According to a recent study performed by Plug in America, Tesla's battery packs in its Roadster model are retaining more of their capacity than what Tesla had initially thought.

Battery packs in Tesla Motors' Roadster electric cars will retain an average of 80- to 85-percent of capacity after 100,000 miles driven, according to a study published by Plug In America, the nation's leading plug-in vehicle advocacy organization. The finding, released at the Teslive Tesla users conference, bests initial projections set by Tesla Motors in 2006 which suggested a 70-percent capacity retention after five years and 50,000 miles driven, said Tom Saxton, Plug In America's chief science officer.

"Our study also found no discernable effect of climate on battery-pack longevity," said Saxton, who lead the research. "Roadster owners in hot climates are not seeing noticeably different battery capacity profiles than owners in moderate climates."

Plug In America launched its first-ever survey in January 2013, receiving data from about 4 percent of Roadster owners worldwide who completed a form on the independent, nonprofit organization's website. This was combined with anonymous data from OpenVehicles.com, an aftermarket vehicle monitoring system.

Studying the Nissan LEAF EV, Plug In America conducted the first-ever evaluation of plug-in battery performance last year. The purpose of the studies was 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's goal has always been to educate as well as to advocate," Saxton said. "We continue to accelerate the shift to cleaner, more affordable, more fun electrified personal transportation by disseminating information that's useful to consumers and vehicle manufacturers."

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.