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Many comments here seem to be confusing the accessories circuit voltage with the electrical vehicle motor and drive system. The vehicle motors have a combined p

Many comments here seem to be confusing the accessories circuit voltage with the electrical vehicle motor and drive system. The vehicle motors have a combined power output of 450KW which requires a high voltage 800V system or the wiring would be so heavy it would not be feasible. This spec appears to be the accessories circuit with Tesla pushing for a new 48V standard. That’s super smart because it immediately means you need roughly a quarter of the metal in wiring that you needed previously. It’s cheaper, lighter and more environmentally friendly. And it enables the use of higher power devices. If you’ve ever had to wire your 1000W sub woofer amp directly to your battery you know what I’m talking about. It’s worth noting that newer GA aircraft like Cessna 172 use a 24 volt system. So non 12V systems for accessory power isn’t a totally new idea. And older 172s have 12 volt systems. So the transition was made in that space anyway.

bigdict 3y ago
Dumb question but why does deciding to run at a higher voltage decrease the amount of wiring necessary? And why not increase it even further then?
- kalleboo 3y ago
  Watts = Amps * Volts (or, more scientifically, P = I * V) If something draws 48 Watts of power, you can either supply it with 4 Amps @ 12 Volt, or 1 Amp @ 48 Volt. It's the amps that determine how thick the wiring needs to be, so by lowering the amperage, you lower the amount of wiring needed. I believe the only reason cars are 12 V is because that's a practical voltage to build lead-acid batteries at (which is actually closer to 14 V in practice). Early car electric systems were only 6 V back when the only accessories were the front lights and the horn, but as more stuff was added on cars moved to 12 V.
  - ace2358 3y ago
    Things like stator motors were also probably quite difficult to do at 6v. I know the older Honda CT110 postie bikes came in 6v in the old days. That was a kick start.
  - grepfru_it 3y ago
    Car electrical systems typically run at 13.8-14.4V while the alternator is charging and drops slightly after the battery is topped off. When the engine is off the battery regulates a 12v output
  - loufe 3y ago
    Resistance is proportional to the square of current. This would mean 1/16th the loss instead of 1/4th.
    - 05 3y ago
      Nope. Consumed power is voltage*amperage, and it’s constant, so 4x more voltage means 1/4 the amps. Wire heating is I squared R, so with both I reduced and R increased the losses are just 1/4, not 1/16 of the 12V system. Because you wouldn’t keep the same wire gauges you’ve had in the 12V system, you’d reduce them to actually have some benefits from conversion.
    - ahepp 3y ago
      Isn't the resistance of a wire usually modeled as a constant value? Power dissipated in the wire as heat is proportional to the square of the current. It's equal to the voltage drop times current (P = I * V). Voltage drop is current times resistance (V = I * R). So P = I^2 * R.
    - deleted 3y ago
      [deleted]
  - LoganDark 3y ago
    This makes such little sense to me. I'm probably just stupid, but if the same amount of power is flowing through the wire, don't you need the same size of wire? A higher voltage system will push more energy through the same component, so you need thicker wires. If you up the resistance of the component to consume the same amount of energy as before, you arrive at the same size of wire you were using before, don't you? What exactly is magical about higher voltages that makes them suddenly able to carry more power across the exact same wire? I know you just said it's "lower amperage" but I don't understand how amperage can be the only thing dictating the required size of a wire.
    - hansvm 3y ago
      A higher voltage system enables the same energy at a lower amperage, so constrained to wanting to do a particular thing (loud stereo, bright headlights, ...), higher voltages result in lower amperages. I'll opt to not try to explain that intuitively, hoping that somebody else chimes in. For the other half, "why do amps determine wire size?" Amps are coulombs per second -- how much electrical stuff moves through a cross section in a second. The amount of heat produced is directly proportional to amperage (every bit of movement has a chance to whack into something and produce heat, and more movement per second or more seconds generates more heat). Wire size is ultimately constrained by being able to release that heat while still acting like a wire (not melting, not sublimating, still conducting, not setting anything else on fire, ...). The amount of heat released is proportional to surface area (proportional to the square root of diameter, but that difference doesn't matter a ton right now), and the amount of heat produced is proportional to amperage and approximately nothing else. The point where those two terms are equal is the limit for the system (engineers add in huge safety factors to account for conduit and insulation and other imperfections), above which your wire melts and below which it behaves reasonably. Higher amperages require higher surface area to release the extra heat (so much higher thicknesses), so a given amount of power (stuff you want your electrical system to do) requires lower amps or thicker wires to work appropriately, because you can achieve the same power by increasing the voltage on the same amperage, thereby solving your power problems without extra heat.
      - hansvm 3y ago
- dgfitz 3y ago
  Higher the voltage the lower the amp draw, so smaller wires can be used.
- pertymcpert 3y ago
  For your first question, power = voltage x current. To transmit the same power, if you double the voltage you only need half the current. This is important because the resistance losses in conductors are related to the current not voltage, so you can use thinner wires without overheating.
- maxerickson 3y ago
  60 volts DC is a safety threshold. You start to move current through tissue at higher voltages.
  - exclusiv 3y ago
    Yes but state of your body/skin (R) and current (I) is what matters with regard to 60 volts being really dangerous (also the current path as the article below states). 60 volts is nothing UNLESS you are completely wet or sweaty. "It is estimated at 150 ohms for completely wet skin (in water), 1000 ohms for sweaty skin, and 100,000 ohms to 500,000 ohms for dry skin." Assuming a worst-case scenario with dry skin providing a resistance of 100,000 ohms, fatality becomes a possibility if the current exceeds 50 mAmp. Therefore, the lethal voltage would be above 0.05 (50 mAmp)×100,000=5000 Volts. [1] So if you are wet or sweaty, it could be 7.5V to 50V that gets dangerous. So it makes sense why 60 volts is a safety threshold, especially for those that live in Florida or Arizona. [1] https://www.scienceabc.com/humans/how-many-volts-amps-kill-you-human.html https://www.scienceabc.com/humans/how-many-volts-amps-kill-y...
    - SV_BubbleTime 3y ago
      > 60 volts is nothing UNLESS you are completely wet or sweaty. Ha, it’s always easy to tell when people have experience with these things. Doorbell wiring is 48V. Go hold those in each hand and tell me how impossible it is to feel if your hands are dry.
      - sokoloff 3y ago
        Doorbell voltage in the US is typically 12V or 24V AC in the US. I can’t feel that with dry hands. You might be confused with POTS (landline phone service) which is 48V DC with all phones not in use.
        nomel 3y ago
        Just don't be holding on when it rings! That's painful, with 90 VRMS at 20 Hz.
- stephen_g 3y ago
  48V is good because a lot of safety standards have the cut off for 'extra-low voltage' at 60V DC, gives you a bit of margin (when battery charging it pushes up to be in the 50s for example). Some battery systems I've seen at 56 volts nominal too. so sometimes people do have a bit higher-voltage ELV systems. 48V still lets you touch both conductors with dry hands and it's still very unlikely for you to be able to get any current to flow through you, you can't even feel it (although with safe working practices you'd only touch either positive or negative at one time, not both). Obviously it's very much not the case to be able to touch the 800V conductors safely in the traction systems in EVs, that side is extremely dangerous and requires extreme caution and safety procedures!
  - formerly_proven 3y ago
    If you can't feel 48 V with your hands I'd recommend moisturizer, because you have very dry skin.
  - jeffrallen 3y ago
    > requires extreme caution and safety procedures As long as the battery pack is still sealed and the interlock systems are undamaged and working correctly, it's no big deal. You follow the manufacturer's instructions to make it safe, then work on it like there's no voltage present. If the battery pack is broken or a prototype or whatever, yeah, then you need to think things through carefully before doing them. And have a plan for what you're going to do when things go wrong. Source: I work in electric aviation and work on battery packs.
    - sokoloff 3y ago
      GP was talking about the low voltage system (the 12-volt system in most cars) and choosing a voltage for it, not anything about the traction battery.
      - carstenhag 3y ago
        No, they were comparing the difference in precautions between 12/48v accessory battery and the 400/800V traction battery.
- fastball 3y ago
  You've already gotten a bunch of answers but to be honest I find all of them a little incomplete if you don't have any electrical background, so here is my attempt to be quite thorough from (almost) first principles. --- The equation for power delivered by an ideal system is P = IV, or power (P) = current (I) * voltage (V). Power is measured in watts, and that is generally the overall number that matters, in terms of what you can run off of your system at the same time. So to increase the wattage (power) of your system you can either increase the voltage or the current. - Increasing the voltage of a system increases the amount of resistance it can "break through". In "danger to human" terms, our skin is generally not a great conductor, so voltages lower than 50V usually won't enter the body (read: vital organs) at all. Voltages above 50V will start to enter the body depending on conditions, which is when electricity becomes much more dangerous. - However even if the voltage is high enough to enter the body, if the current (I) isn't very high it still won't be dangerous. Current is measured in Amperes (A), and the usual number at which a current inside the body becomes dangerous is above 30mA. 30mA can cause respiratory failure if it passes through the lungs, current as low as 100mA can cause cardiac arrest if it passes through the heart. In a car's electrical system, the currents we're operating with are definitely going to be over the 30mA threshold we just established, so we want to keep the voltage under 50V instead. Anyway back to cars and ignoring danger to humans for a second. Resistance is the main thing you want to overcome when it comes to the efficiency of such a system. The equation for power loss is P = I^2 * R, or "power dissipated (as heat) is proportional to the square of the current times the resistance". So if you increase the current of your system (in order to deliver more watts) you will also increase the amount of power you lose as heat. You can decrease the loss by decreasing the Resistance. The equation for resistance through a material is: R = ρ * (L/A), or Resistance = resistivity (an innate property of the material) * Length / Area. In other words, the longer your wire is the more current you will lose to resistance. But if you increase the cross-sectional area of your wire (A) by making the wires thicker, you decrease the resistance. So in short: if you have a high current (amperage) system, you use thicker wires in order to ameliorate resistive heat loss. But you can alternatively just decrease the amperage to reduce your resistive heat loss, which means thinner (and therefore much lighter) wires. But then you need to increase the voltage of your system in order to offset the power you're losing by decreasing the current. If you increase the voltage by 4x, from 12V to to 48V, this keeps it under the human danger zone (of 50-60V) and means your wires can be up to 16x thinner, taking up less space and less weight. Having it be a nice multiple of the previous system (4x) should make upgrading the relevant circuits a little more straightforward as well.
- elif 3y ago
  The reason to not keep going higher is purely practicality because stepping down bigger voltage gaps for lower voltage applications begins to become less efficient/more waste heat. If you design 100v computers, monitor, headlights, seat adjustment motors, window/windshield motors, pumps, etc. you could cut these 48v wires in half again but if you end up stepping everything down anyway it loses its utility. The safety rules about voltage and the human body are often poorly overstated, as is the addage that 'its the amps that kill you, not the volts.' in reality it takes a whole lot of both [0]. As further research, here[1] is styropyro touching 2 contacts which have more current than the largest bolts of lightning (enough to almost instantly vaporize a crowbar) with his bare hands, but because it is 12V it is not able to pass through him. [0] https://youtu.be/BGD-oSwJv3E https://youtu.be/BGD-oSwJv3E [1] https://youtu.be/ywaTX-nLm6Y https://youtu.be/ywaTX-nLm6Y
  - ianlevesque 3y ago
    Just don't try that with wet hands.
    - Dylan16807 3y ago
      Even with wet hands it's hard to get much out of 12v.
- injb 3y ago
  As an analogy, if you have a hose squirting out water at a certain rate, and you increase the pressure, then all other things being equal you'll get a higher rate of water. But then that means you could now reduce the size of the hose, keeping the new higher pressure, and achieve the original rate of water delivery with a smaller, cheaper, lighter hose. It's very similar with electricy: water pressure is voltage and the rate of water delivery is power.
- the_gipsy 3y ago
  Bad analogy: for the electrons, Volts are (inverse) latency, and wire thickness is maximum bandwidth (or max amp). That is, make electrons go faster, instead of pushing more electrons at the same time. Of course none of this really makes sense, for example you can't increase wire thickness to increase amps etc.
- deleted 3y ago
  [deleted]
hatsunearu 3y ago
A lot of commercial trucks use 24V systems also.
maxerickson 3y ago
Stuff like buses have used 24 volt for ages. Basically anywhere with larger accessory loads. 48 volt has been 'on the radar' in automotive spaces for a couple decades.
DannyBee 3y ago
48V has been considered for automotive for decades. The problem was not know how. It simply has never been worth it before to retool things in practice. It's also electrically a lot more noisy. The limit of what is considered low volt is like 60v or something.
- klysm 3y ago
  Why is it inherently noisier?
  - horacemorace 3y ago
    Faster slew rate for all switch supplies in the system. Nothing directly runs 48v.
    - klysm 3y ago
      I’m still skeptical. Why can’t it be filtered out
      - yetihehe 3y ago
        It can, it just requires up to a dollar more per device.
    - ferongr 3y ago
      Nothing "directly" runs on 12V either. All electronics will have stepdown converters in both the 12V and 48V use case. Motors, solenoids and incandescent bulbs can be made to work with either voltage (the same way we do for 24V systems in heavy goods vehicles here in Europe).
      - hulitu 3y ago
        > Nothing "directly" runs on 12V either. Motors, bulbs, relays.
    - bsder 3y ago
rkagerer 3y ago
48V is also used on some boats (eg. certain trolling motors)
user_named 3y ago
"super smart" - more like completely obvious to everyone for decades
- szundi 3y ago
  That’s what Elon said, no innovation, just made the step
  - avalys 3y ago
    [flagged]
    - panick21_ 3y ago
      As usual whenever Elon is involved people turn of their brains just so they can not admit that Elon or his companies did something well. Nobody says this is the first time V48 has ever been used for anything. That literally just missing the point. The point is that the whole architecture of the car will use PoE 48v. And if you go and take apart these Mercedes cars you will see that the waste majority of all of the devices and the wiring use the exact same 12v architecture that all the ICE cars also use.
    - davedx 3y ago
      That seems to be the engine not the entire electrical loom?
    - memish 3y ago
      He's an engineeer that worked through this with other engineers from first principles. You're spreading bullshit with 2 seconds of effort.
      - margalabargala 3y ago
        > He's an engineer that worked through this with other engineers Who? Because you can't mean Elon, not if your objection is to people spreading bullshit...
hulitu 3y ago
> Tesla pushing for a new 48V standard. That’s super smart 48V is a dangerous voltage. Better not touch any wire.
teknico 3y ago
> 1000W sub woofer amp Your car subwoofer is surely not so stiff that it needs so much power. But after using it, your ears will definitely be.
MrBuddyCasino 3y ago
48V is also currently used on the supporting electric traction motor of "Mild Hybrid" cars.
lmpdev 3y ago
I get this a lot I work at an electronics store in Australia This week I had a customer dumbfounded why his 3,500W inverter almost caught fire He was using 8-gauge wire (~56A max) He needed 00-gauge (almost 300A max) If he’d have run the system at 48V instead of 12V he’d have saved $200+ per metre on cabling his inverter My advice on voltage selection for a vehicle is once you’re needing more than a few metres of 0-gauge, you should possibly increase the voltage of the system High current cabling means expensive ANL, mega or other high performance fuses or other expensive circuit protection methods I strongly believe that 48V systems are inherently safer than 12V systems when it comes to the public doing DIY work
- hef19898 3y ago
  Good look doing DIY work on the 48V network of a modern car, ICE or EV or 12, 24 or 48 V doesn't matter. And the past 12 V systems are dead easy to work on. On older cars that is, as soon as you have electronics and bus systems DIY basically requires deep car electronics knowledge and skills.
  - jakewins 3y ago
    Do you have examples of what kind of DIY work is hard? I haven’t done a ton of work on newer cars, but at least adding wiring for a hitch on my Kona EV was just as easy as any wiring I’ve done on my old truck. Is a job like adding amps and better speakers trickier?
    - RamblingCTO 3y ago
      I have a defender and added a bunch of stuff to it: cruise control, motor pre-heater, rear camera, working lights in the rear. I'll also add a diesel heater and more electrics at some point. On 12V it's easy, can't imagine doing work like that with 48V. Not sure how this works, and if you maybe just can use 12V anyway. But wanted to make the point of camper vans and overlanders and offroaders.
      - hef19898 3y ago
Panzer04 3y ago
Given the choice almost any EE would probably pick the higher voltage system for cost saving reasons, but I imagine they were mostly constrained by legacy compatibility requirements here. It’a “smart”, but there are probably reasons it hasn’t already changed everywhere else.
- grecy 3y ago
  > but there are probably reasons it hasn’t already changed everywhere else Yes, the legacy OEMs are constrained by their suppliers, who dictate what they can (not) do.
  - hef19898 3y ago
    In automotive, it is usually the other way round. Regardless, what exactly was wrong with the existing 12 and 24 V systems for auxiliaries that absolutely needed to be changed?
    - mpreda 3y ago
      High current (amperage) requiring thicker wires, or more wires, thicker/heavier connections, more expensive switches, more cooling etc.
    - grecy 3y ago
      The move to 48V means much less power is lost (power loss = (IxI)xR ), cables can be much smaller, cables and terminals can be much lighter and much less copper can be used used. Basically it's just a more efficient way of doing things. Elon has said it's not a revolution, or ground breaking in any way, it is simply a step improvement. Anyone could have done it. Tesla did it.
      - hef19898 3y ago
        As others pointed out lready, others did it, too.
        3y ago
kwhitefoot 3y ago
Do Tesla have 800 V batteries yet? I thought they were still using 400 V for all the cars.
- crishoj 3y ago
  Cybertruck is the first Tesla vehicle with 800v architecture. The S3XY lineup uses 400v.
coffeeblack 3y ago
Musk did an interview with Sandy Munro about it. Worth watching.
cashsterling 3y ago
And FWIW... 24V is very common for DC circuits in industrial control systems. 48V is used too but less common; however, 48V is very common in telecom.
merb 3y ago
up till now most ev‘s used 400v there are some with 800v and more premium models do get 800v

bigdict 3y ago

Dumb question but why does deciding to run at a higher voltage decrease the amount of wiring necessary? And why not increase it even further then?

kalleboo 3y ago
Watts = Amps * Volts (or, more scientifically, P = I * V) If something draws 48 Watts of power, you can either supply it with 4 Amps @ 12 Volt, or 1 Amp @ 48 Volt. It's the amps that determine how thick the wiring needs to be, so by lowering the amperage, you lower the amount of wiring needed. I believe the only reason cars are 12 V is because that's a practical voltage to build lead-acid batteries at (which is actually closer to 14 V in practice). Early car electric systems were only 6 V back when the only accessories were the front lights and the horn, but as more stuff was added on cars moved to 12 V.
- ace2358 3y ago
  Things like stator motors were also probably quite difficult to do at 6v. I know the older Honda CT110 postie bikes came in 6v in the old days. That was a kick start.
- grepfru_it 3y ago
  Car electrical systems typically run at 13.8-14.4V while the alternator is charging and drops slightly after the battery is topped off. When the engine is off the battery regulates a 12v output
- loufe 3y ago
  Resistance is proportional to the square of current. This would mean 1/16th the loss instead of 1/4th.
  - 05 3y ago
    Nope. Consumed power is voltage*amperage, and it’s constant, so 4x more voltage means 1/4 the amps. Wire heating is I squared R, so with both I reduced and R increased the losses are just 1/4, not 1/16 of the 12V system. Because you wouldn’t keep the same wire gauges you’ve had in the 12V system, you’d reduce them to actually have some benefits from conversion.
  - ahepp 3y ago
    Isn't the resistance of a wire usually modeled as a constant value? Power dissipated in the wire as heat is proportional to the square of the current. It's equal to the voltage drop times current (P = I * V). Voltage drop is current times resistance (V = I * R). So P = I^2 * R.
  - deleted 3y ago
    [deleted]
- LoganDark 3y ago
  This makes such little sense to me. I'm probably just stupid, but if the same amount of power is flowing through the wire, don't you need the same size of wire? A higher voltage system will push more energy through the same component, so you need thicker wires. If you up the resistance of the component to consume the same amount of energy as before, you arrive at the same size of wire you were using before, don't you? What exactly is magical about higher voltages that makes them suddenly able to carry more power across the exact same wire? I know you just said it's "lower amperage" but I don't understand how amperage can be the only thing dictating the required size of a wire.
  - hansvm 3y ago
    A higher voltage system enables the same energy at a lower amperage, so constrained to wanting to do a particular thing (loud stereo, bright headlights, ...), higher voltages result in lower amperages. I'll opt to not try to explain that intuitively, hoping that somebody else chimes in. For the other half, "why do amps determine wire size?" Amps are coulombs per second -- how much electrical stuff moves through a cross section in a second. The amount of heat produced is directly proportional to amperage (every bit of movement has a chance to whack into something and produce heat, and more movement per second or more seconds generates more heat). Wire size is ultimately constrained by being able to release that heat while still acting like a wire (not melting, not sublimating, still conducting, not setting anything else on fire, ...). The amount of heat released is proportional to surface area (proportional to the square root of diameter, but that difference doesn't matter a ton right now), and the amount of heat produced is proportional to amperage and approximately nothing else. The point where those two terms are equal is the limit for the system (engineers add in huge safety factors to account for conduit and insulation and other imperfections), above which your wire melts and below which it behaves reasonably. Higher amperages require higher surface area to release the extra heat (so much higher thicknesses), so a given amount of power (stuff you want your electrical system to do) requires lower amps or thicker wires to work appropriately, because you can achieve the same power by increasing the voltage on the same amperage, thereby solving your power problems without extra heat.
    - hansvm 3y ago
dgfitz 3y ago
Higher the voltage the lower the amp draw, so smaller wires can be used.
pertymcpert 3y ago
For your first question, power = voltage x current. To transmit the same power, if you double the voltage you only need half the current. This is important because the resistance losses in conductors are related to the current not voltage, so you can use thinner wires without overheating.
maxerickson 3y ago
60 volts DC is a safety threshold. You start to move current through tissue at higher voltages.
- exclusiv 3y ago
  Yes but state of your body/skin (R) and current (I) is what matters with regard to 60 volts being really dangerous (also the current path as the article below states). 60 volts is nothing UNLESS you are completely wet or sweaty. "It is estimated at 150 ohms for completely wet skin (in water), 1000 ohms for sweaty skin, and 100,000 ohms to 500,000 ohms for dry skin." Assuming a worst-case scenario with dry skin providing a resistance of 100,000 ohms, fatality becomes a possibility if the current exceeds 50 mAmp. Therefore, the lethal voltage would be above 0.05 (50 mAmp)×100,000=5000 Volts. [1] So if you are wet or sweaty, it could be 7.5V to 50V that gets dangerous. So it makes sense why 60 volts is a safety threshold, especially for those that live in Florida or Arizona. [1] https://www.scienceabc.com/humans/how-many-volts-amps-kill-you-human.html https://www.scienceabc.com/humans/how-many-volts-amps-kill-y...
  - SV_BubbleTime 3y ago
    > 60 volts is nothing UNLESS you are completely wet or sweaty. Ha, it’s always easy to tell when people have experience with these things. Doorbell wiring is 48V. Go hold those in each hand and tell me how impossible it is to feel if your hands are dry.
    - sokoloff 3y ago
      Doorbell voltage in the US is typically 12V or 24V AC in the US. I can’t feel that with dry hands. You might be confused with POTS (landline phone service) which is 48V DC with all phones not in use.
      - nomel 3y ago
        Just don't be holding on when it rings! That's painful, with 90 VRMS at 20 Hz.
stephen_g 3y ago
48V is good because a lot of safety standards have the cut off for 'extra-low voltage' at 60V DC, gives you a bit of margin (when battery charging it pushes up to be in the 50s for example). Some battery systems I've seen at 56 volts nominal too. so sometimes people do have a bit higher-voltage ELV systems. 48V still lets you touch both conductors with dry hands and it's still very unlikely for you to be able to get any current to flow through you, you can't even feel it (although with safe working practices you'd only touch either positive or negative at one time, not both). Obviously it's very much not the case to be able to touch the 800V conductors safely in the traction systems in EVs, that side is extremely dangerous and requires extreme caution and safety procedures!
- formerly_proven 3y ago
  If you can't feel 48 V with your hands I'd recommend moisturizer, because you have very dry skin.
- jeffrallen 3y ago
  > requires extreme caution and safety procedures As long as the battery pack is still sealed and the interlock systems are undamaged and working correctly, it's no big deal. You follow the manufacturer's instructions to make it safe, then work on it like there's no voltage present. If the battery pack is broken or a prototype or whatever, yeah, then you need to think things through carefully before doing them. And have a plan for what you're going to do when things go wrong. Source: I work in electric aviation and work on battery packs.
  - sokoloff 3y ago
    GP was talking about the low voltage system (the 12-volt system in most cars) and choosing a voltage for it, not anything about the traction battery.
    - carstenhag 3y ago
      No, they were comparing the difference in precautions between 12/48v accessory battery and the 400/800V traction battery.
fastball 3y ago
You've already gotten a bunch of answers but to be honest I find all of them a little incomplete if you don't have any electrical background, so here is my attempt to be quite thorough from (almost) first principles. --- The equation for power delivered by an ideal system is P = IV, or power (P) = current (I) * voltage (V). Power is measured in watts, and that is generally the overall number that matters, in terms of what you can run off of your system at the same time. So to increase the wattage (power) of your system you can either increase the voltage or the current. - Increasing the voltage of a system increases the amount of resistance it can "break through". In "danger to human" terms, our skin is generally not a great conductor, so voltages lower than 50V usually won't enter the body (read: vital organs) at all. Voltages above 50V will start to enter the body depending on conditions, which is when electricity becomes much more dangerous. - However even if the voltage is high enough to enter the body, if the current (I) isn't very high it still won't be dangerous. Current is measured in Amperes (A), and the usual number at which a current inside the body becomes dangerous is above 30mA. 30mA can cause respiratory failure if it passes through the lungs, current as low as 100mA can cause cardiac arrest if it passes through the heart. In a car's electrical system, the currents we're operating with are definitely going to be over the 30mA threshold we just established, so we want to keep the voltage under 50V instead. Anyway back to cars and ignoring danger to humans for a second. Resistance is the main thing you want to overcome when it comes to the efficiency of such a system. The equation for power loss is P = I^2 * R, or "power dissipated (as heat) is proportional to the square of the current times the resistance". So if you increase the current of your system (in order to deliver more watts) you will also increase the amount of power you lose as heat. You can decrease the loss by decreasing the Resistance. The equation for resistance through a material is: R = ρ * (L/A), or Resistance = resistivity (an innate property of the material) * Length / Area. In other words, the longer your wire is the more current you will lose to resistance. But if you increase the cross-sectional area of your wire (A) by making the wires thicker, you decrease the resistance. So in short: if you have a high current (amperage) system, you use thicker wires in order to ameliorate resistive heat loss. But you can alternatively just decrease the amperage to reduce your resistive heat loss, which means thinner (and therefore much lighter) wires. But then you need to increase the voltage of your system in order to offset the power you're losing by decreasing the current. If you increase the voltage by 4x, from 12V to to 48V, this keeps it under the human danger zone (of 50-60V) and means your wires can be up to 16x thinner, taking up less space and less weight. Having it be a nice multiple of the previous system (4x) should make upgrading the relevant circuits a little more straightforward as well.
elif 3y ago
The reason to not keep going higher is purely practicality because stepping down bigger voltage gaps for lower voltage applications begins to become less efficient/more waste heat. If you design 100v computers, monitor, headlights, seat adjustment motors, window/windshield motors, pumps, etc. you could cut these 48v wires in half again but if you end up stepping everything down anyway it loses its utility. The safety rules about voltage and the human body are often poorly overstated, as is the addage that 'its the amps that kill you, not the volts.' in reality it takes a whole lot of both [0]. As further research, here[1] is styropyro touching 2 contacts which have more current than the largest bolts of lightning (enough to almost instantly vaporize a crowbar) with his bare hands, but because it is 12V it is not able to pass through him. [0] https://youtu.be/BGD-oSwJv3E https://youtu.be/BGD-oSwJv3E [1] https://youtu.be/ywaTX-nLm6Y https://youtu.be/ywaTX-nLm6Y
- ianlevesque 3y ago
  Just don't try that with wet hands.
  - Dylan16807 3y ago
    Even with wet hands it's hard to get much out of 12v.
injb 3y ago
As an analogy, if you have a hose squirting out water at a certain rate, and you increase the pressure, then all other things being equal you'll get a higher rate of water. But then that means you could now reduce the size of the hose, keeping the new higher pressure, and achieve the original rate of water delivery with a smaller, cheaper, lighter hose. It's very similar with electricy: water pressure is voltage and the rate of water delivery is power.
the_gipsy 3y ago
Bad analogy: for the electrons, Volts are (inverse) latency, and wire thickness is maximum bandwidth (or max amp). That is, make electrons go faster, instead of pushing more electrons at the same time. Of course none of this really makes sense, for example you can't increase wire thickness to increase amps etc.
deleted 3y ago
[deleted]

hatsunearu 3y ago

A lot of commercial trucks use 24V systems also.

maxerickson 3y ago

Stuff like buses have used 24 volt for ages. Basically anywhere with larger accessory loads. 48 volt has been 'on the radar' in automotive spaces for a couple decades.

DannyBee 3y ago

48V has been considered for automotive for decades. The problem was not know how. It simply has never been worth it before to retool things in practice. It's also electrically a lot more noisy. The limit of what is considered low volt is like 60v or something.

klysm 3y ago
Why is it inherently noisier?
- horacemorace 3y ago
  Faster slew rate for all switch supplies in the system. Nothing directly runs 48v.
  - klysm 3y ago
    I’m still skeptical. Why can’t it be filtered out
    - yetihehe 3y ago
      It can, it just requires up to a dollar more per device.
  - ferongr 3y ago
    Nothing "directly" runs on 12V either. All electronics will have stepdown converters in both the 12V and 48V use case. Motors, solenoids and incandescent bulbs can be made to work with either voltage (the same way we do for 24V systems in heavy goods vehicles here in Europe).
    - hulitu 3y ago
      > Nothing "directly" runs on 12V either. Motors, bulbs, relays.
  - bsder 3y ago

rkagerer 3y ago

48V is also used on some boats (eg. certain trolling motors)

user_named 3y ago

"super smart" - more like completely obvious to everyone for decades

szundi 3y ago
That’s what Elon said, no innovation, just made the step
- avalys 3y ago
  [flagged]
  - panick21_ 3y ago
    As usual whenever Elon is involved people turn of their brains just so they can not admit that Elon or his companies did something well. Nobody says this is the first time V48 has ever been used for anything. That literally just missing the point. The point is that the whole architecture of the car will use PoE 48v. And if you go and take apart these Mercedes cars you will see that the waste majority of all of the devices and the wiring use the exact same 12v architecture that all the ICE cars also use.
  - davedx 3y ago
    That seems to be the engine not the entire electrical loom?
  - memish 3y ago
    He's an engineeer that worked through this with other engineers from first principles. You're spreading bullshit with 2 seconds of effort.
    - margalabargala 3y ago
      > He's an engineer that worked through this with other engineers Who? Because you can't mean Elon, not if your objection is to people spreading bullshit...

hulitu 3y ago

> Tesla pushing for a new 48V standard. That’s super smart 48V is a dangerous voltage. Better not touch any wire.

teknico 3y ago

> 1000W sub woofer amp Your car subwoofer is surely not so stiff that it needs so much power. But after using it, your ears will definitely be.

MrBuddyCasino 3y ago

48V is also currently used on the supporting electric traction motor of "Mild Hybrid" cars.

lmpdev 3y ago

I get this a lot I work at an electronics store in Australia This week I had a customer dumbfounded why his 3,500W inverter almost caught fire He was using 8-gauge wire (~56A max) He needed 00-gauge (almost 300A max) If he’d have run the system at 48V instead of 12V he’d have saved $200+ per metre on cabling his inverter My advice on voltage selection for a vehicle is once you’re needing more than a few metres of 0-gauge, you should possibly increase the voltage of the system High current cabling means expensive ANL, mega or other high performance fuses or other expensive circuit protection methods I strongly believe that 48V systems are inherently safer than 12V systems when it comes to the public doing DIY work

hef19898 3y ago
Good look doing DIY work on the 48V network of a modern car, ICE or EV or 12, 24 or 48 V doesn't matter. And the past 12 V systems are dead easy to work on. On older cars that is, as soon as you have electronics and bus systems DIY basically requires deep car electronics knowledge and skills.
- jakewins 3y ago
  Do you have examples of what kind of DIY work is hard? I haven’t done a ton of work on newer cars, but at least adding wiring for a hitch on my Kona EV was just as easy as any wiring I’ve done on my old truck. Is a job like adding amps and better speakers trickier?
  - RamblingCTO 3y ago
    I have a defender and added a bunch of stuff to it: cruise control, motor pre-heater, rear camera, working lights in the rear. I'll also add a diesel heater and more electrics at some point. On 12V it's easy, can't imagine doing work like that with 48V. Not sure how this works, and if you maybe just can use 12V anyway. But wanted to make the point of camper vans and overlanders and offroaders.
    - hef19898 3y ago

Panzer04 3y ago

Given the choice almost any EE would probably pick the higher voltage system for cost saving reasons, but I imagine they were mostly constrained by legacy compatibility requirements here. It’a “smart”, but there are probably reasons it hasn’t already changed everywhere else.

grecy 3y ago
> but there are probably reasons it hasn’t already changed everywhere else Yes, the legacy OEMs are constrained by their suppliers, who dictate what they can (not) do.
- hef19898 3y ago
  In automotive, it is usually the other way round. Regardless, what exactly was wrong with the existing 12 and 24 V systems for auxiliaries that absolutely needed to be changed?
  - mpreda 3y ago
    High current (amperage) requiring thicker wires, or more wires, thicker/heavier connections, more expensive switches, more cooling etc.
  - grecy 3y ago
    The move to 48V means much less power is lost (power loss = (IxI)xR ), cables can be much smaller, cables and terminals can be much lighter and much less copper can be used used. Basically it's just a more efficient way of doing things. Elon has said it's not a revolution, or ground breaking in any way, it is simply a step improvement. Anyone could have done it. Tesla did it.
    - hef19898 3y ago
      As others pointed out lready, others did it, too.
      - 3y ago

kwhitefoot 3y ago

Do Tesla have 800 V batteries yet? I thought they were still using 400 V for all the cars.

crishoj 3y ago
Cybertruck is the first Tesla vehicle with 800v architecture. The S3XY lineup uses 400v.

coffeeblack 3y ago

Musk did an interview with Sandy Munro about it. Worth watching.

cashsterling 3y ago

And FWIW... 24V is very common for DC circuits in industrial control systems. 48V is used too but less common; however, 48V is very common in telecom.

merb 3y ago

up till now most ev‘s used 400v there are some with 800v and more premium models do get 800v