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Two Years of Nissan Leaf Mileage Data

I've had a Nissan Leaf for two years now, and it continues to be an exceptionally fun car to drive. The acceleration is incredibly smooth, the car is ridiculously quiet, and it's been completely trouble free so far. The relatively short range of 75 miles has been no problem, as I primarily use it for my 23-mile round trip commute to work and for random errands in town. The trips are short enough that I almost always charge to 80% to extend the life of the battery.

In the summer I can easily go back and forth to work twice on one 80% charge, but in the winter I would barely make it home on the second day, so I charge everyday. This winter has been especially cold, with quite a few subzero mornings and a couple of days where the temperature went below -10℉.

Because the heater on the 2012 Leaf sucks so much power from the battery, I've taken to using the heated steering wheel and heated seats to keep me more comfortable. The amount of power they use is negligible, and they do an adequate job as long as I have a winter coat, hat, and some driving gloves. I probably could use the heater with no problem because my commute is so short, but the car wouldn't really warm up until I was almost to work anyway, so I skip it and use the much faster heated wheel and seat.

On the first especially frigid day that hit us, I was a bit worried about having the Leaf sit outside all day without plugging it in. When it gets too cold, the battery needs to heat itself to keep from freezing. I didn't want to head home after work and find that the battery had drained itself too much for me to get home. It turns out that I had nothing to worry about. Even after sitting in -11℉ temperatures for nine hours, the battery only lost about 5 miles worth of charge, so there was plenty left to get home.

That's all well and good for winter performance, but I like to dig a little deeper into the quantitative measures of how the Leaf has been doing. I have tons of data on this car, so let's see what it can tell us.

The Daily Trip Log

I've kept a log of every trip I've taken in the Leaf, and I've already written about what I found from the first year and a half of data. For my initial methodology, you can read the beginning of that incredibly long article. A few things have changed for the most recent data I've taken, so I'll talk about that briefly before going through the data.

First, my Leaf's software upgrade near the end of June, 2013 did have a significant effect on the accuracy of the GOM (Guess-O-Meter, i.e. the range estimator). The GOM used to have large variations in two different ways. The estimated miles-to-empty would change significantly while driving in response to changes in speed or inclination, and the initial guesstimate after charging would vary markedly depending on the previous day's temperature and the temperature during charging. The software upgrade dramatically reduced the first type of variation, and also somewhat reduced the second type.

Because the software upgrade changed the GOM behavior, I decided to divide the 2013 data into two halves at the date of the software upgrade. This split works out rather well because that allows for estimating battery capacity changes in roughly six month periods while still having a full range of temperatures represented in each period. Doing one year periods is less useful because the battery capacity could change more significantly within one year than within six months.

The second methodology change has to do with how I recorded the day-to-day temperatures. Initially I was recording a maximum and minimum temperature for each charging cycle, but when analyzing the data I found that this caused a problem when there were more than two trips between charges. Only two temperatures were represented in the charging cycle when there were actually more, and since the range is highly dependent on temperature, I felt this setup was not adequate.

For the last five months of data I logged the temperature for every single trip and then calculated a mile-weighted average temperature for each charging cycle. This more accurate data logging combined with the software upgrade effects resulted in range vs. temperature data with substantially less variance.

Finally, I got a P4460 Kill A Watt electricity usage monitor for Christmas, and I put it into use in January, measuring the amount of energy the car uses from the wall outlet. The intention here is to measure exactly how much electric energy I'm paying for to drive a certain number of miles, and to get a more realistic estimation of the Leaf's miles/kWh efficiency value than its own optimistic estimate.

Interestingly, the Kill A Watt meter was working wonderfully, but I had it plugged into an outlet extender so that I could use the other outlet plug that would otherwise have been covered up. Apparently, the cheap-o extender couldn't handle the 12 amps of current pulled by the trickle charger, and it fried itself and took the meter with it. Luckily, the charger was unharmed, and I got the meter replaced at no charge (thanks Menards!). The new meter's been working great now that it's plugged directly into the wall.

With this new set of data, I'd like to answer two main questions. How much capacity has my Leaf's battery lost so far, and how much am I really paying per mile in electricity costs? Let's start with the harder of the two.

Capacity Loss After Two Years

Figuring out how much battery capacity my Leaf has lost is hard to do because I'm depending primarily on the GOM, which is highly variable depending on previous and current conditions. I could get a special meter that hooks into the car's CANbus and reads raw charge information coming off the battery controller, but I would really rather not. That may be something early adopters and people that love messing with their cars like to do, but that's not the average car owner. I chose to figure out what information I could from the instruments available without making any modifications to the car.

The newer 2013 Leaf remedies the variability problem with the GOM somewhat because it also has a battery level meter that shows the state of charge as a percentage. Having that extra resolution would be exactly what I need to do more accurate calculations here, but until I trade in for a newer Leaf, this is the data I have (click for a larger image):

Post-Charge GOM Vs. Temperature

This scatter plot is separated into 2012 (blue), first half of 2013 (red), and second half of 2013 (yellow) data. If I do linear regressions on the 2012 and 2013H2 points, I get two linear equations that estimate the temperature behavior of the battery for those two time periods. If I assume the battery is at 100% capacity at 75℉ in 2012, then I can solve those equations for what the battery capacity is at 75℉ in 2013H2, and for this data I get 95.0%.

That's not too bad. If that trend continues, my battery wouldn't hit 70% capacity for another 10 years. I'm not sure what kinds of effects impact battery life in the long term, but I've read a number of articles that claim that capacity loss is more significant in the first year or so and then tapers off in the medium term. It would be great if the battery could last for 10+ years and still be usable in a car, but I really can't say how it will behave at that age because that data doesn't exist, yet (or at least isn't available to us common folk).

For the sake of argument, I went further with the calculations in the same way that I did in my last data analysis. I used the linear equations for the capacity-temperature behavior to calculate what the GOM should have estimated the range to be for each charge cycle, had it been able to predict the future average temperature for the trips taken on each charge. Then I calculated what the total range would have been for each charge given that starting point and assuming the rate of decline of the GOM per miles traveled would continue until the battery was empty.

To make this data transformation useful, I had to remove the first five months of data because it was so variable that linear regressions were useless. As I speculated before, this behavior could be due to the GOM being in a learning period for temperatures it doesn't have historical data for, yet. Given that caveat, the scatter plot looks like this:

Linear estimation of range vs. temperature scatter plot

In this plot it's hard to tell if the range decreased at all in 2013H2. Running regressions on the 2012H2 and 2013H2 points and then plotting the resulting linear equations with 3-sigma error lines gives the following plot:

Linear estimation of range vs. temperature line plot

Here you can clearly see the reduced variance in the 2013H2 data points by the significantly narrower error lines for that linear estimate. Improving the calculation of the average temperature per charging cycle probably helped tighten up the variation of the data. The software upgrade likely increased the accuracy of the GOM as well. The more realistic GOM reporting is possibly also the cause of the significantly lower range at low temperatures, although there is much more 2013H2 data at temperatures below 25℉. That extra data at low temperatures is likely pulling the 2013H2 lines down closer to their real values.

If I assume 100% capacity for the 2012H2 range at 75℉, and do the same estimate of capacity loss as before, I get a capacity degradation to 94.5% for 2013H2, or a 5.5% loss of battery capacity. This is in good agreement with the previous estimate, so it's fairly likely that that's where my battery capacity is at after two years and 12,000 miles of use. Compared to my estimate of about 4% capacity loss from five months ago, it seems possible that the loss is slowing down as well. I'm pretty happy with that, and qualitatively in real day-to-day use, I haven't experienced any noticeable loss of capacity. Seeing the numbers confirm that gives me good confidence that this Leaf's battery could last a good long time. I do try to take good care of the battery, and I would like to think that it's paying off.

The Real Cost of Driving Electric

I've written before about how I suspected that the Leaf's mileage efficiency calculations were grossly optimistic. Now that I have an energy meter at the wall outlet, I can measure exactly how much it costs per mile to drive the Leaf. After all, I don't really care if the battery-to-wheels efficiency is 4 miles/kWh or 6 miles/kWh. I'm not paying for the electricity used at the battery. I'm paying for the electricity used at the wall.

Using the energy meter, figuring this out is now pretty easy. In addition to other measurements, the Kill A Watt meter keeps track of the total kWh of electricity that have passed through the meter since it was last reset. All I do is note the kWh on the meter in the morning after each charge and record it for the end of the previous charge cycle in my trip log. Then dividing the change in miles on the odometer by the change in kWh from the energy meter gives me the total energy efficiency for that time period.

After about a month, I've driven 569 miles and charged for 217 kWh, giving me an efficiency of 2.62 miles/kWh. Over that same period, the Leaf reported an efficiency of 4.0 miles/kWh on its display. I was not surprised that the Leaf's measured efficiency was higher. I expected that. I was surprised at how much higher it was, though. If that difference was fully accounted for in charging losses, that would mean that charging with the trickle charger is only 65% efficient. I don't believe that's true, and I figure there are some other possibilities for charge loss.

January was a bitterly cold month, and when the battery has to heat itself to keep from freezing up, that energy usage is probably not factored into the Leaf's efficiency measurement. Losing a few miles of charge to the battery heater when the car is parked outside all day can make a significant difference in the efficiency value. The battery also needs to heat itself up to a certain temperature before charging and it will continue heating itself from outlet power as long as it's plugged in and the ambient temperature is cold enough. I'm quite sure that electricity use is not counted towards the Leaf's efficiency measurement.

It's possible that charging efficiency is also lower at cold temperatures. It will be interesting to see if the two efficiency measurements get closer together when the weather warms up. If the wall measurement doesn't get within 85% of the Leaf's measurement, I would be fairly skeptical of the efficiency numbers that the Leaf is reporting.

Regardless of what the Leaf is saying, now I know how much electricity I'm using to drive the car, at least in the winter. At $0.18/kWh for electricity, distribution, and wind power offsets, an efficiency of 2.62 miles/kWh results in a cost of $0.0687/mile. The average new ICE car has a fuel economy of about 25 mpg, and loses about 20% of that in these freezing temperatures. My wife just filled up her tank for $3.30/gallon, and at that price it costs $0.165/mile to drive the average new car.

That makes the Leaf 2.4 times cheaper to drive on a per mile basis than the average new car. Of course, the ratio will change if any of those number change, but it would take an awful lot to tip the scales in the ICE car's favor. I would expect the Leaf to look even better in the summer because its efficiency increases dramatically, and the price of gas normally goes up as the weather gets warmer and people are driving more.

After Two Years, I'm Quite Happy With A Leaf

The severe cold we've had in Madison this winter has certainly tested the Leaf in new ways, and I'm quite pleased with how it's performed. The heated seats and steering wheel are enough to keep me comfortable without draining the battery on super cold days to keep the cabin warm. By taking good care of the battery through garage parking, charging to 80%, and not charging when I don't need to, it looks like I've kept the battery capacity loss to about 5% so far. Finally, I'm paying a little more than I thought to charge the car, at least this winter. But that will improve in the spring, and it's still 2.4 times cheaper per mile than the typical ICE car. Consider me one very happy Leaf owner.

If you're interested in the other stuff I've written about my Leaf, it's all here:
Part 1: The Acquisition 
Part 2: The Summer Drive 
Part 3: The Winter Drive
Part 4: Frills and Maintenance
Part 5: The Data
Part 6: The Future
Part 7: The Energy Efficiency Meter

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