Better Mileage Data with the 2013 Nissan Leaf

I've now had a 2013 Nissan Leaf for nearly 8 months, and temperatures here in Madison, WI have gone below zero, so I have a nice amount of data to share on this newer model. The 2013 Leaf S replaced the 2012 Leaf SL I had previously, and it includes a state-of-charge (SOC) percentage display on the dash that was lacking in the older models. This SOC reading is a major improvement to the unreliable GOM (Guess-O-Meter, or miles-to-fully-discharged meter) that I used before in my data collection.

I haven't invested in any other kind of meter for measuring the Leaf's battery state because I wanted to treat the car more like a normal driver would. Measuring internal battery messages off of the car's CANbus was decidedly outside of normal driver behavior. I did purchase a P4460 Kill-A-Watt power meter for measuring the amount of electricity that is consumed by charging the car. The on-board energy efficiency meter doesn't take charging losses into account, and I wanted to know exactly how much electricity the car is using. I'll report on those numbers as well.

Before getting into the numbers, I will say that I still greatly enjoy driving the Leaf. The 80kW electric motor is nice and torque-y, with zippy performance for around town and pleasing acceleration when jumping on the freeway. The power is especially evident when scaling steep hills, as the Leaf tears up inclines as if they aren't even there. And the ride is always smooth and super quiet.

The handling so far this winter has been pretty good as well. The traction and stability control and the ABS all work when they need to, and the car's low center of gravity from the under-carriage mounted battery helps quite a bit, too. The one thing that could be improved in snow is the stock tires. They don't have the best traction, and the other safety systems have to compensate when the tires slip. I'll probably finish out the winter with them since the tread is still pretty new, but next winter I'm going to switch to snow tires. We're using winter tires on the Prius this year, and they've had an insignificant effect on mileage, so I expect the benefits of using them on the Leaf to greatly outweigh the minor range hit that I'll take.

Data Collection Methodology

Collecting data on the Leaf was fairly straightforward. After every charging cycle, I would log the date, the charge percentage, and the accumulated kWh on the Kill-A-Watt meter. I nearly always charge to 80% unless I know I'm going on a long drive the next day. I was under the impression that the lower charge level was better for the battery. Although, I now hear that new Leafs will no longer have the 80% setting option. I'm not sure if that's because Nissan is trying to avoid consumer confusion, or because there really is no negative impact to the battery when charging to 100%. It seems reasonable that the latter could be true as long as the battery isn't charged until it gets below 80% SOC. The use case for a car battery is much different than for a laptop battery, where keeping it plugged in wears out the battery because it continually charges to 100% during use.

After each drive I keep a record of the %SOC, the odometer reading, and the outside temperature as reported on the dash. I won't charge every night if I don't need to, and I often bring the battery down to 10-20% before charging. I don't normally go below that since there isn't much useful range left for me to get to work and back again at that point. I have a new job with a shorter 16-mile round-trip commute through town instead of my old 23-mile round-trip commute on the beltline, so I can now go three or four days between charges in the summer. There's probably a trade-off between less depth-of-charge and less charging cycles for better battery life, but I have no idea where the optimal point is so I go for less charging cycles.

Once I have a good amount of data logged, I transfer it to Google Sheets to calculate range, average temperature, and miles/kWh. Estimating range is much easier with the %SOC numbers because all I have to do is subtract start and end odometer readings and divide by the %SOC used for those miles. I'm assuming that the miles/%SOC is linear for this calculation, but I'm not likely to push the limit to squeeze a few extra miles out of the battery at the end of the range, so assuming the same miles/%SOC over the entire range is acceptable to me. I'm still getting a reasonable estimate of range over many charging cycles.

I calculate an average temperature for each charging cycle by taking the average of temperatures for each driving segment weighted by miles driven in each segment. Temperature has a big effect on range, so getting an accurate value amidst big Midwest temperature swings is important. I'm sure that the temperature during charging also has an effect, but I don't know how I would estimate this effect without monitoring temperature during every charging cycle. I'm not set up to do that so I ignore that effect. Besides, charging temperature is heavily correlated with driving temperature, even though the Leaf is always charging in a garage. The garage is unheated so it's always a milder form of the outside environment.

The miles/kWh are calculated two ways. The car's measure of efficiency is recorded from the dash, and I let the meter run a measurement for a month before recording the value and resetting the meter. I also calculate the wall-to-wheels efficiency by dividing the miles travelled in a month by the kWh usage for that month from the Kill-A-Watt meter. I can then divide the wall-to-wheels value by the battery-to-wheels value to get an estimate of charging efficiency.

That's how I collect and massage the data, so let's take a look at what we've got.

What is the Real Range of a Leaf?

That is the most common question I get when I talk with people about the Leaf, and for good reason. Everyone knows EV ranges are limited right now, and the answer is it depends, of course. One thing it depends greatly on is temperature. Here's how much my Leaf's range changed with temperature in the last 8 months:

It's an interactive chart so you can zoom and get info on specific points with the mouse. Clearly, temperature is the dominant effect, and the range is cut in half over the temperature range. While I was getting about 100 miles of range at 75°F, I am getting only about 50 miles at 0°F. Luckily, I don't have to drive far to work. This plot does include a mix of stop-and-go city driving and free-way driving at around 55 mph. I did take the Leaf on the interstate once at 65 mph, but only for a short while and it doesn't noticeably show in the chart.

Two features to note in this chart are the outlier at 86°F and the wider variation in driving range at both ends of the temperature range, especially below 40°F. Regarding the outlier, this point happened to be a trip I took with the rest of the family to a high school graduation party. It was hot and raining so there was extra road resistance and I ran the A/C to keep everyone comfortable. The combination of extra weight, road resistance, and constant A/C resulted in about a 20% drop in range, which doesn't surprise me.

The wider variation at the high temperature range is likely due to there being more data points over a wider range of driving conditions. Then as the temperature dropped in the fall, the points followed a more linear curve into the colder temperatures of winter.

The main reason for the wide variation at cold temperatures is probably due to a number of reasons. Depending on temperature and humidity, I have to use the defroster more or less and sometimes I use the heated seats (although the seats don't seem to impact range much). If there's snow on the roads, that adds resistance and lowers efficiency. Lower temperatures happen to coincide with less daylight and inclement weather, so I use the headlights more and usage varies a bit more depending on the weather. Since I have normal headlights instead of the LED headlights that come as an option, they use more battery power. Finally, traffic varies more in the winter, and if I'm stuck in traffic, that amplifies all of the other losses, resulting in even more variation at cold temperatures.

Despite all of these variations, I was amazed at how much more linear this data is than the data from my 2012 Leaf, using the GOM to estimate range instead of the %SOC on the 2013 Leaf. Here is the scatter plot of the 2012 Leaf data for comparison:

Scatter plot of 2012 Leaf estimated range vs. temperature

This plot has many more data points, but it still looks like it has much more variation than the 2013 Leaf data does. It will also be interesting to see if the 2013 Leaf maintains its approximately 100 mile range next summer, since that seems to be better than the 2012 Leaf was while the lower temperature ranges are roughly equivalent. The regular headlights of the 2013 Leaf could be part of the reason for it not being more efficient than the 2012 Leaf in the winter.

Overall, I'm quite happy with the data I'm getting from the 2013 Leaf. I'm a bit less enthusiastic about the steep drop in range over temperature, but when I compare it to our Prius, the change in efficiency is not all that different. We normally get 55+ mpg from the Prius in the summer, but on one of those bitter cold winter days I only got 28 mpg. The big difference with the Prius is that its normal range is about 500 miles on a full tank. Cutting that range in half still leaves plenty of range to get where you need to go. When EVs have 300+ mile ranges on a charge, it won't be as big of a deal when the range drops in the winter.

How Much Does it Cost to Charge?

This is the second most common question I get about the Leaf. So far I've driven 3,811 miles and measured 983 kWh of electricity use from the wall. With an electricity usage rate of $0.18/kWh, it's cost me $177 to drive that 3,811 miles. If I compare that to a car that gets 30 mpg, it would be like paying $1.39 for a gallon of gas. The price of gas has dropped quite a bit, but it hasn't dropped quite that far. Also, paying for electricity has the advantage of being a relatively fixed rate. It doesn't change nearly as much as the price of gas, and gas prices have been much higher in the past and probably will be higher in the future.

Beyond the absolute cost of charging the Leaf, it's interesting to look at the charging efficiency. I always charge with the 110V trickle charger (except once) since I have plenty of time at night, and I've never had a problem finishing a charge before driving the next day. Using a Kill-A-Watt power meter at the wall outlet and the on-board energy efficiency meter in the Leaf, I can measure the wall-to-wheels and battery-to-wheels efficiency, respectively. After doing this for 8 months and grouping the data by month, I get the following results (September and October are combined because of a long vacation where the Leaf sat idle):

Leaf Energy Efficiency bar graph

The charging efficiency is easily calculated by dividing the wall-to-wheels efficiency by the battery-to-wheels efficiency, and it hovers around 80%, dropping slightly to 75% in November. I'm not quite sure why that happened. Another behavior that this chart shows is that the drop in energy efficiency does not fully explain the drop in range at lower temperatures. If that were the case, then the Leaf should have a range of about 80 miles in the winter, but I was averaging more like 60 miles for the last couple months. This discrepancy must mean that both the energy efficiency and the battery capacity drops with temperature. While the usable battery capacity is 19-20 kWh in the summer, it dropped to 15 kWh or less in the cold, accounting for about half of the range loss.

Because of the range loss in the cold, the Leaf is definitely not the best car choice for everyone. If you live in a cold climate, you have to be careful to make sure you have enough range to get where you need to go or have a backup plan when the temperature drops too far. My commute is plenty short, so it works quite well for me. I love driving around in a smooth, fast, quiet car. I look forward to driving it everyday, and I couldn't imagine going back to an ICE car willingly. Once battery capacity catches up with our needs, we'll be looking to get out of our Prius and into a longer-range EV. In the mean time, I'll be enjoying the Leaf and will continue to collect data to see how it performs over time. It will be interesting to see what next summer brings.

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