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More than 400V systems, where do things go sour?

Discussion in 'Technical' started by emir-t, Oct 4, 2016.

  1. emir-t

    emir-t Member

    Oct 28, 2013
    So as you might have heard Porsche announced they were working on a new charging system for their upcoming Mission E and Audi, VW EVs down the pipeline. They claim a 15 minute charge rate for 80% using a 800V system.

    I don't know much about batteries or EVs other than what I research and try to learn but, to my understanding simple pros of a 800V hypothetical Tesla Model S would be;

    • Lighter, thinner cables for both power electronics and Supercharging equipment. As you'd need less current for the same amount of power. No need to use space grade inconel at the fuse to support up to 1500 Amps. Same power can be achieved with only 650Amps. Or use the same thing and double the power.
    • AFAIK charging a battery pack is limited at what the cells can get, not delivering the power. If so a 20 stall supercharger already has 1.3mW of power available spread across 10 banks, 20 stall. Be it an iPhone or a car lithium ion batteries can only charge up to 80% in a fast way (Depending on your cells' max. C rates) and the rest, afer the cells reach peak voltage is CV stage where current tapers off. Suppose Tesla changed the layout of their current pack to use same cell count but connecting 37 cell parallel making a 122Ah cell. Making 12 groups per module and having 16 modules. Result would be a 700V nominal, 122Ah cell. Still around 85kWh. With the current C rates Supercharger delivers it could start its depleted charge at 615V, 270A = >165kW. Faster charging?

    There has to be some downsides to it too or else why would virtually ervey EV is 400v? If not, why not make a kilovolt EV? Electric hyper car Rimac Concept One's pack is more than 600V I think.

    I am also guessing that the chargers' work would be more difficult too as they'd work as transformers? Also DC-DC inverter would have much more work too I guess.

    Someone who knows this better than I do could explain.
  2. chillaban

    chillaban Member

    May 5, 2016
    San Jose, CA
    Actually, when I worked in a few other industries, we used voltages higher than 400V all the time, including nominal voltages in the 500 to 800V range. I'm not going to say exactly what voltage because that more or less gives away the project.

    And in that regard, it's more that ecosystems form based off standardized voltages. I think there's a nice PHEV/HEV/EV cluster around the 200V and the 400V bus voltages, but Porsche be paving the way for an alternate ecosystem around 800V.

    Of course you have to design everything and source every part with your target voltage in mind, but that's a very tractable task.
  3. 3mp_kwh

    3mp_kwh Member

    Feb 13, 2013
    I recently read a Barron's fluff piece, on Borg Warner, saying Mild Hybrid would yet be one of the pieces of business the power train company was going to thrive on. "Ecosystem" seems to be precisely why its failing. Nobody has gotten others on the 48V bus. All those 12v parts bins are tough to leave behind (even for Tesla).

    800v means thinner wires, for what that's worth. It also probably makes kinetic recovery more robust. When racing formulas go up from 1-4kwh of lap storage caps, to something possibly much higher, it may be Porsche who's system is out there recovering the tremendous amount of energy wasted in braking. I think it's safe to say higher voltages are better with higher energy jolts. The R&D into battery uptake will be another gain.

    Having had a GT3, I remember how Porsche went with a mechanical ~40lb flywheel in the passenger seat of their GT3 hybrid. Brakes went from electrical, to mechanical spinning of that wheel (1000's RPMs), which then fed electricity back to the motor as the car exited turns. 800 volts are perhaps a better way to get those transient punches on, and back off-board, again.
  4. Ed Hart

    Ed Hart Member

    Nov 5, 2014
    Yorba Linda, CA
    The higher voltage is a big benefit in many ways. The smaller charging cable is obvious, but not so obvious is the potential for smaller wiring throughout the vehicle, assuming higher voltage inverters are practical and affordable. But the vehicle still needs to store the same amount of power in its batteries, so what is not obvious to me is why batteries would be able to absorb more power at a higher voltage. I will let a battery expert weigh in on this. Thanks.
  5. Jeff N

    Jeff N Active Member

    Oct 31, 2011
    The Toyota Prius and Honda Accord hybrid and perhaps others use DC-to-DC voltage converter circuitry to step up their batteries to 600-700V at the motor.
  6. jkn

    jkn Member

    Nov 29, 2013
    Cell voltage will remain same, about 4 V. So charging it will not be any faster. Copper in cables will be thinner in 800 V system, but insulation must be thicker. Main purpose of 800 V is to create incompatible charging system. 800 V is more than twice as dangerous as 400 V. 12 V does not kill anybody. 50 V AC will not kill me. (tested).

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