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How does DC charging work?

Discussion in 'Charging and Infrastructure' started by SSonnentag, Oct 12, 2017.

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  1. SSonnentag

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    I'm looking for technical details on how Teslas can fast-charge using various random voltages? How are the differences in voltages handled?
     
  2. Badback

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    DC charging bypasses the car's internal battery charger and is managed by the charger electronics in the DC supply. The actual DC voltage supplied to the car is independent from the supplied AC as the DC charger is a buck-boost converter.
     
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    • Frank99

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      This isn't necessarily 100% correct in terms of a CCS or Supercharger system, but it is the common approach to the problem, so take the following as a "broad-brush" approach to answering the problem:

      The Supercharger is designed to take the incoming power from the Electric company (say three-phase 480V AC), and convert it to a variable DC voltage between, say, 250VDC and 450VDC. You can treat this operation as a black box - it's a well understood problem, with quite a number of different engineering approaches to solving it. If this is the area you're interested in, I can go into more detail, but it gets boring quickly.

      When the Supercharger is connected to the car, they have a little conversation. The Supercharger tells the car what it's capable of ("I can supply between 300VDC and 450VDC at up to 300 amps"), and the car tells the supercharger about itself ("My full charge voltage is 398.4V, and I can accept up to 200 amps"). They go back and forth a bit, and agree on what the charging parameters are going to be. Then the car connects the battery pack to the charging terminals in the cable, and the charge starts.

      I'm not clear on what happens next in the Tesla system. There are two possible scenarios:
      1. The Car/Battery knows the precise charging algorithm (maximum charge rate vs. state of charge, etc). In this case, the Car/Battery monitors the voltage and current going into the battery, and tells the Supercharger what to do ("Ramp up the current to 200 amps", "Reduce current to 100 amps", "Stop charging"). This seems most likely to me, as the Car/Battery has the connections to monitor voltage across each group of batteries (and there are 96 groups), to monitor the temperature of the cells, and to adjust the charging parameters based on the precise chemistry of the cells in the battery.
      2. The alternative is that The Supercharger knows the precise charging algorithm, perhaps as part of the initial conversation, and controls the complete charging process. The Supercharger monitors battery pack voltage and charge current, sets the appropriate charge currents, and tapers the charge current as the battery nears full. The Car would still need to monitor and report battery temperature and individual battery group voltages to the Supercharger so that it could adapt to individual battery conditions.

      Charging a Lithium-Ion battery is an...interesting process. The charge current that you start with depends on the battery's current state of charge and temperature. Assuming a fully discharged battery (below 3.0V / cell, about 288V for the battery pack) and normal temperatures, the charge current is kept low until the battery reaches 3.0V / cell, then is ramped up to the full current the battery is capable of. The voltage measured across the charging battery will start climbing until it measures about 4.15V / cell - but the battery isn't fully charged yet; it's about 75% charged. The charge current is then smoothly reduced while keeping the cell voltage at 4.15V, until the charge current falls below a certain level (say, 3% of the maximum current) - at that point, the battery is considered fully charged.

      Did BadBack and I answer the question you had?
       
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      • SSonnentag

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        Thanks! I guess "Buck-Boost Converter" is the term I was wanting to know. Now I can read up on how they work on Wikipedia. :D
         
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        • SSonnentag

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          Pretty much. The actual mechanics of efficiently raising or lowing DV voltage is the part I want to read up on. I understand AC transformers, but have never read up on how the same thing is done with DC.
           
        • garsh

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          What mechanism is used to control the supplied current?
           
        • Frank99

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          SSonnentag -
          To take a high DC voltage, and convert it to a lower DC voltage, you'd use a buck converter:
          https://en.wikipedia.org/wiki/Buck_converter
          To take a low DC voltage, and convert it to a higher DC voltage, you'd use a boost converter:
          https://en.wikipedia.org/wiki/Boost_converter
          To take a DC voltage that may be higher or lower than the desired output voltage, you'd use a buck-boost converter:
          https://en.wikipedia.org/wiki/Buck–boost_converter
          Note that all of these require and inductor capable of handling the charge current - 300A or so. That's a might big hunk of iron (although a Supercharger divides this across 12 different sub-chargers).

          garsh -
          it's a feedback mechanism on the converter. I think in terms of voltage - so you get that view of it. You can think of the converter as a device that adjusts it's operation to keep a constant output voltage. In the Buck/Boost converters referenced above, the converter adjusts the frequency or duty cycle of the waveform that it feeds into the inductor to keep the output voltage constant. For regulating current, the converter measures the output current and uses that to adjust the output voltage - if the current is too high, that implies the output voltage is too high, so the regulator changes frequence/duty cycle to reduce voltage, or vice-versa.
           
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          • SSonnentag

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            So all of the voltage/current conversion is done on the charging station, not the car, right? The car just communicates what it needs and the charger confirms and delivers? I guess that means we have to trust all of the various chargers to do what they're told? I see charging stations can deliver anywhere from 300V on up to 800V. It would be bad news to have a charging station malfunction and start dumping 750V to your Tesla. :eek:
             
          • JWardell

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            Also keep in mind that the superchargers are supplied with AC, not DC, so it's not a DC-DC buck/boost converter.
            There are a number of ways to arrive at the controlled DC voltage and current output, but in a typical modern switching power supply, the AC is multiplied by a much higher frequency to bring it to around 400kHz (or whatever the design is), through a physically smaller transformer thanks to the high frequency to an even higher voltage (120v usually goes to 400v, but these may be much higher). That 400v (or maybe 1000v here) high frequency AC is then dropped down and rectified to the desired DC voltage. That's what is going on in your tiny iPhone charger, and could be similar in the 6ft high superchargers (not the part where the car plugs into, but the big enclosures off to the side).
             
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            • SSonnentag

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              That's a really good point, J. I guess I was under the false impression that any given charger (CCS, Tesla, CHAdeMO) delivered a fixed voltage and that the individual cars modified the voltage and current. Having this function offloaded to the charger does make a whole lot more sense.
               
            • Frank99

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