The Monkey House

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This is an update / continuation of my initial impressions post about purchasing a Nissan LEAF.

One thousand days update! Nearly three years, 5MWh in 364 charges, and almost 17,000 miles. With a bigger data baseline, I can ask some new questions and get interesting answers.

Usage and Efficiency

Graph of energy use per mile on my 2015 Nissan Leaf in the first 1000 days of ownership

Let's compare my vehicle usage in the LEAF versus my previous gas-powered car. In 1000 days I've used an average of 4.98kWh to drive 16.7 miles at an energy cost of 48 cents every day. In my previous car, I used 0.87 gallons of gasoline to drive 17.6 miles at an energy cost of $2.77 per day during its last 1000 days of service. Assuming lifestyle changes or mere randomness don't account for the discrepancy, the 0.9 mile per day reduction might be attributed to the difference between how far I would drive with no range limitations (i.e. how far I drove in the past) and how far I can now drive due to the ~80mi range of the 2015 LEAF (MY2019 offers up to 226mi range). That works out to about three trips per year that just barely exceed the LEAF's range, or fewer longer trips to make up the same average. I would probably need to rent a car for perhaps 1-3 days per year to 'fill in the gaps' left by the commuter-car range that the LEAF provides. Even paying myself a handsome wage for my inconvenience, that only puts a dent in the nearly $1000/yr I save by driving with electricity. Nice to have early estimates validated.

As a reminder, the instantaneous values assume that I charge the battery to 100% with every charge. This is not true: I sometimes only partially charge before my next trip. As a result, charge n will under-estimate energy-per-mile, while charge n+1 will over-estimate. The rolling average is accurate as long as both the under- and over-estimated charges are included in the averaging window; when just one is, there's a slight error, which can be seen in the residual spikiness in the yellow line.

Battery Size Estimation

Graph of estimated battery capacity on my 2015 Nissan Leaf in the first 1000 days of ownership

With about three years of estimated battery capacity, I can start to see seasonal effects as well as begin to estimate early battery degradation. PushEVs wrote last year that the average 24kWh LEAF battery has declined in usable capacity by about 9% over its first three years. Either I'm lucky and somehow got an especially 'good' battery, or my usage - no DC fast charging; almost no L2 charging either - is particularly gentle, despite the 125 degree F temperature swings from Wisconsin winters to summer. My battery, based on the linear trendline of estimated battery capacity (kWh/charge percentage * 100%), is degrading at about 328Wh, or 1.37%, per year. Over 3 years it's down about 4% from its new state.

In winter, the estimated battery capacity is about 10% lower than the summer estimate. Based on the starting and ending points, my 1000-day graph would underestimate degradation by ending in a summer, so I included some post-1000-day data to make a better approximation.

As a reminder, these data are the energy sent into the car, not the recoverable energy stored in the battery. Battery capacity is over-estimated by the amount lost in charging inefficiency, which is probably somewhere around 10%, but the seasonal trends and overall degradation are not affected by this estimation error.

Version 1.0     |     Originally written: March 10, 2019     |     Latest revision: March 10, 2019     |     Page last generated: 2019-03-10 13:00 CDT