Range Confidence: Charge Fast, Drive Far, with your Electric Car

By David Herron

How does EPA estimate electric car driving range?

Driving range is one of the key electric car measurements we look at. The longer driving range the more useful is that car. Traditionally longer range made the car cost more, because of the large battery pack, but today the falling cost per kiloWatt-hour means newer cars with large battery packs cost the same as older cars with smaller battery packs. The car buyer must know if the electric car they're has enough range to reach their office (and back), or the shopping center, or the church, or over the hills and through the woods to Grandma’s house. In other words, we need a good reliable estimate for electric vehicle driving range and likewise, we need to understand how EPA driving range estimates are calculated.

As we discussed elsewhere, fast charging and sufficient fast chargers boosts effective trip speed. Fast charging boosts effective electric car range. In case Grandma’s house is a little bit outside the total driving range, you can always pop by the fast charging station for a boost.

Electric car buyers need a reliable and accurate range estimate. They need to know which car will take them the distance they need to go. And, ones effective trip speed depends on the range, since the longer range car will tend to require less charging during a day of driving around.

Ratings like “Range” or “MPGe” are meant to be used to compare cars. Earlier we went over the contents of the EPA information label for electric cars, now it’s time to delve into the process to calculate those figures.

Fortunately Car And Driver [CarAndDriver] published an informative article going over how the EPA generates the data on these labels. Unfortunately that article focuses on gasoline powered cars, but the EPA follows a similar process for electric cars.

The most important fact is that the EPA does only a small portion of the certification testing. Instead, the manufacturers do the majority of the testing, with the EPA reviewing results documents submitted by manufacturers. The same is done with other regulatory agencies around the world. Since the manufacturers do the testing, there's some room for finagling. That's led to occasional instances of manufacturers having to restate their range estimates or fuel economy estimates (in gasoline vehicles). The big example of this sort of finagling is the Dieselgate problem causing the Volkswagen Group so much troubles. [DieselGate]

Another important fact is that we're talking about a range ESTIMATE. The number on the EPA label will be different from what you see in real life. Personal driving habits have a big impact on the results you get. The range estimate is computed, as we'll see, by running a car through a series of standardized tests.

Estimating electric vehicle range with EPA test cycles

There are several test cycles to calculate the City, Highway and Combined efficiency ratings. The EPA details the test cycles on the (fueleconomy.gov) fueleconomy.gov website. [FuelEconomyGov]

What’s shown here is the City cycle. The total test time is 1874 seconds (31 minutes), covering a distance of 11 miles at an average speed of 21 miles per hour, and a maximum 56 miles/hr speed. The test is not performed on a road, but in a lab with the car strapped to a dynamometer. Also called a dyno, a car is strapped down such that its drive wheel(s) are on giant rollers. It allows engineers to mimic driving the car on the road, without actually taking it on the road. It’s done this way to make the test more repeatable than if it were performed on the road. [DieselGate2]

As you see from the chart, the test procedures require a constantly changing speed. Since the test shown here is for City driving, it’s obviously designed to mimic the occasional stops at intersections while you’re waiting for the light to change. The Highway test cycle doesn’t have speeds quickly ramping up and down, instead one accelerates up to speed and while the speed will vary a bit the car is kept at a fairly high speed the entire test.

The EPA website has more information including a full list of dynamometer test driving schedules. [Dynamometer] EPA’s National Vehicle and Fuel Emissions Laboratory (NVFEL) [NVFEL] is charged with performing the testing. Its page on testing and emissions measurement covers not only range testing, [NVFELTesting] but the other tests administered by the EPA.

Other countries have different test procedures than these. As one might expect, the different test procedures produce different fuel efficiency results.

The EPA tests described above measure energy consumption (a.k.a. MPGe) from which is calculated the estimated range. Namely, the test determines the kiloWatt-hours per 100 miles figure. From that it’s easy to derive the estimated range with simple mathematics. See Energy storage and driving range

The other EPA test cycles

The following is the complete list of test cycles published on (www.epa.gov) the EPA website as of February 2017.

The EPA Inspection and Maintenance (IM240) is often used for road-side vehicle testing.

The EPA Urban Dynamometer Driving Schedule (UDDS) is commonly called the "LA4" or "the city test" and represents city driving conditions. It is used for light duty vehicle testing. The UN/ECE Regulation 53 refers to the EPA UDDS as the "Test Equivalent to the Type 1 Test (verifying emissions after a cold start)."

The Federal Test Procedure (FTP) is composed of the UDDS followed by the first 505 seconds of the UDDS. It is often called the EPA75. The dynamometer portion of the test procedure has a very complex timeline of events.

The Highway Fuel Economy Driving Schedule (HWFET) represents highway driving conditions under 60 mph.

The New York City Cycle (NYCC) features low speed stop-and-go traffic conditions.

The US06 is a high acceleration aggressive driving schedule that is often identified as the "Supplemental FTP" driving schedule.

The SC03 is the Air Conditioning "Supplemental FTP" driving schedule.

The EPA Heavy Duty Urban Dynamometer Driving Schedule is for heavy duty vehicle testing. Be careful! Do not confuse it with the usual UDDS for light duty vehicle testing.

Test schedules attributed to the California Air Resources Board (CARB)

CARB has the authority to develop its own environmental testing standards. The following are copied from the EPA website.

The Air Resources Board LA92 Dynamometer Driving Schedule, often called the Unified driving schedule, was developed as an emission inventory improvement tool. Compared to the FTP, the LA92 has a higher top speed, a higher average speed, less idle time, fewer stops per mile, and a higher maximum rate of acceleration.

The LA92Short contains the first 969 seconds of the LA92 (Unified) Dynamometer Driving Schedule.

Economic Commission for Europe Dynamometer Operating Cycles

Official guidance is found in the United Nations Economic Commission for Europe (UN/ECE) WP.29 1958 Agreement and its Addenda. Guidance is specifically found in Regulation 83 of the Regulations for the Construction of Vehicles. This is again copied from the EPA website.

The UN/ECE Elementary Urban Cycle is Part 1 of the ECE Type 1 Test.

The UN/ECE Extra-Urban Driving Cycle is Part 2 of the ECE Type 1 Test.

The UN/ECE Extra-Urban Driving Cycle (Low Powered Vehicles) is an alternative for Low-Powered Vehicles for Part 2 of the ECE Type 1 Test.

Driving schedules specified in Japanese Technical Standards

Official guidance is found in the Japanese Industrial Safety and Health Association (JISHA) Technical Standards. Again this is copied from the EPA website.

The Japanese 10 Mode Cycle is used as a component of the total driving schedule for the 10.15 Mode Exhaust Measurement and Fuel Economy Test Procedures.

The Japanese 15 Mode Cycle is used as a component of the total driving schedule for the 10.15 Mode Exhaust Measurement and Fuel Economy Test Procedures.

The Japanese 10.15 Mode Driving Schedule for Exhaust Measurement and Fuel Economy Test Procedures are specified in Jisha Technical Standards (Jisha 899, 1983).

Footnotes
2. [DieselGate]: In Sept 2015, the Volkswagen Group was accused of committing fraud regarding Diesel emissions. The company lied repeatedly to countries and consumers around the world that Clean Diesel somehow made their cars fast and fun while slashing pollution. It was a sham, and the Volkswagen Group is being forced to pay huge fines.
4. [DieselGate2]: The Volkswagen TDI Diesel fraud relied on detecting test conditions, and altering engine characteristics while being tested. Theoretically it's possible for automakers to commit similar fraud with electric cars. It's an open question how trustworthy are the EPA test procedures.

Range Confidence is Copyright © 2016-17 by David Herron

How does EPA estimate electric car driving range?

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