Two-Thirds of Earliest Tesla Drivetrains To Need Replacement In 60,000 Miles, Owner Data Suggests Page 2


Tesla Model S lithium-ion battery pack in rolling chassis [photo: Martin Gillet via Flickr]

Tesla Model S lithium-ion battery pack in rolling chassis [photo: Martin Gillet via Flickr]

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Plug-In America's big (enough) data

The failure estimates above come from an analysis on data from Plug in America's survey of Tesla Model S vehicles. Links at the bottom of the PIA page allow the data set to be downloaded.

In the survey, respondents provided a variety of information on their Model S ownership experience, including total miles driven, whether they've had a motor swap (drivetrain replacement), and if so, what the odometer reading was at the time.

In October, when this analysis was run, the data set had 370 respondents. Not having any reliability software handy, I asked a reliability engineer to crunch the numbers--and received the startling reply that the "characteristic life" of the drive train was about 50,000 miles.

In a reliability context, the so-called characteristic life is the age at which 63.2 percent of parts are expected to fail. This represents a survival rate of 36.8 percent, or 0.368. That value is the inverse of e, the base of the natural logarithm. In math terms, 1/e = 1/2.718 = 0.368.

Weibull Analysis of earliest 2012-2013 Tesla Model S drivetrains [plotted by Matthew Klippenstein]

Weibull Analysis of earliest 2012-2013 Tesla Model S drivetrains [plotted by Matthew Klippenstein]

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The reliability engineer was careful to emphasize that the results were only valid if the data was correct, had no selection bias, and was random. While selection bias was definitely in play (only those customers who knew about the survey could choose to fill it out) the sample covered more than 1 percent of the total population of 2012 and 2013 Teslas (327 respondents for about 25,000 vehicles).

So selection bias and randomness were probably acceptably low. But on a line-by-line review of the Excel data file, I noticed a few transcription errors which indeed compromised the result.

When cleaning the data set, to give Tesla the benefit of the doubt--where respondents had reported a motor swap but didn't specify when it happened--I used the final odometer reading. (This would push up the characteristic life slightly.)

I repeated the analysis using free reliability software found online (I chose Reliasoft, which offers a free trial period; there are other packages). For 2012 and 2013 Teslas, the characteristic life rose to about 57,000 miles. (The probability function predicted that 66 percent of drive trains would fail within 60,000 miles, giving the title of this article.)

Weibull Analysis of earliest 2012-2013 Tesla Model S drivetrains [plotted by Matthew Klippenstein]

Weibull Analysis of earliest 2012-2013 Tesla Model S drivetrains [plotted by Matthew Klippenstein]

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By and large, calculations are only as credible as they are transparent, so the raw data and calculation method have been outlined at www.tinyurl.com/TeslaWeibull, along with the author's contact information.

This should be enough for readers to duplicate the drive train analysis, and assess the Model S battery pack and onboard charger as well, the latter of which apparently has a characteristic life of about 1,000,000 miles.

Only 43 people in the data set owned 2014 and 2015 Teslas. And while this verges on a big enough sample--a statistical rule of thumb is that 30 to 50 random samples allow you to make accurate inferences about large populations--only a handful had even reached 20,000 miles.

That means that an analysis on that data would have been invalid. Far from measuring Tesla's success at mitigating the "wear-out" failure mechanism(s), it would reflect quirks in infant mortality and/or random mid-life failures.


 
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