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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

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    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

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    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

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    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

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    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

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    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

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    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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  7. Peter Egan

    Peter Egan Member

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    The bus and large truck industry will pioneer higher voltages. It will be interesting to see what are the standard vehicle voltages in a years time.
     
  8. sandpiper

    sandpiper Active Member

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    Is 800V really twice as dangerous? Dead is dead. A 1000 ft fall is no more dangerous than a 500 ft fall.

    I've been hit (once! :eek:) with 240VAC and 120VAC many times. A good hit on 400VAC would probably be the end of it.
     
  9. nwdiver

    nwdiver Active Member

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    Higher voltages can start finding their way through insulation... IMO >400v is a bad idea.
     
  10. jkn

    jkn Member

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    Note that 230 VAC varies between -325 V and + 325 V. In 3 phase system (230 V single phase) voltage between 2 phase wires goes from -563 V to + 563 V. AC also blocks nerves better than DC, so you cannot disconnect your hand from that wire. So I think 400 VDC is safer than 230 VAC.

    Current through body kills, not voltage! Doubling insulation cuts current to half. Doubling voltage doubles it. Less than 0.01 A is not a problem, over 0.1 A kills. You likely have some insulation: your shoes and whatever you are standing on. That could be enough with 400 V and not with 800 V. Even 110 V can produce deadly current through body. Body's resistance is enough to stop 12 V causing deadly current.
     
  11. Jeff N

    Jeff N Active Member

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  12. AntronX

    AntronX Member

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    800V is nothing to be scared of. It will be just as safe as 400V via prudent safety engineering. If you want to have 200+ mile EVs and charge them in 15 minutes (once batteries can handle the charge rate), higher than 400V systems will be necessary, unless you are ok with wrestling a fire hose sized charging cable carrying 1000A.
     
  13. PtG62901

    PtG62901 Member

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    I'm a software guy, so be gentle with my stupidity.

    Isn't this about what happens to the batteries? We know they can engineer a system, and deliver that level of power, given time and enough money. It is a known problem.

    Isn't the interesting problem is what happens to the batteries, over time, charging that way? They are going to get hot. What happens to the chemistry?

    At least we know why no one else has licensed their cars into the Tesla charger system yet, they are hoping a meteor hits Tesla, so they don't have to do EV's ever. Lol
     
  14. jkn

    jkn Member

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    When accident happens, safety engineering breaks and rescue crew has more V to be afraid of. With 800 V current can travel easier along dirty moist surfaces. I remember reading that Tesla tests liquid cooled charging cable. With it they can use higher current without melting cable.

    1000 A * 400 V = 400 kW 100 kWh battery in 15 minutes. 400 kW would heat charging cable 16 times faster than 100 kW 400 V. Water can transfer heat 4000 times more than air (same volume), so no problem.

    Motor with higher voltage might be slightly more efficient. Battery would have less cells parallel. So one broken cell would reduce capacity more.

    For really fast charging we need to swap battery.
     
  15. emir-t

    emir-t Member

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    IMHO providing or delivering the power isn't the bottleneck here. Providing power to a location is no issue, 20 station 10 bank Supercharger that opened in Norway a few months ago can deliver a total of 1.35 megawatts DC.

    If Tesla wanted to be able to provide 400kW in the start of a charge they'd need ~1300Amps. This wouldn't be impossible with a liquid cooled cable larger in diameter than today's non liquid cooled cables, albeit would be very unnecessarily expensive. But the problem is putting all that power into a battery.

    First you need to dissipate heat when charging. Tesla improved on this over time so tapers kick in later than 2013 packs. But there's the chemistry issue. When you start charging batteries their voltages increase. To be able to pump more power in you need to also up your charging voltage. More power you put in faster the voltage increase. When a cell hits 4.2V you don't want it to go to a higher voltage as it would harm the battery so you start the constant voltage phase and start dropping the power a.k.a. mandatory taper. If cooling is no issue you can pump 120kW without taper into a Model S pack until it reaches per cell voltage of 4.2V. From then on you have to start the taper to protect the battery. And faster you get to 4.2V, equally longer it will take for the rest to fill up.

    So if there was a magic chemistry that gained voltage slower despite added power that would enable fast charging. Also software limited batteries would fast charge too as they never need to be 100% charged physically. But that wastes storage capacity. So everything is a tradeoff.

    I wonder if stopping charging when they reach 4.2V per cell and shocking the batteries with immense peak draw to drop voltage and start charging fast again would have any net gain on charging times.
     
  16. Colby Boles

    Colby Boles Member

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    As you get into these higher voltage ranges, the transistors and other components you need for the motor inverters and battery chargers that have rated breakdown voltages that are some multiple of your operating voltage start to become more exotic and expensive, and less reliable. One might counter-argue that you will need fewer of them, but this isn't true practically as the highest power rated IGBTs tend to be at the lower end of the breakdown voltage range, for example. Voltage = electrical stress. It's much harder to build reliable high voltage solid-state electronics. Certainly not impossible, but keeping the voltage around 400 and using more copper might be simple by comparison.
     
  17. AntronX

    AntronX Member

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    When accident happens, safety engineering kicks in. Inertia triggered circuit breakers embedded inside the pack would break up the battery into 4 sections of 200V each. Liquid cooling takes up space in the cable, making it thicker or as thick as simply adding more copper.

    New SiC Fets are rated at 1500V breakdown. They will get cheaper by the time batteries mature to handle 5C charge rate (Tesla is at ~1.2C presently).
     
  18. jkn

    jkn Member

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    No. Doubling voltage doubles wire length inside motor. This doubles resistance. Since space is limited wire must be thinner. This also doubles voltage. So resistive loss does not change. Higher voltage motor is not more efficient. Since insulation needs more space 800 V motor would be very slightly less efficient.

    Rescue crew cannot be certain that this works. So they cower burning car with foam. Foam prevents cooling of the battery making fire more intense. Battery cell can self destruct without any air. Only heat is needed.

    Water has heat capasity 4.187 kJ/kgK. Warming one liter of water by 50⁰C in a minute takes 3.5 kW power. So water cooling with one liter per minute is certainly enough. That will not make cable thick.

    We have electronics for 800 V, but is it cheaper or more efficient than 400 V system with double current? I believe it is not.
     
  19. AntronX

    AntronX Member

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    Their problem. They should use water to cool the battery. Very unlikely they can get shocked since the battery is isolated from ground. IF they are afraid, they either need to get educated or quit. Anyway, none of these arguments are valid against use of higher voltage.
     
  20. jkn

    jkn Member

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    Risk of death with 800 V is higher than with 400 V. Of course nobody will die, if things work perfectly.
     

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