Calculating the charge rate of a public charging station

There are many different companies providing L2 and DC fast chargers. Unfortunately, the rate at which they can charge your car, especially for DC fast chargers, can vary from location to location.

For now, I’ll focus on DC fast chargers typically found with a CHADeMO connector, CCS, or both.

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This image from Plugshare shows two EVgo fast chargers and 4 Tesla Superchargers.

If you look at the (usually left) side of the charger, there should be a conformance label that shows the certifications as well as power input and output specifications.

Screen Shot 2018-08-03 at 6.15.13 PM
The power output is circled in red above.

This charger is listed on Plugshare as being a 50 KW charger, but will only charge my Chevy Bolt at up to 35 KW. Let’s look into that.

The voltage output range is from 300 to 500 volts of direct current, with a maximum current of 100 amps. To convert this to kilowatts, or KW, you multiply the volts * amps, then divide by 1000 to convert watts to kilowatts.

300 volts * 100 amps = 30,000 watts = 30 KW
500 volts * 100 amps = 50,000 watts = 50 KW

So effectively, this charger can provide between 30 and 50 KW of charging power, depending on the voltage it’s putting out. The voltage will depend on the vehicle being charged, and is negotiated between the vehicle and the DC charger based on the car’s needs.

My Chevy Bolt operates at around 350 volts, which is why I was seeing about 34 KW (out of a max of 35) when I charged.

Wikipedia has a post on EV Batteries showing the capacity of many cars. The capacity will impact the length of charge, but how the battery back is constructed of it’s smaller component cells dictates the total pack voltage. Some examples:

Nissan Leaf: 360v
Chevy Bolt: 350v
Tesla Model S: 375v
Tesla Model 3: 350v
BMW i3: 360v
VW e-Golf: 323v
Jaguar i-Pace: 450v

So you can see there is some variability across these cars, which will impact the rate of charging at the same charging station. The Jaguar i-Pace should charge noticeably faster than my Chevy Bolt, for example.

2 comments

  1. Good idea for an article. It would be good to clarify that the relevant peak charging rate depends on the charging voltage and that, in turn, depends on and increases with the battery state of charge. Different cars will begin to restrict their charging current at different states of charge. Some, like the Bolt EV, begin to ramp down the current at around 52-55% state of charge but others like the Audi e-tron don’t ramp down until a higher state of charge (and thus charging voltage).

    The numbers you give for different vehicle battery packs seem to mostly be “nominal” pack voltages or, in other words, the open circuit voltage when the pack is about 50% full. Some numbers seem a little off. The Tesla Model is listed as 375V but should be about the same as the Model 3 since both use similar cell chemistries have have the same number (86) of cell groups in series. The number should be around 350V for both Model S and Model 3. Some smaller Model S packs like the 60 kWh version actually have fewer cell groups in series and are lower than 350V.

    Also, the number cited for the Jaguar of 450V is a peak voltage when 100% full. It’s nominal voltage is just under 390V.

    Liked by 1 person

    • Typo…

      I said:

      “Tesla Model is listed as 375V but should be about the same as the Model 3 since both use similar cell chemistries have have the same number (86) of cell groups in series.”

      I meant to type 96, not 86 cells groups in series.

      Like

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