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What limits rate of SuperCharging?

Discussion in 'Model S: Battery & Charging' started by Evbwcaer, Aug 8, 2014.

  1. Evbwcaer

    Evbwcaer Member

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    What I mean is why can't they do 200kw for example? Is it the batteries, the SC hardware, or the limit of the grid connection?

    I had been thinking that if a 60kwh is take 90kw, and an 85kwh can take 120kw, could a 110kwh take 150kw?

    I also had the idea that maybe there will be Supercharger travel plazas one day that could have battery coolers, that cool from underneath, and maybe that could help to charge faster, if heat were dealt with externally.

    Answers? Thoughts?
     
  2. ItsNotAboutTheMoney

    ItsNotAboutTheMoney Active Member

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    Likely the cells and battery design have limits, and then they'll have charging hardware that also has an upper limit.

    A battery cooler underneath probably wouldn't help much: you need to keep the temperature down for all cells and the whole of each cell.

    What could be possible would be to replace the swapping idea with cooled charging. Instead of giving you a new battery, they dunk the battery into an environment with more powerful cooling and then charge it faster. But obviously that'd only be worth it if it allowed _very_ fast charging.
     
  3. Todd Burch

    Todd Burch Electron Pilot

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    In the Model S, the battery's ability to accept power for charging is the limitation. Certainly Lithium Ion batteries can charge faster, but beyond a point this will degrade the life of the cells at an unacceptable rate. Tesla is choosing a charging power that is as fast as the battey can support without significantly shortening the life of the battery cells. Everything else in the charging system (charging cable, etc) was built around this main limitation. Now, Tesla certainly has a buffer built into the "other hardware." We've seen them slowly increase the max charging rate (initially at 90kW, then to 120kW and 135kW) as they've tweaked the batteries, done additional testing, and gathered more real-life data. Likely Tesla will increase it more in the future as other chemistries and larger capacity packs come online.

    Having said all that, the key limitation is the battery pack's ability to accept power without degrading significantly. For more information, you may want to Google a battery's "C" rate. Here's a link for starters.
     
  4. Johan

    Johan Took a TSLA bear test. Came back negative.

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    ... Also we do not know the exact upper limit for the inlet/plug combination.
     
  5. ThosEM

    ThosEM Space Weatherman

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    The expense of the charging connection to the grid is also a major factor. 100 kW ain't cheap to set up, or we could all have that in our homes. A typical home is wired for 200 A service, or perhaps 400 A if it is all electric like mine. That current rating is at 120V, so it's only 24 or 48 kW, for the *entire home*. One has to leave some power for the lights and refrigerator (and stove and dryer, and heat pump)!

    But how high can a supercharger go if money is no object? Is a megaWatt possible? Maybe, but that is starting to be a pretty big load on the grid at a single locale, and it will take time for the upgrades to support that.

    Why 1 MW? Because pumping 10 gal of gas in 6 min is 3.3 MegaWatt. But if we are only filling a 100 kWh battery instead of a 333 kWh gas tank, 1 MW will fill that up in 6 minutes!
     
  6. Todd Burch

    Todd Burch Electron Pilot

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    ThosEM is right in that higher powered connections to the grid are more expensive, but I don't believe that is the current reason for the limitation on the Model S. Future systems with higher power ratings should be able to avoid this with battery storage systems located on-site.
     
  7. mitch672

    mitch672 Active Member

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    The existing SuperCharger consists of (12) 10KW Model S onboard chargers. They are grouped/switched three at a time. (To keep the 3 phase power balanced). So you get 30KW, 60KW, 90KW or twelve of them 120KW. That's the limitation. Also each SuperCharger is wired to 2 charging spots, so the above power can be split, if 2 cars are plugged into the same SuperCharger. They get to 135KW by running the modules at 277V, which they can technically put out 11KW. Tesla designed the SuperChargers to use components they already have/manufacture, hence the use of the Model S onboard charging modules.. I don't see that changing. They could design a larger SC with more than 12 modules, perhaps 15 or 18, but the issue becomes larger cabinets, more cooling, larger cabling, larger utility transformers (they already use 500KVA transformers at (most SC sites).
     
  8. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    But could you lift the cable? :)
     
  9. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    A small error in your numbers. The current rating to a house on split-phase, 240-Volt service is at 240 Volts. That means that 200A*240V = 48 kW and 400A*240V = 96 kW. Because most conventional circuit breakers can only be run at 80% of rated current without a risk of tripping, those are really upper limits of 38.4 kW and 76.8 kW. Utilities know that the vast majority of homes don't come close to using their maximum power, and they follow different rules that allow significantly more power to be drawn than rated from a transformer and more current through a wire than the NEC allows. That's why my house with 600 Amp service (max 115.2 kW) and two neighbors with 200 Amp service (38.4 + 38.4 kW) could take a total of 192 kW, but we all happily share a 50 kVA transformer, and I have no problems pulling 70 Amps charging my Roadster and 80 Amps charging my MS at the same time with only 6-8 Volts drop on the 240.

    Even at today's Supercharger power levels, it's a big draw at each site, but not huge on the grid. Hotels or office buildings draw similar levels of power. Typically 4-Stall Superchargers have a 300 kVA transformer, 8 stall locations have 500 kVA and 10-12 stalls should have 750 kVA. BTW for comparison, Wolf Creek ski area put in a new high speed quad Super-chair in for the Treasure Lift last year. That ski lift has its own 750 kVA transformer to power the lift motor!
     
  10. mgboyes

    mgboyes Member

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    The limitation is that Tesla as a company will live or die on whether their batteries can last as long as the typical life of a car, which means absolutely no worse than 10% capacity loss over 100k miles IMO. So far, things are looking good!

    With the current battery chemistry that means that charging significantly above 1C is risky, even with thermal management, hence they're OK running an 85kWh battery at 1.4C (120kW) for a short time, at low SOC, but no more.

    If we see a 120kWh battery we might see a 170kW supercharger, but we won't see anything that improves on the "half a charge in 20 minutes" rate with the current battery chemistry.

    In the EU I'd be surprised if they can go much above 120kW with the current connector, since it's a modified version of a connector that in standard form maxes out at around 50kW, and the modifications are quite subtle. I've never seen a US charge connector up close to have any view on how "chunky" it really is and therefore what its current limit might be.
     
  11. ItsNotAboutTheMoney

    ItsNotAboutTheMoney Active Member

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    Ultra-fast would be done by machine, I'm sure. (Hey, maybe that's what Tesla's big Fremont batteries are really for..)

    But double or triple the current Supercharger rate I'd bet would be be done with multiple cables.
     
  12. TonyWilliams

    TonyWilliams Active Member

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    Exactly. When a future Tesla can take 270kW, very little change would be required to the Supercharger station. Just plug in two cables, one on each side.

    I imagine driving over the charging plug that would automatically plug itself into the bottom of the car. There wouldn't be any visible indication that the car was even charging.
     
  13. J1mbo

    J1mbo Member

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    I would prefer a higher average charge rate than a higher peak. No point in charging at 270kw peak if the average for a full 85kwh battery charge remains around 70kwh.
     
  14. ThosEM

    ThosEM Space Weatherman

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    Thanks for the correction, but is it right? I am genuinely curious, as I was basing my statement on the fact that it requires a pair of breakers, each rated at 50A, to support my NEMA 14-50. That suggests to me that the current ratings are charged against 120 V individual lines, rather than against the 240 that results with two spit phase breakers at 120 V. So when one of my two breaker boxes is rated at 100 A, is it full when it has 100 A worth of breakers in it? Or can I get away with 200 A as long as I place half of them on each phase of the 240? For normal intermittent household loads it may not make much difference but for a car soaking up 10's of kW for hours, getting the ratings right is critical to prevent meltdowns or fires.

    Also, I didn't mean to suggest that this is the only reason for the current limitation of the Model S, which of course is a factor of 10 above what has been typical. Going a factor of 100 above what has been typical is going to require some upgrades along the lines that are done for bigger energy sinks like ski lifts and hotels. It's going to take time to get adapted to the idea that a single individual can legitimately soak up that kind of power (albeit briefly) when we are accustomed to sharing it among numerous hotel rooms or chair lift chairs.
     
  15. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    Yes, the current limit on household systems is for the combined 240 Volt system. If you look at a typical 200 Amp circuit breaker panel for a house, there is a main 200 Amp dual circuit breaker (Line 1 and Line 2) connecting the panel to the electric meter feeding the house. Continuous operation below 80% of rating (160 Amps) at 240 Volts will usually not trip the main breaker, and if the 120 Volt loads are perfectly balanced 160 Amps on each line side, you can operate up to 320 Amps of 120 Volt load on a household split phase system.

    Another good example is my charging setup in Boulder. I have a 200 Amp sub-panel in the garage for car charging. In many homes, this would be the main panel. That panel has three loads, a 100 Amp, 240 Volt, dual breaker feeding an HPWC that can output 80 Amps to my Model S, a 90 Amp breaker feeding a Roadster HPC at 240 Volts that can put out 70 Amps, and a 50 Amp breaker that feeds a 14-50 outlet at 240 Volts for testing, etc, that can draw 40 Amps. I know that I have to keep the total load on that panel to less than 160 Amps at 240 Volts. Many times, I have charged the Model S at 80 Amps, 240 Volts and the Roadster at 70 Amps, 240 Volts for hours at a time. That is a total load of 150 Amps at 240 Volts and the 200 Amp breaker feeding the panel has never tripped.
     
  16. TEG

    TEG TMC Moderator

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    I think factors are well covered in this thread, but recap for the original question:
    #1: for battery longevity they don't want to charge above a certain rate.
    #2: they design the battery cooling system to handle charging heat up to some limit.
    #3: The wiring in the charge cables and charge port is engineered to be just enough for the chosen max rate.
    #4: The supercharger modules are 'stacked' to support the max rate (although they could likely add more modules if other things weren't limiting)
    #5: Availble grid power capacity could start to become a factor in some locations.
     
  17. Matias

    Matias Active Member

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    Considering point 4, they could stack more modules to get 240 kW, so that 2 cars could simultaneously charge at full rate.
     
  18. ThosEM

    ThosEM Space Weatherman

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    Great summary. 1 and 2 may overlap a bit if the battery heating losses are the main factor affecting longevity.

    The comment about whether the cable can be readily lifted was a good one, too! It often amazes me that 100 kW can be put through the Model S plug and socket without apparent issues, while there have been issues with 10 kW on the other end of the UMC cable. The pin spacing and dimensions there don't seem that different from what is found on the car end of the cable, though admittedly, those supercharger cables are a lot heftier than the UMC. Perhaps AC vs DC makes a difference as well?
     
  19. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    OTOH, the 135 kW max out of a cabinet, probably offers max rate to 99%+ of the situations and is more cost effective for Tesla. I would much rather have 20% more Supercharger locations and stalls at 135 kW than get 240 kW everywhere.
     
  20. PoweredByRain

    PoweredByRain Member

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    I agree. The probability that two cars will arrive simultaneously, both at very low state of charge, and both connect to the same Supercharger, is low. And even if it does happen, the effect is probably felt for only 10 minutes or so, as the power ramps down once the battery gets to half or so. 67.5 kW is still a pretty impressive power to be charging at.
     

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