Leaf Mileage Update with 14 Months of Data

It's time for my bi-yearly update on my Nissan Leaf experience. I'm on my second Leaf, having owned a 2012 Leaf SL for two and a half years before trading it in for a 2013 Leaf S. I've written extensively about both Leafs already, so I won't repeat myself too much here. Check out the EV tag for all the gory details. Suffice it to say, I love the Leaf, and after having driven an EV for three and a half years, I can't imagine going back to an ICE car. The Leaf is fun, torque-y, quiet, and oh-so comfortable to drive.

On Range and Battery Degradation

The one major issue with the Leaf is the capacity of the battery, coupled with how long it takes to charge it back up. It has a range of about 85-100 miles on a full charge in the summer, depending on driving conditions, so I'm limited to the city and the immediately surrounding area unless I do careful planning and have a lot of time. Those stars have not yet aligned, but I do enjoy zipping around Madison and coming back home to charge up in my garage. It's the essence of convenience. We have a Prius for the longer and far less frequent trips we need to take beyond a 40 mile radius of our house.

Because EVs are still a new and interesting technology, I keep records of my driving and charging so I can plot things like the change in range over temperature, battery efficiency, and estimated battery degradation over time. To read about the methodology I use, take a look at the two-year update of my 2012 Leaf or the first report of my 2013 Leaf. Basically, I track driving temperature, battery state-of-charge (SOC), mileage, and kWh consumed at the wall outlet. I always trickle charge off of a 110V outlet through a P4460 Kill-A-Watt power meter so I know exactly how much electricity I've used to charge the car.

Since the main question to answer about the Leaf is what kind of range it gets, I use my data to estimate the range I could get on every charge. I scale up the miles I drove to what it would be if I charged the battery to 100% and drove the car until the battery died. This is assuming the SOC is linear over the entire range, even though that doesn't seem to be exactly true. In my experience a 1% change in SOC will take you farther when the battery is mostly discharged than when it is mostly charged, but I don't have a good way to account for this so I assume it's linear. Then I plot these estimated ranges against the average temperature for each discharge cycle, and I get the following plot for 14 months worth of data:

This plot is interactive, so you can hover on points to get details and zoom in by selecting an area with the mouse. Clearly, the range has a significant dependence on temperature, with a range as low as 42 miles at sub-zero temperatures and as high as 110 miles in perfect summer weather. I very rarely use the air conditioner or heater, so range would be reduced from this data if climate control was used on especially hot or cold days. In fact, the outlier at 86°F and 78 miles of range was a day when I drove the family 53 miles with the air conditioner running to keep them comfortable. It was also a trip that was about half freeway driving at 65 mph, which further reduced the range. (For a great set of charts on the Leaf's range dependence on driving speed, check out the range charts at MyNissanLeaf.com.)

I split the data between the first 8 months and the last 6 months so we can see how the range has changed over time. Trend lines are shown for both sets of data, and the few outliers—one in 2014H2 and two in 2015H1—were ignored when calculating the trend lines. The two lines are practically indistinguishable at the warm end of the temperature range, and those points are further apart in time, taking place in the summer in 2014 and the beginning of summer in 2015. The 2014H2 range at the low temperatures is actually lower than the 2015H1 range even though the points at that end of the graph happened closer in time and the lower range points happened earlier, likely because it was slightly colder at the end of 2014 than at the beginning of 2015. Overall, it appears that the battery has had a negligible amount of degradation in the past 14 months.

I do what I can to keep my battery as healthy as possible, since a healthy battery will have a longer range over a longer period of time. To take care of the battery, I generally follow these guidelines:
  • Charge to 80%.
  • Do not charge if SOC is at 80% or above.
  • Avoid hitting the low battery warning at 17%, the very low battery warning at 8%, and turtle mode at the end of charge.
  • No DC Quick Charging.
  • Reduce the number of charging cycles by not charging every night.
  • Store the battery at a lower SOC, if possible, by not charging every night and delaying a charge if I know I'm not driving the next day.
  • Limit the depth of discharge (DOD) by charging before a trip that would take the SOC below 20%.
Limiting the number of charging cycles and limiting the DOD are in direct conflict, so it's a balancing act. I'm not sure what the best trade-off is between charging cycles and DOD, but I tend to err on the side of shallower DOD. My average DOD over the last 14 months has been 51%, meaning I normally drive until around 30% SOC and then charge up to 80%. I've gone as deep as 71% and as shallow as 17%. The following histogram shows the distribution of DOD cycles that my Leaf has had:

Leaf DOD Distribution Chart

On Energy Efficiency

That leaves the Leaf's energy efficiency left to look at. I measure the energy used at the wall outlet as well as keep a record of the on-board energy efficiency meter for each month of driving, so I can plot those over time. I can also calculate the charging efficiency from these two energy efficiency numbers, and all three series are plotted in the next chart:

Leaf Energy Efficiency bar graph

You can see that all three efficiencies got worse during the cold months of winter and have now recovered with the Leaf's efficiency meter reporting well over 5 miles/kWh as the weather has warmed up. I even set a new monthly record of 5.5 miles/kWh this month, as measured by the Leaf or 4.5 miles/kWh from the Kill-A-Watt meter at the wall. Charging efficiency for trickle charging is also up over 80% with the warmer weather. I'm not sure what the oscillating behavior of the charging efficiency was about last year, but it seems to have gone away for now. It possibly has to do with the coarseness of the Leaf's efficiency values.

I'm getting fairly good efficiency numbers with the type of commute that I have through the city of Madison, and so far I've used 1,740 kWh of electricity to drive 6,644 miles. Since I pay $0.18 per kWh, that's $313.20 total, or $0.047 per mile that I pay to charge my car. That's the equivalent of paying $1.41 per gallon of gas for a 30mpg car or $0.94 per gallon for a 20mpg car to drive the same distance. That's pretty nice even with the higher than national average price I pay for electricity (to support wind power).

Future EVs

The Leaf has been a great first EV experience for me, and I'm excited to see what the future holds for electric cars. So far the Nissan Leaf, Chevy Volt, and Tesla Model S have been the only practical EVs widely available, and they each serve different markets. The Leaf, being a pure EV with limited range, is a city commuter car. The Volt, with its gas generator, is the PHEV for people that need a full-range vehicle. The Model S is the EV for those lucky individuals that have $100k+ to burn on a car. Now the BMW i3 has entered the ring as well, and it's a combination of the other three cars—the electric range of the Leaf, the gas generator of the Volt, and some of the luxury of the Model S at a little higher price than the Leaf or the Volt.

These EVs have made some significant advances over the past four years, and soon it looks like there will be some bigger leaps forward. Rumors are surfacing that Nissan will increase battery capacity in the Leaf 25% for the 2016 model year, and double it for the 2017 model year. Chevy is getting ready to release the all-electric Bolt with a 200 mile range, and they're increasing the battery capacity of the Volt as well. Tesla is getting close to releasing the Model X SUV, and the mass-market Model 3 with a 200-mile range and a $35k base price will follow, hopefully in 2018. The next couple years are going to be interesting for EVs with at least three affordable cars becoming available with a 200-mile driving range. Hopefully other manufacturers will get in the game, too, and we'll have even more options to choose from. That kind of range could be a game-changer for EVs. I can't wait.

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