Tesla Model S lithium-ion battery pack in rolling chassis [photo: Martin Gillet via Flickr]Enlarge Photo
This brings us back to our request for additional Tesla Model S owners to upload their data to the survey. As a neutral, non-profit third party with an interest in electric vehicles, Plug In America is probably the ideal steward of this information.
There should now be enough 2014 Model Ses with enough road miles that we can refine our drivetrain reliability trend analysis.
That will move us from discussing the yes/no question of whether or not the Model S is reliable to using crowdsourced data to evaluate its past reliability rate, and the pace of improvement in newer vehicles.
The calculations above are generally called a Weibull analysis, after the eponymous probability distribution. Its utility comes from the fact that it measures survival rates over time, which means inferences can be drawn from parts which haven't yet failed (called "suspends" or "suspended tests").
This is very important for long-lived parts, where it can take an impractically long time to run all samples to failure. (Calculations on early Teslas consisted of 77 failures and 250 suspends.)
2015 Tesla Model S 70D in new Ocean Blue colorEnlarge Photo
In short, the Weibull analysis docks you points for items that have failed, and gives you credit for items that haven't.
Weibull distributions are power functions, meaning that they can be highly curved when plotted on a linear scale. For this reason, they're often plotted on a logarithmic scale, where they form nice straight lines.
Both linear and logarithmic charts have been included here. Failed tests are shown in blue, and suspended tests in red (along the bottom axis). In the bottom left, three values are given. Beta is the shape parameter.
Values above 1.0 represent failure rates that increase over time, indicating a component that wears out over time. Eta is the characteristic life noted above, at which point 63.2% of components are expected to fail. Finally, Rho is the correlation coefficient, which should ideally be close to 1.
No Tesla answers to questions
Before publishing this article, Green Car Reports reached out to Tesla with a list of specific questions to help us put this analysis in context.
Specifically, we asked:
Tesla Store Los Angeles [photo: Misha Bruk / MBH Architects]Enlarge Photo
Tesla declined to answer any of these specific questions. A company spokesperson sent the following statement:
Close communication with our customers enables Tesla to receive input, proactively address issues, and quickly fix problems. Over-the-air software updates allow Tesla to diagnose and fix most bugs without the need to come in for service. In instances when hardware needs to be fixed, we strive to make it painless.
Despite the problems, our data show that Tesla owner satisfaction is still very high: Ninety-seven percent of owners said they would definitely buy their car again. It appears that Tesla has been responsive to replacing faulty motors, differentials, brakes, and infotainment systems, all with a minimum of fuss to owners.
And Tesla’s attention to customer service has been effective. Almost every survey respondent made note of Tesla’s rapid response and repair time, despite the lack of a traditional dealer service network. For its early adopters, Tesla has made a practice of overdelivering on service problems under the factory warranty…