Why no 800V?

Cosmacelf

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Since Rivian isn't coming out with their vehicles for 2 years, and they are starting with a clean sheet, I wondered why they weren't going to leapfrog the current generation of EVs and do what Porsche is doing with their upcoming Taycan and outfit their vehicles with an 800V architecture.

All EVs other than Taycan use a 400V battery pack. The advantages of an 800V system is that is allows thinner cables inside the car, more efficiency, and thinner cables on a fast charging station. It doesn't, however, allow faster charging by itself. Taycan will supposedly support 350kW fast charging, which is at least double what any other EV can or plans to do. To charge that fast Taycan must use a different cell chemistry that can support higher charging rates (or have one heck of a really good battery cooling system, or both).

The disadvantage of an 800V system is that you need some way to be compatible with the huge number of existing 400V fast charging stations in existence. That means some sort of extra circuitry inside the car.

I asked at the Rivian booth why they aren't using 800V and the answer was to be compatible with existing charging stations, but that isn't a valid answer as Porsche is going to be backwards compatible by using extra circuitry.

I suspect the real answer is that they just don't feel the need to be a trailblazer in this particular technology aspect. There are pioneering enough new things like four motors and a better integrated skateboard.
 

lookmtb

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How does doubling the voltage affect the number of cells required to get the same capacity?
 

Cosmacelf

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How does doubling the voltage affect the number of cells required to get the same capacity?
It doesn’t. Capacity in kWh is the same with the same number of cells. An 800v pack’s cells are just wired differently.
 

lookmtb

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So is it Amp-Hours goes down but Watt-Hours stays the same due to the voltage increase?
 

lookmtb

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After doing a bit of research I suggest that the reason for sticking with 400v is compatibility with current chargers. I believe it is quite difficult to convert 400VDC to 800VDC with an on vehicle system, and maybe it was too complex to reconfigure the battery to charge on 400v on the fly.

Also I know the standards for Chademo and CCS are up there in that higher voltage range now, but I can't think of a single charging station that actually uses it except for the Electrify America network that is currently under construction. Surely these will come in the future though. And I believe it's easier for the station to down regulate in this case for other vehicles to utilize these.

If compatibility was indeed the driving factor I agree. I would rather have more chargers available to me. All this makes me wonder what is involved with teaming up with tesla for their network...

Also would love for an electrical engineer to chime in on this.
 

Cosmacelf

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Well, I wouldn’t say converting 400v to 800v is “quite difficult”, but it does require more circuitry. A simple way to do it would be to reconfigure the pack voltage on the fly with a couple of extra high voltage, high current contactors. Or you could use electronics that works up to a lower power limit like 50kW since most existing 400v stations don’t go much past 50 kW. Porsche Taycan is doing one of these two things. But as I said, 800v doesn’t buy you all that much. Just lighter wiring and slightly more efficiency.

Personally, if I were designing an EV from scratch, I wouldn’t mess around with the 400v standard. Instead, I’d replace the 12v low voltage system inside the car with something else, like a 48v system. Lights, horn, electronics, little actuator motors, small pumps, all use the cars 12v bus. But the current requirements keep going up, and the 12v wiring is getting heavy.
 

lookmtb

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I am very curious to learn how the Porsche is achieving this compatibility. If they do it well they have definitely moved the ball forward!

I couldn't agree more about the 12v system. Leftovers of a bygone era... I really don't know anything about what's stopping them from removing it, but my guess is just a supply of parts that are compatible with another standard?
 

Cosmacelf

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I couldn't agree more about the 12v system. Leftovers of a bygone era... I really don't know anything about what's stopping them from removing it, but my guess is just a supply of parts that are compatible with another standard?
Yes, that’s my understanding. It’s quite ridiculous. You look at a golf cart, which has either a 36v or 48v DC battery pack for the motor and what do you find? A DC-DC converter to 12v for the horn, lights, and other accessories. Argghhh.
 

theredviper

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12V seems silly, but with EVs, isn't there a significant amount of power controls that are eliminated from the overall system? Plus LED lighting is much less power intensive.
 

Cosmacelf

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12V seems silly, but with EVs, isn't there a significant amount of power controls that are eliminated from the overall system? Plus LED lighting is much less power intensive.
Not really. The compressor is run off the 400V, but then in an ICE, the compressor gets power directly from the engine (via the serpentine belt), as does the water pump, and power steering pump.

So in an EV, 12V powers the power steering, brake booster, HVAC blowers, glycol pumps and valves, door locks, door handles, windows, horn, interior and exterior lights, entertainment system, computer systems, rear window defroster, and maybe the air compressor for air suspension (not sure if that is a 400V device).

So there is a lot powered by 12V, and I'm sure I've missed some. Just take a look at your ICE car fuse box...
 

Laurent

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Does it make sense to buy a 400V Rivian in 2020 if they introduce 800V models two years later? Any chance the early adopters would get some kind of credit towards the upgrade?

See Rivian electric pickup, SUV ready for future 800V upgrade—possibly in 2022

Rivian will be ready for it, on a timeline that might just be two years after the original launch. Scaringe makes it sound like that's when the company’s products hit their full potential.
“Power levels go up, charging speeds go up, everything gets better,” added Scaringe, in a much more enthusiastic tone. “As good as the vehicle is today, when we double the voltage it’ll be better.”
 

Cosmacelf

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Does it make sense to buy a 400V Rivian in 2020 if they introduce 800V models two years later? Any chance the early adopters would get some kind of credit towards the upgrade?

See Rivian electric pickup, SUV ready for future 800V upgrade—possibly in 2022
CEOs just can't help themselves, can they? There was no reason to boast about being 800V ready when it's at least five years away. The problem with 800V is that there is no charging infrastructure for it. It'll be at least five years before there is a usable 800V charging system available.

More importantly, 800V doesn't buy you all that much. Charging up twice as fast sounds cool, but Rivian's 160 kW charging speed is no slouch. It's more than what Tesla can do at 120 kW, and you don't hear too many Tesla owners complaining about charging speed. The reality is that road trip charges occur while you eat lunch or dinner. Charging up in 20 minutes versus 40 minutes isn't hugely useful.
 

Cosmacelf

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Oh, and no, there wouldn't be an upgrade path from 400v to 800v. As that article said, there are devices on board that are built for 400V like the air conditioner, air compressor, etc. You'd have to swap those out too.
 

theredviper

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My 2 cents: 800V will be niche for a long time or forever. Electrify Amerca and Tesla's networks will dominate. Porsche has some plans for fast charging, but it's a huge investment in a parallel technology that will be catching up on years of deployments.

The 350kW and similar speeds are marketing hooplah anyway.
 

Cougs

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I agree with Cosmacelf that keep the 400V sice we are not gaining much. I plan to charge before long trips and plan my next charge during my long breaks on the road like lunches, shopping, take my laptop and do work etc....

I believe that the advances in technology will bring us better energy storage than Lithium ion and Tesla and others will find it as long as $$$$ is pumped into the research programs.
 

azjohn

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I do like the idea of a 800v system but for a different reason than for charging. With the higher voltage for the battery and motors it would make for a cooler running system between the motors and energy storage due to the lower amp draw, of course they would need different motors to accommodate the higher voltage
 
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