There are various limitations throughout the process that forces the charging rate to be N rather than Z. We want the charging rate to be the highest possible. The higher charge rate means a higher effective trip speed, and therefore we have more flexibility/freedom.
Tesla Motors recognizes this to a degree other car companies do not. They have always focused on vehicles that would blow up stereotypes and cover the most acceptance criteria. In the area of charging that meant Tesla always pushed the charging rate as high as possible.
While a 120 kW charging rate is pretty darn fast, it's not as fast as the equivalent gasoline charging rate.
Let's tour the electric car charging process and the technical limitations at each step.
Battery Pack charge rate limitations
Batteries while being charged heat up, due to inner resistance and other factors. The heat can cause damage or worse. Batteries have a maximum safe charging rate which varies based on the specific battery.
A 120 kW charging rate into an 85-100 kWh battery pack is beyond a 1C charging rate, which is pretty high rate. But I think Tesla quickly ramps down the charging rate rather than keeping it steady 120 kW throughout the process.
For a 200 kWh battery pack the 1C charging rate is 200 kW, and for a 100 kWh pack 1C is 100 kW. Hence, the 200 kWh pack in the recently announced Tesla Roadster 2 means a doubling of the potential charging rate. Ditto for the Tesla Semi - supplying 500 miles range to a fully loaded class 8 truck means a very huge battery pack, giving the possibility of a charging rate high enough to supply 400 miles range in 30 minutes as Elon promised. It's the larger battery pack which enables the higher charge rate, while keeping the charge rate within 1C.
Ampacity and the capacity of charging cords, connectors and wiring
The wiring and connectors has a maximum number of amps it can handle. When I learned Solar Design they said the Ampacity of a wire depends on its thickness, and as the name implies Ampacity is measured in the number of Amps the wire can safely carry. The same holds for connectors.
Generally speaking the higher the Amps required the thicker the wiring and connectors must be.
In the previous section we talked about the charge rate in kiloWatts.
Watts = amps * volts Amps = Watts / Volts
Typical electric cars use a 400 volt (or thereabouts) system, and the DC Fast Chargers are designed with that voltage in mind. DC Fast Charging connects the DC source directly to the battery pack, and therefore the DC Fast Charge station must match the voltage of the pack.
It means that a 50 kiloWatt DC Fast Charge station at 400 volts runs at 125 Amps. if you look at Ampacity charts, you'll see 120 Amps requires really thick wires.
AC/DC Converter capacity
What we just said about the capacity of the wiring applies to every other component in the charging station. The primary componenet in a DC Fast Charging station is the AC-to-DC converter.
The stations receive 3 phase AC directly from the utility grid. For DC charging that must be converted to DC power, because it's being sent directly to the battery pack. The highest efficiency circuit for AC-to-DC conversion is a bridge rectifier coupled with capacitors to smooth out the voltage. At 50 kW or 125 kW or whatever, the size of those components must be huge.
Electrical service limitations
When you buy service from the power company, it’s rated for a maximum capacity. At a Supercharger facility, they have to split the capacity across each stall. The electrical code says the sum of electrical demand within a facility must (of course) be less than the maximum capacity of the electrical service.
The "80% rule"
For a continuous load the power demand must be limited to 80% of the rated capacity of the wires and other equipment. Charging an electric car fits within the definition of continuous load.
Doing the math the other way around, the ratings on the equipment must be 125% of the continuous power requirement.
Because we're talking about Ampacity, the units here are Amps. For the 50 kW charger we mentioned earlier, that's running at 125 Amps, the components must be rated for at least 156 Amps. The electrical code also says to round the component rating upwards.
These are fees charged by utility companies based on the demand pattern of the device. DC Fast Charging stations often fall afoul of this and see high demand charges.
A charging station operator might limit the charging rate to avoid demand charges.