Commercially available electric cars are labeled with an EPA Fuel Economy window sticker, displaying the vehicle’s range as a single number of miles. As can be seen in Table 2, the range of typical BEVs is up to 100 miles, with the Tesla models being big exceptions with driving ranges up to 300 miles.
Table 2 : Overview of 2011-2012 Electric Vehicle models. Marked with an asterisk are manufacturer-estimated ranges, not rated be the EPA.
Because of various factors influencing the range of electric vehicles, it would have been much more appropriate to display a range or ranges instead of giving that single number. This paragraph aims to provide more insight into how actual range might differ from labeled range and it shows how significantly they may differ. Before we go into that, we will describe the method which is used by the EPA to determine EV ranges.
The SAE-J1634 test procedure
The range of electric vehicles is determined by the Environmental Protection Agency (EPA), following a test procedure by the Society of Automotive Engineers called ‘Electric Vehicle Energy Consumption and Range Test Procedure’, or: SAE-J1634. The procedure is similar to that used for determining gasoline cars’ mileage, except it includes some additional practices specific to BEVs and PHEVs, regarding the battery charge, operating temperature and calculations for the range and mileage (mpge, explained below).
The testing is conducted as follows: the electric vehicle is fully charged, parked overnight, and then the following day driven over successive drive cycles until the battery becomes discharged and can no longer follow the driving cycle. Some vehicles enter ‘limp-home’ mode when the battery is almost empty (limited velocity to maximize the leftover range to reach a recharge point) and this rule implies that the EPA does not add this reserve to the vehicle’s range label. After, the vehicle is recharged with a normal AC source and the energy consumption determined by dividing the kWh AC consumption by the miles driven.
The EPA currently applies the ‘5-cycle’ method, which includes five city and highway driving cycles (FTP, HFET, US06, SC03, Cold FTP; the same cycles used for establishing mpg-ratings for conventional cars). In order to calculate mileage and range estimates, weighting factors are applied to the results of each of the driving cycles (EPA, 2011). The same document states that the estimate also incorporates an additional 30% adjustment factor ‘to more accurately reflect the energy consumption and driving range that customers can expect to achieve in the real world’. In other words, the range found in the tests is adjusted with a factor of 0.7.
In order to compare conventional gasoline cars to alternative fuel vehicles, the EPA has introduced a metric mpge, miles per gallon equivalent, based on the energy content of different types of fuels. We can describe the energy content of a gallon of gasoline as 115,000 BTUs, but also in other units: 121 MJ, or 33.7 kWh. The latter comes in handy when we compare electric vehicles to gasoline cars: miles per kWh and miles per gallon.
As an example, observe the FE-label of the 2011 Nissan LEAF in Figure 4Figure 2. As a result of the SAE J1634 test procedure, the LEAF was labeled 99 mpge. This was calculated using the following formula:
Where x is the ‘adjusted’ AC electricity consumption divided by the miles travelled until the electric car was not able to follow the cycle any longer. x (or rather 100x) can be found on the sticker as well: 34 kWh per 100 miles.
Figure 4 : EPA Fuel Economy sticker for the Nissan LEAF.
Based on the displayed mpge values of city and highway, the range would be between 78 and 68 miles, respectively. However, many LEAF drivers have achieved ranges of up to 113 miles on a single charge. The difference is probably a result of the 30% adjustment the EPA factors in. Nissan states a range for their LEAF between 62 and 138 miles, giving scenarios with some more insight into the driving conditions:
Given the LEAF’s battery capacity of 25 kWh, the DC electricity consumption varies from 0.18 kWh/mile to 0.40 kWh/mile, over a factor of 2 different. With such a variety in possible ranges, it hardly makes sense to put a single number on an EV. Especially considering that the above scenarios are based on brand new batteries and thus do not take into account battery degradation. For future drivers, this uncertainty in expected range may be addressed by estimating EV consumption based on temperature and GPS readings from trips made in conventional gasoline cars (like a personal EV range simulator; see 'Discussion').
In the next chapter, we show what factors lead to the wide variety of range; things important to know if you'll ever own an electric car.