Ok. I can't take it anymore. I try not to post to these forums because it enrages all the poseurs and wannabees. But I can't take it any more on this supercharger thing.
There is no magic. they are 12 chargers JUST like the ones in your car. When you connect AC to your car on your home charger, the J1772 circuitry in the car detects the pilot signal and starts the EVSE (the part on the wall) to the charger (the device in the car).
On the Supercharger, the pilot signal is somewhat different. The car detects this and enters a 33.3kbps data conversation using Single Wire CAN. It is VERY close to a standard CHAdeMO exchange with a couple of very minor differences. It passes your VIN and it has the ability to restart the entire conversation on changes in the power source caused by things like other cars plugging in next to you, clouds passing over solar panels, etc.
In the event of a successful CHAdeMO exchange, the CAR connects the two large pins of the connector directly to the battery pack through a couple of contactors in the junction box ENTIRELY BYPASSING your onboard charger. And from there, the DC power is provided by the 12 "chargers" in the Supercharger station. But everything about that is pretty much controlled by the battery management system in the car - how much current, what voltage it charges to, etc. and including determining the point at which charging terminates. The EVSE does get to vote I suppose in case of error or as earlier mentioned, lower power availability. But other than that, the CAR is directing all aspects of Supercharger power production.
Now let's talk about the charger. As noted, it is the SAME charger whether in the car or in the Supercharger charging station.
The first stage of the charger is a semiconductor full bridge rectifier. It converts AC, typically 85 to 260 vac, to a DC voltage which is further smoothed by some input capacitors. This is Vi in the attached diagram.
The rest of the charger is an isolated DC switched power supply. The attached diagram would be the simplest possible version and the one in the charger is much more advanced, mostly with more switches and inductors, but really operates in the end almost identically to this very basic buck/boost converter.
Switch S1 is a semiconductor IGBT (insulated gate bi-polar transistor) that basically is switched on and off at a fixed frequency but a variable pulse width. When it switches on, current flows from the negative terminal of the DC input up through the inductor (coil) and through the switch to the positive terminal of the DC input. Note that all the circuitry on the battery side is kind of out of the picture here.
The current through the coil causes a magnetic field that resists this surge in current and grows as a function of the amount of current through the coil. In this way, energy is "stored" in the magnetic field surrounding the coil. When S1 is opened, the magnetic field COLLAPSES inducing a current in the coil in the same direction. But this requires the polarity of the voltage in the coil to reverse. And that causes current flow through the diode D into capacitor C where it is stored.
The switching of S1 is typically at a frequency where the inductor is always expanding or collapsing the field. And capacitor C is more or less continously receiving pulses of current.
The level of energy in C is modulated by the PULSE WIDTH of S1. If we have a frequency of 10 kHz, for example, our period would be 100 microseconds. If S1 is on for 10 usec and off for 90 usec then the voltage level at C would be 10% of the input voltage. The input voltage of rectified 220vac is roughly 310vdc.
We can increase this power output by increasing the pulse width of S1. And we can decrease it by decreasing it again.
If we connect all this to a battery, you would not actually observe much of a change in voltage at the output R. The voltage would gradually increase as the charge level of the battery increases. Changes would be felt as changes in current to the batteries. And so we can increase the current by increasing the pulse width and decrease the current by decreasing it.
When the car battery reaches some previously agreed voltage level, the car changes the CONTINOUSLY transmitted current request to a lower value. S1 pulse width is decreased and the current is decreased in proportion. if the voltage of the pack rises ever so slightly beyond the target voltage, the car simply again revises its currrent request to a lower value. In this way, it maintains a CONSTANT VOLTAGE during this CV phase of the charge. Once current decreases to soem predetermined level, it terminates the charging session.
Understand that all 12 EVSE chargers can have their outputs all ganged together on a single output bus and ALL the S1's are adjusted together. But the Supercharger can also connect any number of chargers to either of two output busses, with two control channels to do CAN. And so it can indeed charge two cars at the same time. But power would be reduced to BOTH cars. And which is favored over the other would be purely a matter of the firmware in the EVSE Supercharger Station. And again, more or less unlike CHAdeMO, the Supercharger can send a renegotiate signal to a car AT ANY TIME forcing the initial negotiation for current available to be revisited. This would happen so quickly and so smoothly you as a car observer would never know it happened beyond a decrease in power which you might observe.
There are no huge spikes of voltage involved at any point. When the output bus is first connected by contactor to the battery pack, it isn't energized at all. Power is produced to that bus only AFTER the connection is made and actually both the EVSE and the car have to "vote" on making the connection. Either can force a disconnect at any time, but would normally drop the power level to zero before disconnecting.
The concept of a magic variable AC transformer, while comical, almost describes a switching DC power supply, which is what this is. But the output is ultimately DC, not AC.
The actual charger has more switches and inductors and actually provides true isolation in this way. So you really cannot be "electrocuted" or anything even if you touch ONE of the high voltage terminals while it is live. There is no path to ground either in the car or in the EVSE. If you get across both output terminals, you can indeed get a headache. Even then not normally a fatal one.
Jack Rickard
http://evtv.me
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