Ramblings of an Electric Vehicle Geek

Well, it is the end of the summer in warm Riverside, CA, and I have an update to report on the performance of our Chevy Volt and brand new data to post on that of our Nissan Leaf.

Because we used air conditioning pretty heavily in the hot summer months since the last time I posted performance data here, we observed a slight decrease in the overall efficiency of the Chevy Volt (in miles per kWh). But this will be my fairest report so far, because I started collecting data precisely in October 2013, and therefore the report will more or less account for the year-round changes in weather. Here is the report:

Once again, of course we have not driven just 2,500 miles in a year. However, in all of these reported 2,500 miles, we made sure that we started from point A with a full charge, and came back to point A before charging again. We excluded those trips where we had to use gas, or those where we had to recharge at locations that do not give kWh information.

The heavy A/C usage is to blame for the drop from 4.01 to 3.94 in miles per battery kWh, and as a result, for every other slight drop. However, as you can see, the CO2-equivalent MPG suffered a much bigger drop: from 116 to 91. That is because I received new data from Riverside Public Utilities about how clean their electricity is. There has been an increase in their CO2-equivalent emissions: It is now 0.857lbs per kWh instead of 0.687, which was the figure I used earlier.

Now, ready for my first Nissan Leaf data ever? Here it is:

The first thing you can notice is that the battery-to-wheel efficiency is 4.34 miles/kWh, which is about 10% better than that of Chevy Volt. Nice! However, since the charging efficiency is lower at 78%, this lead largely evaporates: the wall-to-wheel efficiency is just 3.40 miles/kWh, a mere 4% higher than the 3.27 achieved by Chevy Volt.

I believe the charging efficiency of the Leaf is worse because it is charged with a higher power (about 3.7kW compared to Volt's 3.2kW with Level 2 charging). That is understandable because with a pure EV, you need to maximize your miles per hour of charging to avoid being stranded. Indeed, despite the lower charging efficiency, with 3.7kW power consumption, the Nissan Leaf goes about 12.6 miles per hour of charging, whereas the Chevy Volt goes about 10.5. That is 20% more!

Let me finish this post by comparing the efficiency data side by side for the two cars:

I have been charging my Chevy Volt with ChargePoint at work and downtown Riverside for the last 7 months. As explained in my previous post, I recorded the miles, dashboard kilowatt-hours, and kilowatt-hours measured at the charger whenever I came back to the same point as the dashboard data got reset, and had not in the meantime (a) charged the Volt at any spot which does not give me kWh information, including home, and (b) consumed any gas.

The data is as below:

Of course, I didn't drive just 1826 miles in 7 months. However, the fact that I don't want to leave any room for guesswork limits me to only that many miles. Nevertheless, I believe 1826 miles is long enough to converge to statistically meaningful numbers.

Let's look at the above table again. I guess I owe everyone some explanation about the last three rows there. The energy-equivalent MPG, or more commonly referred to as the MPGe, is the standard EPA measure, where 33.7kWh of electricity is taken as equivalent to 1 gallon of gas. Therefore, the 3.32 miles/kWh performance I calculated above is translated to  

This is actually higher than the EPA’s rating of 98 MPGe. That is probably due to very good weather conditions here in Riverside, and the driving style of yours truly. 

But this is not the most interesting MPG figure one can think of, because at the end of the day, what significance does consumed energy have if all energy is not created equal? Much more interesting are the figures of CO2-equivalent MPG (for the environmentalist) and dollar-equivalent MPG (for the economist). As far as I know, nobody else defined those measures before.  

CO2-equivalent MPG is the MPG of a hypothetical gasoline-burning internal combustion engine (ICE) that would emit as much CO2e per mile as your electric vehicle (EV). Obviously, you need to know (a) the CO2e emitted as a result of burning 1 gallon of gas, and (b) the CO2e emitted as a result of consuming 1 kWh of electricity. Here, by CO2e, I mean equivalent CO2 emitted not just during operation, but also during production and distribution. EPA calculated the CO2 emitted during operation, i.e., from tank-to-wheel, as about 20 pounds per gallon of gas (which is based on solid chemistry). For production and distribution, i.e., for well-to-tank, figures are much more fuzzy. On the net, I have seen well-to-tank numbers all the way up to 8 pounds of CO2e per gallon of gas. I am being more conservative here, and assuming 4 lbs/gallon. Thus, I will take the well-to-wheel emissions as 24 lbs/gallon.

As for the CO2e consumed per kWh electricity, you need to check with your provider. Apparently, Riverside Public Utilities is not very public when it comes to the emitted CO2e per kWh, but luckily I know someone who works there and he has access to that figure for every year since they started calculating. The latest number they have is for 2012, which is 0.687 lbs/kWh.

Therefore, the conversion factor from miles/kWh to miles/gallon in the CO2-equivalent sense can be found as

thereby yielding a CO2-equivalent MPG of

The dollar-equivalent MPG is similarly defined as the MPG of a hypothetical ICE vehicle that would cost as much money to drive per mile as your EV. Again, for this one, you need the cost of 1 gallon of gas, and of 1 kWh of electricity. Luckily, these figures are as transparent as can be, although the price of gas fluctuates a lot. At the time of this writing, it was $4.206 per gallon on average in California, but I took just $4 (to the advantage of ICE vehicles). Riverside Public Utilities charges $0.1035 per kWh, so that’s what I’m using. The conversion factor then becomes

yielding a dollar-equivalent MPG of

The dashboards of both Chevy Volt and Nissan Leaf give numerical information about your miles per kWh performance. However, that only accounts for the battery-to-wheel performance. What about the wall-to-battery performance? How do we account for the losses during the charging process? 

We have a Bosch Power Max Level-2 charger at home. We are happy with it and all, but unfortunately the designers didn’t think of the possibility that some people like me would want to know the total kWh pulled from the grid. Some utility companies put a separate meter on your charger, because they give you better rates when you charge your vehicle. Ours is Riverside Public Utilities, and they do not yet do such a thing. So, no luck there.

Enter the ChargePoint Level-2 chargers we have at work and at many public parking lots in Riverside, CA. They couldn’t have picked a better EV-charger to install. Not only can you control the chargers remotely and check their availability using your smart phone, but it also tells you how many kilowatt-hours you consume on each charging session. That, dear readers, is my gold mine. 

Chevy Volt 2013 automatically resets the mileage and consumed energy after a full charge. For the Nissan Leaf 2012, it is a bit more involved; after a full charge, you need to reset you trip odometer and your miles per kWh performance. After the reset, you drive like you normally would, go to work, go run errands, go home, etc., and when you come back to the same point as you charged before, take a note of how many miles you drove and how much energy you consumed. When you perform another full charge, you can now find out 1) the true miles per kWh, and 2) the “charging efficiency” i.e., what percent of the electrical energy you pulled from the grid actually went into your battery.

For example, during my very last trip for which I could collect data as above, my Volt traveled 33.7 miles, and according to the dashboard, consumed 7.8 kWh. I fully charged it and read 9.683kWh from the ChargePoint app. That means

Wall-to-wheel miles/kWh = 33.7/9.683 = 3.48

Charging efficiency = 7.8/9.683 = 0.806 = 80.6%

This last number varies a lot. Well, both numbers do from trip to trip. But it is somewhat understandable that the miles/kWh does, because there are so many variables like driving style, weather, tire pressure, etc. But it is weird that the charging efficiency varies at all. I have been observing a range of 80% to 85%, with an overall average of about 83%.

By the way, information about charging efficiency was one of the things I struggled a lot to find on the net, to no avail. Some websites talk about charging efficiency as the internal efficiency of the Level-2 charger. However, the true efficiency must take into account all the losses, including those within the vehicle.