?By Tom Saxton, Chief Science Officer
Released: July 13, 2013
Introduction
As the first full-production all-electric vehicle of the modern era, the Tesla Roadster represents a unique opportunity to study battery pack longevity. Deliveries of the vehicles began in 2008 with a total of 2,500 Roadsters produced through the end of production in January, 20121. As of July 4, 2013, Tesla Motors reports that “2,100+ Roadsters” have been driven over 35 million miles2. For this study, Tesla Roadster owners were contacted via social media including the Tesla Motors Club and various other online groups and asked to submit data for their vehicles. As of July 5, 2013, 126 vehicles totaling 3,198,749 miles have contributed to the survey, amounting to approximately 5% of the user base and nearly 10% of the miles driven as reported by Tesla Motors. In addition, data from 106 Roadsters was collected anonymously through the Open Vehicle Monitoring System3 (OVMS), with considerable overlap expected between the two data sets. In 2006, Tesla Motors projected4 that the Roadster battery pack would have more than 100,000 miles of driving range and more than 5 years of useful life. Over this period, the capacity of the battery pack would be expected to decline. As an example, a Roadster driven 10,000 miles per year for 5 years would be expected to have 70% of the original battery pack capacity. The purpose of this study is to determine how the battery packs are holding up and how real world performance compares to the expectations set by Tesla Motors in the timeframe when the Roadster was first announced and described in 2006.
Summary of Results
Of the factors considered – miles driven, vehicle age, and climate – only mileage showed a significant correlation with battery pack capacity. Data collected suggests that, on average, a Roadster battery pack will have between 80% and 85% of original capacity after 100,000 miles.
1 Tesla Motors, Inc, Form 10-Q for the quarter ended September 30, 2012, filed with the SEC on November 7, 2012.
2 http://www.teslamotors.com/enthusiasts/millions-of-miles
3 https://www.openvehicles.com/
4 Martin Eberhard, JB Straubel, “A Bit About Batteries” November 30, 2006 http://www.teslamotors.com/blog/bit-about-batteries
Unlike results from the Plug In America LEAF Battery Survey5, no significant correlation was found between climate and battery longevity. There is significant variation in battery capacity reported; the difference in capacity between vehicles with similar mileage can be as large as the projected loss over 100,000 miles of use. Individual owners should therefore expect variation between their experience and the projected average performance. Because of the variation in battery pack longevity experienced by owners, especially where such variation may be due to factors beyond the owners’ control, it would seem desirable for the manufacturers of electric vehicles to guarantee not only the life of the battery pack, but also the capacity performance over time and miles.
Nissan Motors responded to the climate issues reported by LEAF owners (and confirmed by the Plug In America study) by amending their battery warranty to cover capacity. With the performance of the Roadster battery packs exceeding early expectations, it’s curious that Tesla Motors doesn’t offer any capacity warranty, even on the 85 kWh Model S, which has a warranty good for 8 years and unlimited miles.
5 Tom Saxton, “Plug In America’s LEAF Battery Survey” http://www.pluginamerica.org/surveys/batteries/leaf/Leaf-Battery-Survey.pdf
The remainder of Plug in America’s Tesla Roadster Battery Study is at http://www.pluginamerica.org/surveys/batteries/tesla-roadster/PIA-Roadster-Battery-Study.pdf
