Bolt EV under ideal charging conditions

Korea's Ministry of Environment (ME) has been aggressively expanding its network of DC Fast Charging (DCFC) stations throughout the country, with more than 1,100 new chargers being installed and operating in pilot mode since early this year, accounting for nearly 40% of total. I was fortunate enough to live close to one of such stations (and a 100kW version at that), which let me test out the charging characteristics of my Bolt EV without costing me a dime.

Although the chargers are supposed to switch to normal paid operation starting mid-May as the firmware updates are deployed in a staggered manner, I was able to observe what the close-to-ideal charging situation would be before this happened to the nearby charger. The following graphs plot the data I recorded.

It should be noted that the ME chargers have either a 40-minute or a 41-minute time-out. This was done to prevent a single person from hogging the charger for too long. Therefore, I did these charging sessions during the early hours in the morning when no one else was around in order to have as close to continuous charging as possible. This led to a bit of "blips" in the graph (64% - 41st minute / 88% - 82nd minute), but it did not affect the overall picture that much.

Bolt EV's charging curve on a 100kW station as a function of Displayed SoC

Charging speed is largely dependent on the battery's State of Charge (SoC), so it helps to see the data as its function. As you can see, the charging current remains more or less constant at a given "zone", then drops down a step after a certain level of SoC is reached.

The actual charging power will slowly increase in a zone because the charging voltage rises. This is a direct reflection of the the voltage of the battery cells themselves, which rise as the energy is filled up. The 288 cells are arranged as 96 groups in series of 3 cells in parallel, so there would be nearly a 100-fold difference between the cell voltage and the charging voltage.

Another major factor in the speed is the battery's temperature. Assessing multiple charging sessions, it became apparent that it should be around 24 to 27°C at the beginning in order for the Bolt EV to enter maximum current (roughly 150A before 50%). If it's colder, it will start out a bit slower, then ramp up to 150A as the battery heats up to about 24°C. If it's too hot (more than 30°C), the charging current caps to 95A to prevent overheating.

Bolt EV's charging curve on a 100kW station as a function of time

For someone who's waiting for the car to fill up, the time it takes for each of those charging zones is also quite important. So this is a graph showing the same data, but plotted as a function of time. Key numbers are distilled into the following table.

To make a quick comment about the displayed and actual SoC, the two meet at around 75% mark, with the displayed getting larger above and the actual getting larger below. At the extremes the two differ by about 4%, showing the buffer for preventing over-charging or over-discharging.

SoC Range (%) Duration (mm:ss) Speed (%/min) Power (kW) Current (A)
10 ~ 50 28:30 1.40 53 149
50 ~ 67 16:00 1.06 39 107
67 ~ 83 24:00 0.67 25 66
83 ~ 92 20:00 0.45 18 46
92 ~ 98 17:00 0.35 11 29
98 ~ 100 08:30 0.24 6 ~ 10 17 ~ 28
The power and current values are as seen from the charger. The values from the vehicle's subsystem were about 96% of these, showing the losses inherent in the charging process. Further losses occur as the energy ends up inside the battery, so we end up with a bit more than 10% loss in total.

The advantage of using a 100kW (500V x 200A) charger is apparent only for the first 50% of charge, and is not a huge one at that. 50kW chargers in Korea supply either 110A or 120A maximum current, so the charging speed of 1.06%/minute should extend to below 50% SoC when you use them. Hence, you'll shave about 10 minutes off the session with a 100kW charger instead of a 50kW one if you're starting from 10% charge left. You can thank Bolt EV's highly conservative charging regime for this.

So what's the takeaway from all these information? Probably a good basis for forming a charging strategy during a long-distance trip. To minimize charging times, you should keep the car's SoC between 10 to 20% minimum and 70% to 80% maximum, with each charging session lasting about an hour at most. The last 25% alone takes an hour to charge, so a full charge is not a good strategy unless you're going to a place where the chargers are sparse. Meanwhile, the ambient temperature during charging should be as close to 20°C as possible. Hopefully, you can find a charger within a building or under a shade.
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