How does temperature affect Bolt EV's efficiency?

Efficiency-Temperature Correlation of Bolt EV (February 2019 to January 2020)

I keep a detailed log of my Bolt EV drives to gain insights to the questions I wanted answers to. One of them was this feeling that the drivable range was getting lower on a cold day despite the fact that I drive without having the heater on. Since my EV driving habits became consistent after driving for about half a year, I decided to analyze one full year's of driving from February 2019 to January 2020 to spot a trend between the ambient temperature and the car's efficiency ("fuel economy").

The results above speak for themselves. Even if you don't use a heater, the car's efficiency will certainly drop as the outside air gets colder. This is largely because the air itself becomes more dense, increasing resistance. Using a heater will impact efficiency on top of this. Meanwhile, driving at an average trip speed of about 50 km/h (30 mph) yielded about 1 to 1.5 km/kWh better efficiency than at about 80 km/h (50 mph). Again, less air drag meant better outcome.

If you want to know how the data points were chosen, please read on.

Road Type Speed Limit
(km/h)
Minimum
% of Trip
Trip Range (km) Speed Range (km/h)
Min Max Min Max
Expressway 100 - 110 75 112.7 364.8 70.4 90.0
Intercity 70 - 90 19.6 25.3 40.6 69.0
My two most common driving patterns happen on expressways and intercity roads. The former are usually for the long distance family trips. The latter are used in the routine errands between Naju and Gwangju for groceries or movie-going. Downtown driving is done mostly by my wife and there are no records of time or temperature, so they were not analyzed.

As all trips start and end within cities, the most significant and fastest road type used must take up at least 75% of the entire distance for a sample to be representative of a type. And to minimize impacts of traffic jams, average speed of a trip had to be at least 70 km/h for expressway and 40 km/h for intercity. Driving under rain or snow were also out in order to avoid other weather factors. Additionally, expressway trips had to be at least 100 km long and the intercity trips had to have no significant deviations from the most common 22 km-long route I take. Use of heater was completely avoided, while air conditioning was used very sparingly if needed and took up less than 1% of the battery consumption.

Road Type Total Trip
(km)
Average Data Count
Trip (km) Time (H:M:S) Speed (km/h)
Expressway 9,036.7 251.02 3:10:03 79.25 36
Intercity 1,593.1 22.13 0:26:20 50.41 72
In the end, 108 samples spanning a total distance of more than 10,600 km were chosen out of the 27,000 km total distance covered during the period. The real life driving conditions did still create some variability, but the trends were clearly present. I can now use this analysis to better plan for future trips.
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Assessing one full year of Bolt EV driving

My Bolt EV has travelled 24,099.4km in its first year

On June 1, 2019, my Bolt EV had its first birthday. As it moved around more than 24,000 kilometers, I kept a detailed log to make continuous analysis of the car's conditions and characteristics. Many people including myself are interested in how an electric vehicle fares over the years, so this should provide some good insights.

Bolt EV's monthly statistics from June 2018 to May 2019 - distance, fuel economy, and battery capacity

I used to drive around 1,000km per month on average before getting a Bolt EV. But you can see that it has doubled since. Cheaper fuel costs was a major factor (less than 1/10 that of gasoline), with some "new car curiosity" thrown in. Efficiency suffered in summer and winter due to the extreme temperatures, which affects battery performance and climate control use. Largely speaking though, fuel economy had been improving because I've been adjusting my driving style to be smoother in order to go further before recharging. This proved to be helpful in long-distance trips.

The three lines at the bottom of the graph depict the battery capacity as calculated by various means. The battery degradation is a major concern for many, so I kept track of this closely as well. Going by the reported values, my Bolt EV originally had 58.63kWh of usable capacity (65.14kWh raw) and had 55.98kWh usable (62.20kWh raw) by 24,099.4km. This is a degradation of 4.52%. Assuming linear progression, the battery would have exactly 70.0% of capacity left after 160,000km. This is in line with the industrial average warranty and shows that my Bolt EV's battery is in a reasonably good condition so far.

So why did I have three lines here? It stems from the fact that the Bolt EV doesn't tell you its battery health outright. One of the Parameter ID (PID) readings from the OBD-II port (#2241A3) correlates directly with battery capacity, but interpreting the number has been up for debate. So I decided to find an interpretation that I was comfortable with.
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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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Using 3rd Party Navigation on iOS 12 CarPlay

Kakao Navi appears on the CarPlay screen on the Bolt EV's infotainment display

Apple has included 3rd party navigation application support for CarPlay with iOS 12, which means the cars equipped with CarPlay can use maps other than Apple Maps as long as they make use of the new API. Google Maps and Waze were named when the feature was announced back in June, but one of the major Internet Service companies in Korea, Kakao, beat them to the punch and launched the CarPlay-supported version (3.26.0) of its Kakao Navi app today, September 15. As the iOS 12 GM was already released to the developers and beta testers two days ago, it was possible for me to try it out on my Bolt EV as you can see above.

Default look of the Kakao Navi upon launching

Kakao Navi is no stranger to the car navigation game, as it was selected as the sole navigation app when Google's Android Auto was launched in Korea in July 12 of this year. This happened because the stand-off between Google and the Korean government resulted in a severely crippled Google Maps support in Korea. In any case, Kakao Navi has claimed first 3rd party navigation support on both Google and Apple's car interfaces for Korean users.
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On the road with Bolt EV, part 2

The trunk was packed again to make the return trip to Naju from Ulsan

After arriving in Ulsan and charging the Bolt EV's battery as seen in the last post, my four-member family went about our own business for a couple of days. When it was time to return home, we dropped by a nearby Costco to pack up some items in the trunk. While the space was smaller compared to the one in the cars I used to drive, we were able to fit everything in.

We spent 18% of the battery charge during our stay, leaving 76%. It seemed a bit risky to attempt a full return without a mid-trip recharge since the previous trip used 73% of the battery. Still, I thought it presented an interesting opportunity to see the car's limits and pressed on casually, with the air conditioning on. The following time-lapse video shows what happened in its entirety.


As you can see, the Bolt EV was able to return to the charging station in the parking lot at home just barely. Like the previous trip, this sort of drain-to-the-bottom run should be attempted only if you're sure of the range and the charger is ready at the end. So what was the scariest moment?
Continue reading "On the road with Bolt EV, part 2"

Copyright (C) 1996-2020 Wesley Woo-Duk Hwang-Chung. All rights reserved.