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Comment by saltcured | Hacker News Reader

You want a higher voltage to allow more power without huge cables due to high current. You want low voltage to avoid high-voltage safety issues which increase engineering, assembly, maintenance, and emergency costs.

The 48V system sits right at the boundary where you can deliver enough power for more auxilliary systems and some mild-hybrid techniques including frequent start/stop systems, brief acceleration boost, and regenerative braking. Yet, it's not high enough to require switching to a different electrical wiring safety paradigm.

This is similar to the reason power-over-ethernet runs at the voltage it does, or "low voltage" lighting exists for home applications where consumers are moving fixtures around and interacting with the wiring.

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From my reading of the article, 48V has been chosen to comfortably fit in under some regulation that defines 60V as “high voltage” which required special (read “more expensive”) components.

Keep in mind there’s no such thing as a “48V battery” or a “60V battery” if you have hard requirements on those voltages. If they’re using lithium polymer batteries, they’re rated at 3.7V per cell nominally, but will be at 4.2 or 4.3V when freshly charged - so a 48V lipo battery will probably be 13 cells and 48.1V nominal, but possibly as high as 55.9V fully charged (and one more cell in a 14 cell pack would hit 60.2V fully charged). Even lead acid batteries do the same thing, your 12V battery today is six 2V nominal cells in series, but you’ll commonly see 14V when it’s fully charged, so four current tech car batteries with a nominal 48V rating will have 56V fully charged.

I don’t think there’s anything “optimal” about 48V apart from it being as high as you can risk going using available battery technologies without bumping into the 60V “high voltage” regulations when your battery is fully charged.

I think it’s 50 VDC that is considered HV. Any equipment we buy at work 50 VDC and over requires NRTL (i.e. UL) certification. Also interlocks and safety boxes when working on circuit.

I have no actual knowledge here beyond the article itself:

“Why 48 volts? Settling on this level holds the electrical system under the 60-volt safety threshold of what is considered high voltage, where power cables must be orange and special connectors, costing 10 times as much, are required, said Mary Gustanski, chief technical officer of Delphi Technologies.”

I wonder if your 50VDC limit is driven by the same “typical peak voltages for common battery chemistry” margin of safety I’m speculating exists?

My understanding is that 50-60VDC is approximately the voltage at which electricity will arc through a person's chest (i.e. pass through their heart) if they hold a wire in each hand. That makes it unsafe to handle without special insulating equipment. That doesn't mean that 48V is safe necessarily though. If your hand is wet or even just a little sweaty, you can arc from pinky to thumb at ~48V.

When I was studying electrical engineering, we were taught that at 30V, current may be able to pass through the skin (depending on a number of conditions presumably). Once it's under the skin, you only need a tiny current through the heart to kill you.

> Yet, it's not high enough to require switching to a different electrical wiring safety paradigm.

This is the tldr. Also once you ditch lead acid batteries for higher voltage hybrid batteries there no reason to keep using 12VDC.

The math bit is that the required wire size is related to the inverse square of the voltage. At the same time reduced current means smaller switches.

> Also once you ditch lead acid batteries for higher voltage hybrid batteries there no reason to keep using 12VDC.

Not really. The operating voltages for most of the actual electronic circuits will still be the usual 3V3/5V/12V - and converting from 12V to 5V/3V3 is easier than from 48V.

The switched loads I was thinking of are lights and motors. So 48V headlights, 48v blower motors, 48v door solenoids, etc. All these draw a fair amount of power and thus a lot of current at 12V.

It's not a total win especially for mechanical switches and relays which tend to be rated by their ability to interrupt power. And you do need higher voltage transistors.

Hmm. Definitely an interesting idea. Downside: manage to e.g. swap a 12V and 48V line and while an 48V component shouldn't be damaged by 12V, a 12V component which does not have a voltage regulator in front will get fried. It's nasty enough with a truck with dual voltage and trailers - a 24V trailer will at least be recognizable as the lights will simply have a dimmer light but a 12V trailer on a 24V truck means hours of swapping every goddamn tiny light bulb. Add a 3rd voltage into the mess and the potential for chaos multiplies...

Thanks. I had always understood PoE's relatively high voltage as helping deal with the attenuation from long runs of thin copper wire.

I couldn't find a succinct explanation in the article why cars would benefit from this spot in between merely sending information (digital levels like 1.8, 2.2, 3.3, 5.0, etc.) and using electricity to do "real" work (120V, 240V, 480V, etc.). As I understand your answer, yes, they'd like to use it for work, but not so much as to stray into the range of orange wires. I got the second part of that from the article, but the first part was very hard to extract from it.

The specific number of 48V is presumably inherited from telecommunications:

https://electronics.stackexchange.com/questions/10851/why-do...

There's lots of telecom equipment that works at 48V, so it's presumably cheap and easy for carmakers to find suppliers.

There is practically no overlap between automotive and telecom. When I was at Tesla, we couldn't find even one manufacturer for common use subsystems (e.g. window motors). It's a chicken and egg problem for now. That's why 'everything' is still at 12V.

Canot say in the US, but in EU most trucks have (since I can remember) 24 V circuits, so there is anyway a long list of parts at 24 V manufactured.

All US military vehicles (even COTS ones with civilian counterparts like the CUCV and newer) use 24v. So do most heavy trucks.

I would have assumed it's just because it's a multiple of the current 12V (so you could use shortcuts like wiring 4 existing 12V lamps in series)

At 48 volts you start running engine accessories off of the electrical bus instead of belts and pulleys. That means the engine isn't necessarily responsible for keeping everything under the hood running at all times. You might not need a water pump or the AC when the engine is off, except to keep the cabin comfortable. But you still need things like power steering and power brakes, stability control and other safety systems.

Also, you start getting economies of scale with all-electric car components that pull down the price of EVs over time, because they can use the same components.

> You might not need [...] the AC when the engine is off

Or you might! Driving a car with a start-stop system in the summer, it doesn't stay stopped at a red light for long until the AC wants to kick in and the engine comes back on.

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