Tuesday, January 7, 2014

The EPA Range of the BMW i3: A Prognosis

Originally published on Torque News.

Whenever someone strikes up a conversation about the ActiveE, which I have been driving for nearly two years now, the first and seemingly most important question is about the range. I usually say "The EPA range is 94 miles, but my personal best is 125 miles on a single charge". That seems to satisfy  most people, although I often field additional questions.

Why is the range of an electric vehicle so important? Nobody seems to ask this question about a conventional car with a combustion engine. There appears to be a perception that charging an EV is inconvenient, and that it will take some time. The range on a single charge seems to determine the utility and perhaps even the value of an electric car in the mind of many prospects.

While charging infrastructure and the speed of refueling are certainly an important consideration, I will focus on the question, which appears to be on the mind of every prospective BMW i3 buyer. What will be its range?

Although the official EPA figure is not going to be available for a while, it's worth  noting that BMW went on record to say that the i3 will manage between 80 to 100 miles, depending on driving conditions and style.

That narrows it down a little bit, but that's hardly enough. Range figures seem to be as important and as closely scrutinized like the MPG rating for conventional vehicles and hybrids. Will the i3 have a range over 90 miles? And how will it will compare to the Honda Fit EV or the LEAF? I will do my best to answer these questions.

Before we delve into the specifics, it's important to realize that the EPA rating is determined by following a very specific test protocol. This involves a complex driving profile with particular speeds, ambient temperature settings and test duration. Although it will not be possible to take all these factors into account, and calculate a precise range figure, perhaps we can get a realistic estimate by comparing the i3 with other similar vehicles.

An obvious choice would be the 2013 Nissan LEAF. The LEAF has comparable outside dimensions and weight. Additionally, the LEAF has a similar aerodynamic profile, and it seems to consume approximately the same amount of energy at different driving speeds.

The 2013 LEAF has a rated EPA range of 75 miles. This is an average of test results achieved on an 80% and a full charge (66 miles and 84 miles respectively). The i3 will not have a comparable charge setting, and it's very likely that the EPA will only test the car on a full charge.

Taking this comparison a step further, it might be worth to have a look at the NEDC range figures for both vehicles. This acronym stands for the New European Driving Cycle. The corresponding test protocol emphasizes moderate to low-speed city driving and is quite different from the test cycle the EPA uses on this side of the Atlantic. In short, it's a useful benchmark.

A direct comparison indicates that the NEDC range of the i3 and the LEAF is within 5% of each other. Assuming that the LEAF achieves 84 miles on a full charge on the EPA test cycle, this would put the range of the i3 between 80 to 88 miles on a single charge.

Another interesting metric is the usable battery capacity of each vehicle. The i3 has 18.8 kWh of usable capacity per manufacturer specifications, and the Nissan LEAF has 21.38 kWh based on an NREL teardown analysis. This metric suggests that the i3 should have about 12% less range than the LEAF.

While it makes complete sense that a vehicle with a smaller battery should have a shorter range, this does not consider any differences in driving efficiency. These can be quite important. Take the LEAF, for example. The manufacturer has reportedly tuned various drivetrain components between the 2012 and the 2013 model year, which resulted in a 15% greater range on a full charge, even though the rated battery pack capacity remained the same. Could the i3 be more efficient than the 2013 LEAF? Let's have a look.

While one could examine individual differences between the vehicles, such as the size and weight or regenerative braking, this will not be easily quantifiable. Luckily, the NEDC yields another useful metric: kWh used to travel 100 kilometers. The LEAF reportedly needs 15 kWh, and the i3 12.9 kWh. This translates to 4.16 miles per kWh for the LEAF, and 4.84 for the i3.

The implied 16% better driving efficiency should effectively erase the consequences of using a smaller battery pack. Not only that, the i3 should be able to achieve about 2% more range as well:

1.16 x 18.8 kWH / 21.38 kWh x 84 miles = 85.8 miles

If we only consider the battery size and driving efficiency, the i3 should have a slightly higher range than the LEAF on a full charge. Since the i3 will be available in two trims: one a full electric (BEV), and the second one a range-extended hybrid (BEVx), we might want to come up with an estimate for both.

Per manufacturer specification, the range-extended version will be 4.5% less efficient than the pure electric trim due to higher weight and increased aerodynamic drag. Additionally, the range-extender will engage when the state of charge has reached 5%; effectively protecting the battery from reaching very low charge. This means that the electric range of the BEVx trim will be reduced by about 10%.

0.9 x 85.8 miles = 77 miles

If we assume that the BEV trim will achieve 86 miles on the EPA cycle, then the BEVx should get about 77 miles. That's a shade more than twice the EV range of the 2014 Chevy Volt.

And how about the range-extended mode? The tank in the i3 will famously only carry 9 liters or 2.38 gallons of premium gasoline. The range extender in the Volt is rated 35 mpg in the city and 40 mpg on the highway, and it uses premium fuel as well.

Interpreting the technical specs from BMW, the REx autonomy figure is about 16% lower than the BEV range when driving inefficiently, and about 5% lower when driving conservatively. This translates to a REx range between 72 and 82 miles, and a fuel economy of 30 to 35 mpg. The average of these two range figures is 77 miles, and the total projected autonomy of the BEVx trim is 158 miles:

77 miles + 77 miles + 4 miles (reserve) = 158 miles

I hope that you enjoyed the article. Please let us know what range you might have expected or would like to see from the i3.

2014 BMW i3 Range
2014 BMW i3 Range

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