PIA projections of Roadster battery capacity after 100,000 miles using data (left to right) from its survey, from the subset of owners with the original battery pack, from the subset of owners with replaced battery packs, and with OVMS data. Source: PIA. Click to enlarge.

Unlike the results from the earlier Plug In America LEAF Battery Survey, the Roadster study found no significant correlation between climate and battery longevity. “Roadster owners in hot climates are not seeing noticeably different battery capacity profiles than owners in moderate climates,” said Saxton.

Plug In America launched the survey in January 2013, receiving data from about 4% of Roadster owners worldwide who completed a form on the PIA website. This was combined with anonymous data from OpenVehicles.com, an aftermarket vehicle monitoring system.

Deliveries of the Roadster began in 2008; 2,500 Roadsters were produced through the end of production in January, 2012. As of 4 July 2013, Tesla Motors reported that 2,100+ Roadsters have been driven more than 35 million miles.

Plug In America contacted Tesla Roadster owners via social media including the Tesla Motors Club and various other online groups and asked them to submit data for their vehicles. As of 5 July 2013, 126 vehicles totaling 3,198,749 miles had contributed to the survey, amounting to approximately 5% of the user base and nearly 10% of the miles driven as reported by Tesla Motor; of these, data from 122 were used for the full analysis.

In addition, data from 106 Roadsters was collected anonymously through the Open Vehicle Monitoring System (OVMS), with considerable overlap expected between the two data sets, Plug In America said. (OVMS data was from 106 owners who have installed the OVMS device and chosen not to encrypt their data.)

For the Roadsters analyzed, average miles driven was 25,387 (40,856 km); the median was 21,131 miles (34,007 km); and the most miles driven was 87,111 (140,192 km).

Of the 122 owners in the PIA survey, 23 (18.9%) reported that some or all of their battery pack—comprising 11 equal sheets of 621 cells—had been replaced. Replacement can range from a single sheet to the entire battery pack.

The study evaluated three different measurements on battery pack capacity: Standard mode ideal miles, Range mode ideal miles, and Calculated Amp-hour Capacity (CAC).

Tesla allows the owner to view the car’s estimate of the battery pack’s current stored energy in a unit called an “ideal range mile” (IM). The Roadster has two primary charge modes: range and standard. Range mode allows maximum charge and discharge, and thus maximum driving range. Standard mode charges to about 90% of the pack’s usable level and also hides the bottom 10%. Charging and driving within Standard mode protects the battery from both high charge and discharge levels.

The study found that standard mode IMs correlated very well with CAC; range mode correlated less well, with a few outliers both high and low, lowering the correlation coefficient.

The projections from the various data sets suggest that Roadster batteries will be at 80% to 85% capacity after 100,000 miles, on average, Saxton concluded—i.e., an average loss of about 3.7 ideal miles of range (1.6%) per 10,000 miles driven.

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.

—Plug In America’s Tesla Roadster Battery Study

Studying the Nissan LEAF EV, Plug In America conducted the first evaluation of plug-in battery performance last year. The purpose of these studies is to help consumers better understand battery reliability, support industry-wide adoption of standard battery performance warranties, and inform consumers making extended-warranty purchase decisions. Andy Palmer, executive vice president of Nissan Motor Co., Ltd., encouraged every LEAF owner to read the survey’s results when Nissan announced the addition of capacity-loss coverage to an enhanced LEAF battery warranty.

Plug In America has also launched battery-pack performance surveys of the 1997-2003 Toyota RAV4-EV and the new Tesla Model S. The RAV4-EV survey will document the 10-year experience of owners of these early electric vehicles.

It is premature to glean much about battery capacity for the Model S, which became available in mid-2012, but this research has already shown that survey participants are driving an average of more than 16,000 miles per year, well above the national average of 13,500 miles for conventional vehicles, Saxton reported.

In addition to heading up Plug In America’s battery performance research, last year Plug In America conducted the first performance evaluation of public EV charging infrastructure effectiveness including charging station reliability, availability and average-use levels.