages: present-day semiconductors are not so conveniently flexible.

Simple physics dictates that required inductors to step between voltages increase in physical size (and weight, and material cost) as that voltage disparity increases. Capacitance required, etc. all increases with it. Efficiency plays into both of these as a triangle. Heat increases as this disparity increases. These properties are unacceptable for a myriad of use cases.

[pokey00]

You didn't have to convert voltages all the time for things you did in a car until relatively recently. All the typical 12V stuff could've run at 48V no problem. By the time we wanted to put computers in cars and charge our cell phones, switching supplies were readily available. The only part of the system now that really benefits from lower voltages are semiconductors.

[elihu]

I think another big part of it is that DC switches tend to get expensive above 12V. Cheap AC switches work fine at higher voltage, because the arc is self-extinguishing as it passes through zero twice per cycle, but DC doesn't do that so you can end up with an arc that doesn't extinguish itself, which, aside from not turning the thing off when you want to, burns out the electrical contacts.

[dreamcompiler]

Now we have MOSFETS and IGBTs that can switch DC without sparks. I suspect most DC switching in the Cybertruck is relayed through these. (Except the main contactor and pyro fuse of course.)

[myself248]

And the contactor is usually switched with no current through it. The only time it would open under load is in an emergency, and presumably it only does that once.

Which is to say, yeah, it's a non-issue for pretty much everything. Even in the 12v realm, new BCMs have so few relays anymore, almost everything's done with onboard FETs and software.

[elihu]

And given that this is a Tesla, it wouldn't really have had many mechanical switches in the first place. Maybe the turn indicator? Which could happily run at 5 volts or something and blink the lights via CAN bus messages.

[Kirby64]

None, unless you count the physical buttons as 'mechanical switches'.

The turn signals on a Tesla don't even make the 'clicking' noise if the Infotainment is rebooting, because it's literally just a noise piped through the infotainment.