Let us know if you can figure it out
Some things that we learned in grade school apparently don’t work in the real world. Like basic math for instance. We decided to demystify the confusing EPA mileage range assigned to various manufacturer’s electric vehicles (EVs) only to be slapped down by yet another confusing government process.
Let’s start with the Nissan LEAF, as that is the vehicle with which we are primarily concerned. According to Nissan, a LEAF owner can expect to go 100 miles on a full charge. We know this not to be a reasonable expectation, as Nissan bases their claim on the EPA’s LA4 test cycle – which has an average speed of under 20 miles per hour with a distance traveled of just over 7 miles. Not really representative of the typical driver in the U.S., and we are not really sure how Nissan can even make this claim based on that test. But back to the EPA.
The EPA’s fuel economy portion of the LEAF’s window sticker claims a range of 73 miles – one that we find to be actually very close to what many might reasonably expect to achieve, based on a variety of driving conditions, speeds, and climate. How they obtain this number is where the black magic starts to make itself known.
On the EPA’s fuel economy label applied to the LEAF, they say that it needs 34 kilowatt hours of electricity to travel 100 miles. So, in order to get their estimated range, just do the math. The LEAF has a 24 kilowatt hour battery – 24 divided by 34 equals .71. So, based on this, the LEAF should go 71 miles. But the EPA says it will actually go 73. So one might assume that there might be some rounding involved somewhere along the way to account for the two mile difference. But let’s take a look at the other three vehicles listed on the EPA’s web site.
Ford Focus Electric uses 32 kilowatt hours to go 100 miles. It has a battery capacity of 25 kilowatt hours. 25 divided by 32 equals .78. Based on this one would expect the Focus to travel 78 miles. The EPA says it has a range of 76. Here again we are off by a couple of miles, but in the other direction.
The Coda is not very efficient and uses 46 kilowatt hours to travel the same 100 miles, but Coda has the largest battery here – 31 kilowatt hours. The math would project a range of 67 miles, but the EPA says that it will travel 88. Way beyond a rounding error.
Finally, the Mitsubishi i is the most efficient of the four evaluated here using only 30 kilowatt hours per 100 miles, but it also has the smallest battery pack – 16 kilowatt hours, or just more than half of that found in the Coda. Math would say you could drive it 53 miles. The EPA says you can go 62.
Clearly, we don’t understand grade school math. Either that, or we don’t understand the EPA’s methodology. The EPA is using some basis other than the number of kilowatt hours to travel 100 miles to determine overall vehicle range. If that is the case, why are they even putting the kilowatt hours per 100 mile statistic on the car? Just another mystery of life to ponder.
The amount of electricity consumed is derived from the amount electricity from the wall plug (e.g. from the grid), not from the pack size. So it can be influenced to some extent by the efficiency of the on-board charger. However, the dominate variable is the usable battery capacity, set by the manufacturer. The LEAF for example, has a usable battery capacity of about 20kWh out of its 24kWh total. You are never allowed to charge too high nor discharge to low. The battery will last far longer if kept in the middle sweet spot. That’s why many LEAF owners only charge to 80%.
The EPA testing for EVs follows SAE Recommended Practice J1634 “Electric Vehicle Energy Consumption and Range Test Procedure”. An overview can be found here:
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCwQFjAA&url=http%3A%2F%2Fwww.smidgeindustriesltd.com%2Fleaf%2FEPA%2FEPA_test_procedure_for_EVs-PHEVs-1-13-2011.pdf&ei=BCmUT-OuDaes2wXk8oHwBA&usg=AFQjCNEa1op_ZQLaHTT1_5dlmDr1gkCh4Q
Indy, thanks for the feedback. I had seen the document that you linked to before. The document that you reference is not a government document, but a summary posted by an individual on their blog. That would be much the same as me posting a document and making it publicly accessable on this blog. I’m not saying that there is anything wrong with the document. I’m just saying that it is not the EPA testing procedure, but a summary of someone’s understanding of the testing procedure.
To your point of using grid electricity usage to measure range – from my point of view, as I have stated before, that is the important measurement criteria. In other words, how much does it cost in real dollars to drive X miles down the road. That is determined by the power used from the grid, not by how much power the on-board battery can store. That said, I stand by my math in the post. Because once you are disconnected from the grid, you only have that power available to you that is stored in the battery. Using the Coda – if it takes 46 kilowatt hours to travel 100 miles, and the Coda has a battery pack size of 31 kilowatt hours, it should have a range of only 67 miles. There is one other possibility – Coda first advertised a battery capacity of 34.8 kilowatt hours. If that is indeed the case, the range would be 87 miles – almost spot on with the EPA’s expected range. The only anomaly would then be the Mitsubishi, which I currently cannot account for.
For more information on the ongoing SAE J1634 procedure development, here is a link to an Argonne National Laboratory (ANL) presentation (it’s only 13 slides long). ANL is a U.S. DOE national lab that provides research into energy, environment and national security. As near as I can tell, the original J1634 testing procedure (that procedure used to determine EV range), is undergoing ongoing evaluation to be able to deal with the wide range of new technologies – pure battery electric vehicles, traditional hybrids, plug-in hybrids, and Volt-type hybrids. They are faced with an extremely complex task, and as I understand it, there is no standard currently in place. One thing about this evaluation process did rather stand up and make me take notice – the last bullet point of the last slide states in bold font “Concept not validated with BEV yet”. This is still very much an industry in its infancy – an any number of ways.
Ernie, the size of the pack doesn’t matter it’s the amount usable from the pack that matters. You can stand by your math all you want, but you’re not correct.
Ah, I see the error of my ways. I have a thick head, but when pounded upon long enough and hard enough sometimes new ideas actually make their way in. The EPA is actually measuring the efficiency of several items – the wiring from the electric meter to the EVSE, the EVSE itself, the onboard charger, the battery pack, the controller, and the motor. Beyond that I guess we need to include the transmission and regeneration system since each time the accelerator is lifted it provides an opportunity for regeneration, based on the manufacturer’s approach. If you have ever had an opportunity to drive the Mini E, the term brake regeneration is a misnomer, as regeneration is immediate and aggressive upon throttle lift. All of this must be accounted for in the test procedure. I agree that there is a difference between the usable battery capacity and the stated capacity. The problem arises in that manufacturers publish maximum capacity, not usable capacity and leave us to guess as to the size available for our use. In that sense, the EPA running from a full charge to the car no longer moving is somewhat helpful, but then throwing in that 30% adjustment is I guess their way of saying “your mileage may vary”.
So in reevaluating this information, the most important item to note on the EPA Fuel Economy label applied to an EV is not the range, but the kilowatt hours used per 100 miles. In other words, what is the cost to operate the vehicle. It can be seen on the EPA web site that the Coda costs $850 to cover 15,000 miles while the Mitsubishi i can do it with $550. Granted the Coda is a larger, heavier car. This measure will become more important as more vehicles come to market that are similar to each other in design. Comparing the LEAF with the Focus Electric, two similar vehicles in size and design, we have to acknowledge that the Focus is slightly more efficient than the LEAF. The difference, of course, is that the LEAF starts at $35,200 while the Focus Electric starts at $39,200. $4,000 buys a lot of electrons.