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Comment by Moto7451 | Hacker News Reader

Audi also has a 48v architecture in use for mild hybrids like my SQ5. This is an older article but explains it well https://www.greencarcongress.com/2017/06/20170602-audi.html

My 2021 is fully 48v except for an inverter for necessary 12v circuits (like cigarette outlets and the jumper pins) and a small motorcycle battery that is used for minimal 12v functionality. The mild hybrid battery is under the rear seats and spins the alternator to keep accessories running.

https://www.autoweek.com/news/a36331077/48-volt-hybrid-syste...

My guess is that handing this out is a very nice thing to do but perhaps irrelevant in a world where 400v and 800v drive systems are the state of the art. Those systems can’t rely on the car body as a common ground for all sources of current and have a lot of inherent safety concerns that force a lot of electrical engineering that my old Camaro didn’t need to have.

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Most automotive parts are 12V so that just means due to economy of scale, 48V parts are more expensive. They are trying to reduce their cost basis by getting everyone to move over to a common, more efficient voltage spec. This is good industry advocacy.

There's a crossover where ROI results in lower wiring costs.

> My guess is that handing this out is a very nice thing to do but perhaps irrelevant in a world where 400v and 800v drive systems are the state of the art.

The high-voltage system for the drive motors, whether 400V or 800V, is unrelated to the low-voltage system for accessories. The transition from 12V to 48V is concerning the low-voltage system. Prior to the Cybertruck, no automobile ever had a fully 48V low-voltage system. To learn more about how this is a big deal, check out Jason Cammisa's video on the subject. I've cued it to the specific topic in the [1] link below.

[1] https://youtu.be/L6WDq0V5oBg?si=dbzsPMAkvmGiQQ8n&t=613

> This is an older article but explains it well

https://www.greencarcongress.com/2017/06/20170602-audi.html

From the article:

> The 12-volt system is connected to the main electrical system via a DC/DC converter

> Audi also offers the new MHEV technology with the conventional 12-volt electrical system

> 48-volt vehicle electrical system. In a different layout—without MHEV—the 48-volt constant voltage system already entered volume production in 2016 as the Audi SQ7 TDI. In this vehicle, the alternator still operates on a 12-volt basis, and a DC converter couples the 48-volt electrical subsystem

> Small consumers such as control units or lights will remain in the 12-volt system well into the future, however

The article makes it very clear that what Audi are doing is not similar to what Tesla have done with the Cybertruck. The Cybertruck has no 12v system, everything runs on 48v. The power seats. The seat heaters. The rear defroster, the interior lights, literally everything. They can have much smaller and lighter wires and use way less copper.

Using a 48V system for a mild hybrid is very common, even Jeep do that.

Realistically, using 12 volt and 48 volt for accessory systems are pretty similar. 48 volts just reduces your current, and you have to design everything for ~56 volts instead of ~14. But it's not really a different exercise.

> 48 volts just reduces your current

"Just" is doing an awful lot of work in that sentence.

Yes, moving from 12v to 48v reduces the current. That means you can have much smaller gauge wires, it means you lose a lot less power ( powerloss = (II)R ), it means you use a lot less copper and it means you save a lot of weight on wiring and connectors.

That is literally the point.

Right, the reason to do it is obvious and the development work is straightforward, it's a step forward but it isn't something to get all breathless about.

Absolutely, Elon said that himself in the recent Munroe interview.

It is just bringing car wiring into the 21st century.

At least in the case of Audi it is more than just the starter used for the mild hybrid depending on market and configuration. On my generation/configuration of car it is also used to prespin the Turbo to avoid lag, it powers the active magnetic suspension, the AC compressor, and the power steering pump (annoyingly located between the feet of the driver and the seat where you can feel it when it runs).

While you’re right on the accessories voltage, In other comments I’ve made the point that I’m not trying to say anything about the merits of 48v accessories. Jim Farley and Elon’s bromance aside, the tech exists within car companies. It’s nice they shared but it’s not like NACS where a real issue is being solved. Problems of driving aren’t solved with 48v interior lights.

Supply lines are a real challenge and why my Audi still has 12v heated seats despite having a 48v system available. The head unit is 12v. I’m guessing the speaker amp is 12v even though 48v would make that less challenging. They have a 12v only model that shares parts and they apparently didn’t want to move everything to 48.

Sure, they use the 48v mild hybrid system to act like the ICE when the ice is not running to power some things that would be on the serpentine belt and need to keep spinning when the ICE is off. It's a slightly more fancy engine start/stop system.

> the tech exists within car companies

Of course. Elon said very clearly in the sandy munroe interview the move to 48v everything is nothing revolutionary, its just bringing vehicle wiring into the 21st century. Anyone could have done it, they just have not been able to pull it off before now.

> They have a 12v only model that shares parts and they apparently didn’t want to move everything to 48.

They couldn't move everything to 48v because their suppliers dictate what they can do, and that prevented them.

> except for an inverter for necessary 12v circuits

Nitpick: That's a DC-DC converter, not an inverter.

Not sure how relevant this is, but curious readers might be interested: In those 400-800 volt systems, the low-voltage system is used to wake up the battery's computer and close a normally-open contactor inside the battery that connects the high-voltage system.

So indeed they might not ground HV stuff to the chassis, and the hundreds-of-volts sources are disconnected if the car is turned off and unplugged.

Exactly. Apart from waking up the vehicle and closing the high voltage connectors, a whole myriad of things run off the low-voltage system, including lights, climate, infotainment and most (if not all) actuators.

Its not that same, that Audi still had a 12V system because all the commodity parts (control units, interior lights) are built to run on 12V. What Tesla are doing is getting rid of 12V entirely so that every little window raising motor or interior light or actuator to move the seats is running on a 48V system.

>and the jumper pins

It can be started through the converter? That takes something like a hundred amps. It must be pretty beefy.

No.

For safety, hybrids and electric vehicles physically disconnect the high-voltage battery from everything when it's off with massive relays. So you can end up in the somewhat stupid situation where your high-voltage battery is fully charged, but you can't do anything because the tiny 12v motorcycle battery is flat.

Starting simply requires enough 12v current to close a relay, the amps going though the "jumper pins" are tiny. And once the relay is closed, the high-voltage battery can keep the relay closed and charge the 12v battery via the dc-dc converter.

The amount of current to "jump" a hybrid or electric car is so small that they really should just install a USB-C cable, so you could just use your phone to jump start it.

That describes the situation where the 12V battery is flat but the HV battery is charged. But what happens if the HV battery is flat on a non-plug in hybrid? Can it be charged from 12V?

Wouldn’t you just start the car if the HV was flat and the LV was charged?

If they’re both flat I agree, I’m curious what you do.

There is no 12v starter motor, it always just uses one of the large motor-generator units to start the engine, and in the Prius drive chain, they require 600v (boosted from 200v from the HV battery).

So if the HV battery is flat, you can't start the engine to charge it.

Wild! Thanks!

Yeah, I started wondering the same thing as I was writing that response, as I do actually own a Lexus with the Prius drive chain.

For starters, it would be pretty hard to get into that state in the first place. My understanding is they actually report a 10% or 20% charged battery as "completely empty". And the it's completely disconnected, so it shouldn't discharge much.

Looking though the service manual, there doesn't even seem to be an error code for "HV battery low", nor any mention of the DC-DC converter being able to charge the HV battery from 12v, it only goes from HV to 12v.

So I really suspect if you do happen to end up with a flat HV battery that's too flat to start the engine, it's just going to flash the "Replace HV battery" error code. Theoretically, someone with the right tools could disassemble the battery and charge the individual modules.

And the only speculation I can find on google is that "flat HV batteries are very rare" and probably only happen when the HV battery has other issues and needs replacement.

Edit: After more googling, apparently Toyota dealers have a special charger called the THS that can charge the HV battery in non-plugin hybrids.

No. Nobody is putting in a 12v to 400v dc to dc converter. There would be no point. I haven’t done the math but I’d be surprised if you got more than a couple miles out of it. If your high voltage battery is completely dead, you’re calling a tow truck.

In the case of my Audi, the starter motor is 48v so the 12v system charges the 48v system enough to start.

In EVs the 12v system is used to power a contactor that connects the EV battery to the drive system. That’s the clicking sound you hear on power on and power off and another contactor can be heard clicking during charging.

My understanding was that nearly every production vehicle on the road is 12V and that Tesla and perhaps Audi as you mentioned are doing things differently. Who is using 400-800V systems? That sounds extremely dangerous.

They're getting mixed up, the 400-800V systems are _only_ for the motors, everything else in the car still uses low voltage.

In fact, afaik every single electric car has two separate systems. There's a high voltage system (the main battery and motor(s)) and the low voltage (almost always 12v) system, usually with a traditional lead-acid 12v battery that is charged via a DC-DC converter connected to the high voltage battery.

Not true in most EVs regarding 'only the motors'. The AC/heat pump compressor is run off the HVDC, typically. Since it runs for long periods of time and consumes kilowatts, you want to run it straight off the main HV battery. Also, any vehicle that still uses resistive heating (older EVs tended to do this, Tesla included) use the HVDC for that.

I’m speaking about the power planes. What they drive is irrelevant from the standpoint that higher voltage systems create safety issues that lower voltage systems do not have. No one is going to be ok with “occasionally being shocked by 48v” or “occasionally seeing arcing in their car from a high voltage system.”

Running 48v to a radio is not an engineering marvel. Running it safely and reliably in an automotive environment full of vibration and weather is.

If you can do that with 400v, I argue you can do it with 48v, so the secret sauce they’re sharing is less relevant than if they did this ten years ago.

I feel like you're mixing up points now, running 48V is fine _because_ the voltage is low enough that it's unlikely to kill anyone if you mess it up. The 400V system in the meantime has a lot more care taken and limiting the systems it works on to ONLY high power systems lessens the number of things that can go wrong.

A ton of trucks are 24V, especially in EU. 48V in automotive industry is something new; I can't remember seeing it on any commercial trucks.

Electric bikes use 12V, 24V, 28V and even 56V. Nobody loves the weight and the cost of thick copper wires.

400V or 800V are charging voltages, because it's the operation with the highest current; to keep it manageable while transferring more energy you need really high voltages. AFAICT, motors inside a Tesla use 320V. I suppose that ICE-electric hybrids also use some reasonably high voltage to feed the electric motors.

Tesla uses 800v only to run the Cybertruck's drive motors. Every other electric load in the truck uses 48v. Both of these are much higher in the Cybertruck than all other Tesla models. All other Tesla models use 400v [IIRC] for the drive motors and 12v for everything else.

Newer builds are using 16v lithium batteries for low voltage.

Pretty sure the AC/heat pump on the Cybertruck runs off the 800V pack. Since it consumes so much power for long periods of time, it makes little sense to run it off the 48V. Existing other Tesla's use the main pack voltage for AC/heat pump.

Good point and makes sense.

Mercedes introduced their 48V hybrid in 2016. Kia in 2018. Peugeot just introduced theirs this year.

Jalopnik was talking about the "48V revolution" in 2017:

https://jalopnik.com/everything-you-need-to-know-about-the-u...

At least once delivered early then? Sharing tech 6 years before their annoncement of sharing it!

Alternative take: Everybody else was aware 48 V networks work as well for decades, there was just no real benefot in switching yet. And Tesla has nothing to do with it.

What's next, Tesla annoncing they will share the revolutionary tech of windscreens and doors?

>Who is using 400-800V systems? That sounds extremely dangerous.

Every electric vehicle. And yes, it is dangerous to work on the HV circuit.

Not all of them. The Kia Niro I’m driving as a rental this week has a 240v battery. Level 3 charging it feels wildly slow compared to my car.

But yes, you’re right the increased voltages add a lot of risk that has to be managed.

And, thankfully, high-voltage systems are extremely well-managed in EVs.

Evidence in support of this claim is that EVs are statistically 4-5 times less likely to catch on fire compared to ICE vehicles.

https://gmauthority.com/blog/2023/11/ev-fires-less-likely-th...

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