A Practical Approach for Collecting Electric Vehicle Specifications and Determining Battery Capacity Thresholds
Mehrsa Khaleghikarahrodi1, Jeremy Herrera Santos1, Fatima Lynch-Smith2 & Gretchen A. Macht1
1 Department of Mechanical, Industrial & Systems Engineering
2 Department of Electrical, Computer & Biomedical Engineering
This methodology defines the data required to determine the battery capacity thresholds for categorizing electric vehicles into small and large vehicles.
1 Data Inputs
The data consists of the make, model, year, battery capacity, driving range, charging time, and battery charging rate of plug-in hybrid electric (PHEV) and battery electric vehicles (BEVs). To find the aforementioned elements on electric vehicles (EVs), the public database from Fuel Economy – the official U.S. government source for fuel economy information – is utilized. This website is administered by Oak Ridge National Laboratory for the U.S. Department of Energy and the U.S. Environmental Protection Agency (Fuel Economy, 2023a) and includes specifications on vehicles (e.g., make, model, year, etc.) in the U.S. To determine vehicle efficiency (range-capacity ratio), manufacturers conduct a series of tests in a laboratory as specified by federal law. These tests are usually done on pre-production prototypes, and the results are reported to the Environmental Protection Agency (EPA). The EPA reviews the results and confirms about 15%–20% of them through their own tests at the National Vehicles and Fuel Emissions Laboratory. A dynamometer is used to simulate driving conditions, and adjustments are made for wind resistance and vehicle weight. A driver then goes through several cycles that simulate driving behavior. For electric vehicles, the battery is fully charged, and the vehicle is parked overnight prior to use. The procedure with the dynamometer is the same except for the method of measuring range at the end of the experiment. The vehicle is driven until the battery becomes fully discharged. After running the cycles, the battery is recharged, and the energy consumption of the vehicle is determined by dividing the kilowatt-hours of energy to recharge the battery by the miles traveled by the vehicle. The same process is used for PHEVs (Fuel Economy, 2017, 2023b). By determining the range based on the energy required to recharge, the determined range value includes any losses due to inefficiencies of the manufacturer’s charger. In addition, battery capacity cannot be used to calculate range since manufacturers usually do not design vehicles to use the entire battery capacity. It should be noted that vehicle manufacturers are not required to test every vehicle offered for sale; they are only required to test one representative vehicle, typically a preproduction prototype, for each combination of loaded vehicle weight class, transmission class, and basic engine (Fuel Economy, 2023d).
Figure 1: Battery Capacity Over the Years Per Make
2 Acquisition of Data
2.1 Driving Range, Charging Time, and Battery Charging Rate of Electric Vehicles
For data collection, the Power Search feature on the Fuel Economy website (Fuel Economy, 2023c) is used to determine the make, model, year, driving range, charging time, and battery charging rate of electric vehicles available in the U.S. Since all vehicles available in the U.S. (with few exceptions) must go through the EPA (Fuel Economy, 2023d), this was treated as an exhaustive list of all electric vehicles driven in the U.S. This functions under the assumption that the Fuel Economy has listed all of the vehicles that have been tested under EPA guidelines. The data was entered year by year, model by model. The “Plug-in Hybrid” and “Fully Battery Electric” features were used to differentiate between BEVs and PHEVs in the search process. After the make, model, year, driving range, charging time, and battery charging rate information was determined. Since this source did not include battery capacity data, a separate search process was conducted.
Figure 2: Battery Capacity Comparison Between Types
2.2 Battery Capacities of Electric Vehicles
The collection of battery capacities in kilowatts per hour (kWh) was collected from other databases, car manufacturers, review articles, news outlets, and research-based websites. Nominal battery capacity, the total size of the battery, was collected instead of the usable capacity. There is an inconsistent reporting of usable capacity by car manufacturers, especially for older car models (Reed, 2011). Prioritization was given to databases and pressrooms from the official car manufacturers. When those sources were not available, the other sources were used. The process of collecting battery capacity information was executed from older to newer cars, in alphabetical order of make and model. Each model was found individually by using a search engine and finding the battery capacity (unless the article or database included any other car information).
Figure 3: Battery Capacity per Year & Type for Each Make
3 Threshold
This research paper aims to develop an approach for determining thresholds to categorize the battery sizes of small and large electric vehicles. In order to achieve this objective, it is essential to create a realistic and up-to-date database that reflects the existing and available EVs. The data collection process resulted in a comprehensive database comprising information from 44 distinct car manufacturers, encompassing 351 unique models spanning 11 consecutive years (i.e., starting from 2013 to Spring 2023). The existing literature lacks a definitive methodology for defining battery sizes as small or large. Consequently, this paper conservatively estimated the thresholds to accomplish the aforementioned objective. Since battery capacity is a discrete and sparse variable, the Tertile method was implemented to analyze the distribution of collected battery sizes and identify the thresholds (Figure 4). This method involves dividing the ordered battery capacities (i.e., in ascending order) into three equal groups. The selected thresholds for categorizing battery capacities are essentially the first and third quantiles of the distribution. Accordingly, EVs with battery capacities lower than 14.10 kWh are classified as “small,” while those with capacities greater than 82.00 kWh are classified as “large.”
Figure 4: Distributions and Density Plot of Battery Capacities with Estimated Thresholds
Figure 4 displays the two modes in the distribution: one mode is centered around the first threshold of 14.1 kWh, and the other mode is centered around 82 kWh. The concentration of data around these thresholds indicates that plug-in hybrid electric vehicles (PHEVs), generally, have limited battery capacities of up to 40 kWh, whereas battery electric vehicles (BEVs) have a minimum battery capacity of 30 kWh starting from earlier years (e.g., 2013) and this capacity progressively increases over time. Thus, the first and second thresholds and modes represent PHEVs and BEVs, respectively. Future work is recommended to employ and replicate this approach outlined in this article in order to achieve both similar and updated results.
4 Limitations
This study has limitations in which type of battery capacity was collected. Usable capacity will be more beneficial as it will bring better accurate results to what the user has at their disposal, especially when researching their charging behavior. Reported or public newer model unveilings should come with usable battery capacity and not just the nominal battery capacity. For more accurate results regarding charging behavior, there should be a focus on the distribution of vehicle types among EV users, so it can better reflect real-world patterns. The use of more scholarly sources could be used as well, but this database can serve as an academic source for future EV analysis. The process of collecting data could have been done differently as well. Instead of searching up each car individually, the bigger databases can be used to fill in what they have. When gaps of information occur, that’s when car manufacturer manuals or third-party sources should be used.
5 Conclusion
In the past decade, a significant number of EVs have been released into the market as a response to the shift to a fully electric transportation system. However, an established database where EVs’ specifications (e.g., make, model, year, type, charging time, and charging rate) has been missing. This article has compiled that information through various sources to provide the EV audience a referral source where all this information can be found. This database then assisted with the development of this paper for categorizing EVs battery sizes into small and large batteries by applying the Tertile method to the battery capacities distribution. It is a great starting point for future work to reflect upon equivalent EVs among various makes and models, the increase in battery sizes, the ratio of PHEVs vs. EVs, and many more. Moreover, this article and the database provide a clear guidance on upgrading this database depending on individual needs.
6 Acknowledgement
The authors would like to thank Nicholas D. Bernardo, Jr., Ph.D. for his continued support.
7 References
Fuel Economy. (2017). EPA Test Procedures for Electric Vehicles and Plug-in Hybrids. U.S. Department of Energy. [online] Available at: https://www.fueleconomy.gov/feg/pdfs/EPA%20test%20procedure%20for%20EVs-PHEVs-11-14-2017.pdf.
Fuel Economy. (2023a). Frequently Asked Questions. U.S. Departement of Energy. [online] Available at: https://www.fueleconomy.gov/feg/info.shtml.
Fuel Economy. (2023b). How Vehicles Are Tested. U.S. Departement of Energy. [online] Available at: https://www.fueleconomy.gov/feg/how_tested.shtml.
Fuel Economy. (2023c). Power Search. U.S. Departement of Energy. [online] Available at: https://www.fueleconomy.gov/feg/powerSearch.jsp.
Fuel Economy. (2023d). Which Vehicles Are Tested. U.S. Departement of Energy. [online] Available at: https://www.fueleconomy.gov/feg/which_tested.shtml.
Reed P., (2011). Electric Car Battery Basics: Capacity, Charging, and Range. Edmunds. [online] Available at: https://www.edmunds.com/car-technology/electric-car-battery-basics-capacity-charging-and-range.html.