what is the voltage system for the Rivian compared to Tesla?

ajdelange

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Let's look at a pair of 3 volt batteries powering a 3 volt motor rated at 6 watts. It draws 2 A from the batteries which we would connect in parralell. Each would clearly supply 1 amp. If the battery impedance is 0.1 Ω then I^2R losses in the battery would be 0.1 W each for a total of 0.2 W. If the motor resistance is 0.15 Ω then the I^2R losses in the motor would be .15*2*2 = 0.6 W. Now let's rewind the motor for 6 volts. It would require twice the length of wire and if we use the same size wire the resistance will double to 0.3 Ω but the motor will only draw 1 ampere from a 6 V supply and the losses in this new motor will be 0.3*1*1 = 0.3 W or half what they were in the 3 V motor. But our wire cost has doubled and the motor is more expensive so we would very probably go to a smaller wire size. One with twice the resistance per unit length would result in wire resistance of 0.6 Ω which, with 1 amp current would dissipate 0.6W which is where we were with the 3 V motor. So with respect to motors there is a potential advantage with respect to loss depending on what the designer is willing to spend on copper.

In the batteries deliver 3 watts each whether they are connected in series or parallel so each is delivering 1 A in either configuration and the internal loss is therefore 0.1 W each for a total of 0.2. It's the same with charging.

In the interconnection between batteries and motors if you keep the same amount of copper the current drops in half and the power lost goes down to a quarter. So obviously you are going to look at thinner wire. unlike the case with the motor you don't have to increase the wire length so if you go to wire of half the cross section (with double the resistance you wind up loss reduced by a factor of (2)(1/4) = 1/2. Now I saw a blurb for one 900 volt advocate (can't remember which one) saying that this would save 60 lbs of copper per car. That is appreciable.

Finally we have the inverters. A transistor switching 1 amp will dissipate 1/4 of the heat dissipated by a transistor switching 2. That is a substantial reduction, of course, and could have significant implication on the cooling system. But it would require semiconductors with PIV tolerance twice what the 400 V designs need.

And I guess we need to mention the chargers. The manufacturers want to charge at rates that require so much copper in the cables that they become too unwieldy. The solution (which has been used) of liquid cooling the cables isn't too appealing. Doubling the voltage means, with wire half the size, half the power dissipation. There is clear advantage there.
 

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Thanks. That helps me understand the debate a bit more.

Lucid is going to use MOSFETs in their inverters. Does that suggest any advantage arising from the higher voltage?
 

ajdelange

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Not in and of itself. Several transistor types have been used over the years. What seems to be all the rage these days is the Silicon Carbide MOSFET which can evidently handle voltages pushing 2 kV and handles heat well,
 

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Yep, Lucid mentions silicon carbide specifically. Thanks again.
 

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From an article on the Lucid Air, "The CEO expects the Air to create a new standard for efficiency with 4 to 5 miles of range per kWh for a full-size luxury sedan." The thing is if they launch at 4, that is on par with Tesla's latest products. But from what I've seen so far, it appears they may only launch with a 100 kWh battery good for 300 miles or 130 kWh battery good for 400 miles. This is nowhere close to the 4-5 mi/kWh mentioned by their CEO. It will be interesting to see what they actually deliver. Has anyone seen any more recent specs from Lucid? I'm not in the market for a sedan but I am watching Lucid with great curiosity.
 

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In March Lucid did a highway range test of over 800 miles from San Francisco to Los Angeles and back with only one charge along the way. (In the background of their promotional video, you can hear their CEO Rawlinson remark to the test engineer that the trip was ”almost 1,000 miles”. They don’t say that in the marketing releases, however.) The test was reportedly done with a 110-kWh pack. Lucid has canceled plans for the 130-kWh pack, saying that advances in total drivetrain efficiency enable over 400 miles of highway driving on a single charge with the smaller pack. If these figures prove out in independent testing, it means Lucid has already cleared the 4 miles/kWh hurdle.

Their CEO Peter Rawlinson has said he thinks 400 miles of real world range is sufficient. Once that range is achieved, Lucid is shifting emphasis to reduce the size, weight, and cost of battery packs through greater efficiency rather than by maximing range through ever-larger battery packs. (This focus on weight and efficiency may be a holdover from his pre-Tesla days when he was Chief of Advanced Engineering at Lotus, which has always focused on those things.)

Among the techniques for achieving greater efficiency, Lucid claims:

- proprietary permanent magnet motors that virtually eliminate clogging torque and spin almost as freely as induction motors

- higher system voltage enabling the reduction of cooling equipment

- reduced battery pack impedance

- proprietary inverters with industry-leading efficiency

- use of corroidial transmissions.

A lot of people, myself included, are anxious for independent testers to get their hands on a car to verify these claims. If Lucid delivers, it’s going to be a killer product.

A couple of the more-detailed articles about all this are:

https://spectrum.ieee.org/cars-that...-motors-says-ecar-design-all-about-efficiency

and

https://motortrend.com/peter-rawlinson-lucid-motors-interview/
 
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Bigger packs (longer range) vs faster charging is indeed the ultimate question, and finding the balance for the majority of users is going to be the challenge.

  • If you could drive 300 miles, then recharge in 10 minutes, most would find that very acceptable.
  • If you could drive 200 miles and charge in 30 minutes, a fairly large percentage of people find a problem with those parameters.
  • Some claim they would want to drive 500+ miles before charging - and if it took more than 5 minutes to regain those 500 miles that would be too slow.
Add towing into the mix and range drops roughly in half and charging time basically doubles (per mile of travel).

I personally have no need to haul around (and pay for) 500+ miles of batteries. As it stands today, the sweet spot is likely 400 miles. That allows for plenty of buffer for sparse DCFC charging stations and/or plan changes, and allows efficient charging in the 20-80% window with little to no taper.

Someone towing a large travel trailer that wants to do 1,000 mile days will have different requirements.

Higher charging voltages do much to help cut charging time. VW and Lucid are implementing that higher voltage at pack level, but Rivian is - or at least considering - treating the pack as two packs when charging at higher voltages (essentially splitting the voltage between them).
 

ajdelange

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Higher charging voltages do much to help cut charging time.
Lets say I have two 100 kWh battery packs is a vehicle which can be charged at 2C and I stop at a 350 kW charger. I need to pick up 200 kWh of energy. I put the batteries in parallel and charge at 400 V. That means each will get half at 175000/400 = 437.5 A for a total of 875. It will take 200 kWh/350 = 0.571 h to charge the batteries. So I put the batteries in series and charge at 800 V. The total current from the charger now 437.5 A and that flows through each of the batteries and the voltage across each is now 400 so each is charging at the same rate (400*437.5/100000 = 1.75C) and it will take just as much time (0.571). IOW 350 kW delivered to the car is 350 kW whether the supply is 1 V or 1 kV.

In the 800 V connection the charger cables will run much cooler (1/4 the heat dissipated) but there is no advantage in speed that I can see. What am I missing here?
 

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In the 800 V connection the charger cables will run much cooler (1/4 the heat dissipated) but there is no advantage in speed that I can see. What am I missing here?
This comports with Rawlinson's statement that the 900+ volt system of Lucid does not confer any extra charging speed. Rawlinson has said that charging speed is more a function of battery chemistry than the car's voltage architecture.

However, I see frequent claims that Porsche's 800-volt system enables it to charge faster. For instance, "Car and Driver" published this in an article:

"We suspect the Taycan's primary reason for being able to charge so much more quickly is its 800-volt electrical architecture, which is twice the EV norm. The 350-kW Electrify America stations also operate at 800 volts, and the fact that the voltage is twice as high means that half the current is required to achieve a given rate of charge." (https://www.caranddriver.com/reviews/a30894056/porsche-taycan-fast-charging-tesla-model-s/)

As a non-technical layman, I find this all very confusing.
 

ajdelange

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Yes, I can see how that might be. The statement is true. At double the voltage half the current is required to deliver the same charge rate (with the heat reduction) but a 350 kW charger delivers 350 kWh in an hour whether it delivers it at 437.5 A (800 V) or 875 A (400 V). Perhaps it is because people are less likely to want to build a 350 kWh charger with 800 V architecture than 400.
 

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Amperage is limited to 350 in the CCS charging standard. Doubling the voltage for a given amperage results in faster charging. The 350 kW chargers get that rating from their 1000V capability (@ 350A).
A vehicle using 400V charging would be limited to 140 kW
A vehicle using 900V could charge at 315 kW
 

ajdelange

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So no, doubling the voltage does not increase charging speed. But you can get higher charging speed if your car can take higher voltage because higher power, which does increase charging speed is only available from high voltage chargers (as I surmised at the at the end of #25). In rhe CCS world, anywaty. Teslas (some of them) can charge at up to 250 kW (625 A @ 400 V).
 

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Is there any technical reason the Tesla charge rate slows sooner than the Porsche? (According to the C&D article, the Tesla fell below 50 kW at about 80% charge while Porsche didn't fall below that level until about 94% charge.) Is it because the Porsche battery pack is a bit smaller? Is Porsche just being more aggressive in its programming choices for charging? Some other reason?
 

ajdelange

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Tesla collects information of various sorts from their drivers. Recently they have
1)Started limiting charging if you have charged your car to high SoC too many times recently
2)Lowered the maximum charge rate at which their cars will accept charge from Level 2 chargers and reduced the charge rate from Level 2 chargers they sell
3)Started advising their drivers to charge more frequently to lower levels while on the road e.g. charge from 20% to 60 % twice rather than from 10% to 90% once
4)Use Level 2 chargers rather than Super Chargers to the extent that is possible

One can come up with all sorts of reasons why Tesla does any or all of these things but having been served well by the advise of William of Occam over rhe years I tend to rely on his razor (the simplest explanation is often the best) which, in this case says that Tesla has concluded from the battery data that it has collected that charging fast at high SoC shortens battery life and so tries to discourage fast charging at SoC by various means one of which is tapering the charge.

Porsche 1)has a different battery chemistry than Tesla and 2) does not have the millions of miles of road data that Tesla does 3)may have lab data that indicates that their battery is less susceptible to this effect. Thus they may not believe that what applies to Tesla applies to them and thus do not taper charge.

That's the best I can do.
 
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Tesla does "advertise" fast charging capabilities and "range" to enhance sales and image. I have read of owners having their SC experience throttled. I am aware that they reduced the L2 capacity to 11.3kW(is that the number a 60 amp, 240V line can create?) from some higher number. I'm not positive that I am reading significant degradation of their battery packs (-20%) over time but where is the data for this?
 
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