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Comment by snthd | Hacker News Reader

>40-60V system

What makes that voltage a better trade off?

Siblings covered why 12 V is annoying, let's talk about why 12 V was chosen in the first place.

6 and 12 V electric systems in vehicles came about because lead-acid batteries made sense in this application, and cetpar. a higher voltage lead-acid battery is overall costlier to manufacture, because it contains more individual cells. Another big thing is that there are many switches and relays in a car, and those often switch significant power. E.g. the light switch for the headlights has to deal with around 200 Watts total for incandescent/halogen headlights, indicators are like 15 Watt each front and back, the starter motor requires a tremendous amount of power, and has a very robust power switch built into it.

All of those switches become much more expensive when you increase the voltage. DC at 12 V and sizable currents is something you can reliably switch with mechanical contacts without costs going through the roof.

Being able to use 48 V for everything in a car is more or less dependent on using silicon switches for everything, not something that was possible in the past. The reason why legacy ICE cars (all ICE cars are now legacy) stick with 12 V is because everything is 12 V, and everything would have to change for the new voltage.

I seem to recall, several years ago (when stopping the engine at traffic lights and restarting as soon as the driver released the brake), at least one company was exploring moving to 24V. However, the effort failed precisely because the lifetime of 24V switches was significantly shorter.

Naturally, I can't find any links to the info...

FWIW Almost no switches in a modern car (turn signals, wipers, door switches) carry a significant amount of current as they are controlled via confuzers.

Just built a car without a single relay. I used PMUs (basically boxes of mosfets and current sensors AFAIK).

wtf is a confuzer? I can find nothing via Google.

slang for 'computer' made popular by Youtuber AvE

Trucks and agriculture equipment already use 24V systems, so the point is moot. Vehicle usage during the lifetime of a truck greatly exceeeds the lifetime of a personal passenger vehicle and agriculture equipment is also exposed to dust and grime, yet both continue to operate normally. So I guess it's more of a cost problem than a component issue.

>All of those switches become much more expensive

Wouldn't a more important question be how does the failure mode change?

Above 30V - 50V DC, contact arcing becomes a huge problem. (It's a problem below that, but less huge.)

With AC, the arc will self-extinguish a half-cycle after the switch opens. With DC there is no cycle, and contacts can be completely vaporised in tens of milliseconds.

Commodity switching components are usually rated "30V DC, 250V AC" for this reason.

It is possible to design switches so that even DC arcs self-extinguish, but the result is expensive and not as reliable as one could wish.

If cars had been invented after 1980, they would probably use solid-state switches except for the very high current circuits such as the starter solenoid and headlights. (Transistors were around a lot earlier than that, but engineers are sensibly cautious about new technologies.)

Edit: To answer your question directly: no. Cost is prime.

>To answer your question directly: no. Cost is prime

I meant to the customer or owner, over the normal lifetime of the vehicle, not to corporate accountants that translate pennies into millions of dollars.

"More expensive" presumably is a small amount per car; the average new car in the US is over $40K.

if we are going to 48V then pretty sure all power switching would use solid state relays/MOSFETs, Any mechanical switch would just be the input to a micro at 5V or lower.

You could also just switch the HV (400V+) bus directly down to 48/60V as all the high current devices motor/ac/power steering etc run off the HV bus and you would just be left with lights, entertainment and servos on the low voltage side. The 12V battery is probably a bit silly when you have a 50KWH main battery and 97% switching conversion efficiency regulators and I am guessing it may disappear in the next 10 years

Do BEVs still use a 12V lead acid battery?

nissan leaf does

That's a bit silly. What's it used for? Couldn't they just use a step down switching power supply that draws current from the main battery?

Yes, my Model Y has a "traditional" 12v lead acid battery. Granted its a small battery as it doesn't have to run any heavy loads (starter motor). Furthermore, it's my understanding that newer Tesla vehicles are being delivered with 12v gel batteries instead of the the lead acid design.

Plenty of older diesel road vehicles like Land Cruisers run 48V for everything. They've been that way since the 80s (maybe 70s).

Note: These were never sold in the North American Market, but they're extremely common in Australia, South Africa, etc.

Power is volts x amps. The more amps, the thicker the wires need to be (volts are irrelvant).

So to minise weight/wire size, use the highest voltage possible (and thus the lowest current). Losses are purely related to current (I^2*R) so the incentive is to squeezee the current (so needing to increases the voltage).

There's a reason the high-voltage overheads are 432 kilovolts (or more); 10 amps at 432kV = 4.3MW (MegaWatts) while 10amps from a stock AC outlet is 1.2KW (KiloWatts). The wire thickness required for both is the same (though the HV wires need to be better insulated, out of the way of crazy fools etc).

So a 60V system for a car carries 1/5 the current of a 12v system, and the wires can have 1/5 the cross-sectional area.

(yes, I know there are caveats when it comes to AC).

Recent TIL:

Wire guage follows a 10log10 of ohms per 1000ft, relative to 0.1 ohms per 1000ft:

0ga .. 0.1 ohms per 1000 ft

10ga .. 1.0 ohms per 1000 ft

20ga .. 10 ohms per 1000 ft

30ga .. 100 ohms per 1000 ft

http://www.interfacebus.com/AWG-table-of-different-wire-gaug...

> The more amps, the thicker the wires need to be (volts are irrelvant).

Nitpick: the wire as a whole includes insulation, which technically does need to be thicker at high voltage (though at 40-60V, and maybe even at 40-60kV, it's probably dominated by tolerances for erosion and abrasion and such).

High voltage power lines use air, which is actually a fairly good insulator per se, but has the problem that wires tend to pass through it on their way to a short circuit if not well-restrained.

Another factor I haven't seen mentioned in the responses yet is safety to human bodies. You (or your kid) can stick a wet finger in a 12V DC charger socket in your car and not get an electric shock, including any of the exposed contacts under the hood, including the battery terminals. But once you're up to 40-60V, the risk of electric shock to humans is actually something that needs to be factored in.

You'll get a shock if wet slightly below 9V. It just stays on your skin. Voltage penetrates dry skin at around 50V, and this is the legal definition of high voltage.

Something I learned around 12 years old, licking the terminals of a 9V battery :-)

40V is still fine IIRC. I think 48V is the threshold.

What amperage are we talking about here? Discussions of electrical safety solely in terms of voltage is nonsense.

Amperage is set by the resistance of the human body. That's in the order of 2kOhm, so at 50V, you get a current of 25mA. That's in the zone where it becomes quite intense for you.

At 12V, we are just talking about 6mA, where you can feel the electricity but it should not be hurting too much.

Thanks, that makes sense.

I'm not the OP nor an electrical engineer, but I believe that you can get higher DC power with less resistance losses using higher voltage and lower amperage. Additionally, this allows the use of thinner wire (lighter, cheaper, easier to package in tight locations).

Other comments pointed out why higher is better. The cap at 60v is somewhat arbitrary, but as others said, higher voltage is harder to switch, and above 60v DC it is easy to kill people.

Voltage levels sometimes arise from the geometry of semiconductors, capacitors, etc. 60V is a common cutoff, usually to give some slack to a 48V design voltage.

Less current draw, thinner cables mainly. 40-60V or higher is also great for electrical motors and power electronic, but anything above 50V is high voltage, so 48V is still safe enough. Cables in a passenger vehicle amount to >4 km and ~30 kg.

I'm an electrical engineer working in an automotive related field.

One of the big issues with 40-60V systems, which have been right around the corner for 20 years now, maybe more, has been that typical relay contacts arc over at about 28V DC. Yes, there are ways to adress this. No, none of them are as practical as relays yet.

This isn't really a fundamental problem. It just means using relays with contacts rated for higher voltages. It's probably the easiest part of the bom on a typical vehicle to move to a higher voltage range. I use 125VDC contact/coil rated relays all the time.

It is a fundamental problem (it's physics). The current rating for relays drops off sharply as the DC voltage increases; you might see 125 V DC rating, but the current will be a fraction of the nominal current at the rated AC voltage. If you're using an relay rated for up to 250 VAC/DC, then the breaking capacity with DC might be as low as 1 % compared to AC. Bad relay datasheets don't mention this at all, or maybe only for 30 V. Good datasheets have plots of capacity vs. voltage and current vs. life (rated capacity of relays is generally for around 100k cycles).

I am an engineer in a field that has been using >100VDC relays for a hundred years. There is a sizable supply chain out there for these things. You can buy them in whatever amperage you need, they just use different construction from typical low frequency AC relays. Wipers are often high quality metallurgy to extend life and they make use of simple passive magnetic snubbers to 'blow out' the arc that is self-extinguishing in AC circuits. They aren't substantially more expensive than regular high quality AC relays and use standard form factors.

Speaking of good relay datasheets, some will also show contact resistance vs current. I recall one which noted that for small signals the resistance was very low, but if higher current was applied, the contact coating would burn off and the resistance rises permanently.

Yep, those are gold-over-silver(-over-nickel-on-contact-alloy) plated contacts. Good for both signal and power switching, as long as you use the same relay only for one of those things.

Thinner wires can be used, which significantly reduces weight, complexity, and cost.

Lower amperage.

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