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EV charging for Dummies!

Discussion in 'Charging and Infrastructure' started by Robert Pickel, Mar 1, 2017.

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  1. Robert Pickel

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    Can anyone help me understand or direct me to a "charging for dummies" web page? There are kWhs on the batteries, home charging is often shown in Amps and volts, SuperChargers are listed as 480-volts and we now have "Tiers" Some non-Tesla chargers are rated in KWs. So, how do any of these compare? I sort of know how long charging on a SC will take, but how can I figure the time on an "off network" charging station? Thanks!
     
  2. KennethK

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    Check out Trev's video listed here, about half way through it.

     
  3. Badback

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    OK, I'll try.

    Watts are amps times volts.

    If a charger is capable of 100kW, it would take it 1 hour to charge a 100kWh battery. 100kWh/100kW=1 hour.

    This ignores losses, which are usually minor.

    This assumes that the volts supplied to the charger is sufficient to maintain the charge current as the state of charge of the battery increases. Battery voltage increases with charge.

    The charger contains circuits to boost voltage so that it is possible to charge a 400 volt DC battery from a 120 volt AC source.
     
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    • Twiglett

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      to extend the answer @Badback gave you.
      the watts = amps x volts gets you numbers that may start to make sense
      220Volts * 30Amps = 6600 or 6.6kW which is a really common home charger
      Once you have both in Watts you can start to guesstimate charging times
      You can also easily compare them when they are all in kW

      For me it was most interesting when I realized the 110V 12A is a piddling 1320 Watts which is way the wall socket at home takes so darned long to charge the car.

      You also often see folks talk about "miles per hour" to describe charging rates.
      This is usually based on an estimate of your cars average power consumption.
      I can get roughly 4 miles per KW with my car, so multiplied by 6.6kW means I can charge at about 26 miles an hour.
       
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      • Matthew Morgan

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        Sometimes its easier to explain electricity in terms of water and how fast it can fill a bucket.

        Voltage (Volts) = Water Pressure
        Current (Amps) = Size / Diameter of the water hose (one could argue that its the amount of water passing through the hose but this is simpler and gets the point across - and is technically correct since current is indirectly proportional to resistance which is directly proportional to the diameter of the pipe - Bernoulli Equation)


        Power (Watts) = Water Pressure (times) Diameter of the Hose
        • Power will tell you how fast your jamming electrons into a battery (or how fast your filling up a water tank)
        Energy (Watt-Hours) = Power (times) Time
        • Energy will tell you how many electrons are in you battery. (or how much water is in the water tank)

        When you want to compare apples to apples, you always have to figure out the energy or power of the device because these are the true units of energy. Use the equation below to figure it out.

        • P=IV (Power = current x voltage)
        You increase the amount of electrons flowing by increasing the voltage or the current ( increasing the pressure of the water or making the water hose bigger)
        • Level 1 Charger
          • 120 volts * 12 amps = 1,440 watts (or 1.44 kilowatts) (or 1.44 kW)
            • kilo = 1,000
        • Level 2 Charger
          • 240 volts * 40 amps = 9,600 watts (or 9.6 kW) - 6 times as fast as Level 1 charging!
        • Level 3 Charger (aka DC fast charging)
          • 368 volts * 313amps = 115,000 watts (or 115 kW) - 12 times as fast as Level 2 charging - 80 times as fast as Level 1 charging.
            • There are many levels of DC fast charging, This is a typical Tesla SuperCharger
        Back to our water example
        • Level 1 - Equivalent to having a garden hose connected to your house.
        • Level 2 - Equivalent to have a fire hose connected to the a fire hydrant.
        • Level 3 - Equivalent to running over the fire hydrant with your Tesla and letting it spill all its glory

        I'll leave the AC versus DC discussion for later :)


        Disclaimer: Batteries don't actually hold the electrons, Putting electrons in batteries causes a chemical reaction thus "storing" the electron for later use. When you need the electrons back, the chemical reaction will give it back to you.

        - I don't know why Tesla chooses to show Amps on their charging screen since the actual charging rate depends on the voltage as well.... kinda silly. However, amps are directly tied to the copper losses so maybe the engineers at Tesla were originally focused on this and it never migrated to wattage on the user interface? Who knows...?
         
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        • Badback

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          Please allow me to clarify a couple of points, this will make some things more complicated, but inevitably more clear.
          The water analogy is a very good one as almost everyone can understand how water behaves.

          From a physics POV it is important to decouple a physical attribute from its unit of measure:
          Electromotive force (EMF) is measured in Volts, same as water pressure, measured in pounds/in^2 (PSI) or grams/m^2 in metric, or any other weight per area units.
          Current is measured in Amperes (Amps), An Ampere is 1 Coulomb/sec, same as flow in water is measure in gallons/minute or liters/minute. A coulomb is a quantity of electrons: approximately 6.242×10^18
          Restriction is measured in Ohms, same as pressure loss in a water system caused by the diameter of the pipe, hose, etc. The pressure at the end of the pipe is always less than the pressure at the head of the pipe if there is any flow. Pe=Ph-Flow*Restriction.
          EMF=Flow * Restriction (Volts=Amps*Ohms), this is Ohm's law.
          Power is a rate. It is the rate at which work is done. Power is measured in Watts. 1 Watt is 1Joule/Second, the work required to move one Coulomb through an EMF difference of 1 Volt. It is EMF*current. Watts=Volts*Amps. 1 kilowatt hour = 3.6 megajoules. In a water system, power=pressure*flow. You can use this relationship to calculate how much power you can get from a water wheel. 1 Horsepower is 746 Watts.
          Energy is power*time and is measured in Joules, it is the rate at which work is done for a given time, Watt Seconds), the work required to move an electric charge of one coulomb through an electrical potential difference of one volt, or one '"coulomb volt" (C·V).
          There is so much more if anyone is interested.

          The definition of an Electrical Engineer is a person who has not fallen asleep by now.
           
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