Battery life is the subject of lingering doubt for many electric vehicle owners, but doom and gloom degradation projections don't always come true.

Not for the Tesla Roadster at any rate, which is confounding Tesla's own battery life expectations for the car.

According to data recorded by Plug In America (PIA), Roadster battery packs are retaining around 80 to 85 percent capacity after 100,000 miles of driving--greater capacity after far greater distance than Tesla was expecting. It equates to a loss of only around 0.15 percent on average every thousand miles.

Tesla's own 2006 predictions were for 70 percent retention after just five years and 50,000 miles, according to Plug In America's chief science officer, Tom Saxton.

Better still, climate doesn't seem to have much of an effect on battery life.

"Roadster owners in hot climates are not seeing noticeably different battery capacity profiles than owners in moderate climates," said Saxton at the Teslive Tesla users conference where the findings were released.

Those contrast with the findings of Plug In America's last battery life survey, on the Nissan Leaf.

Nissan's problems surrounding the battery capacity loss of Arizona Leaf owners were backed up by PIA's findings--showing that cars experiencing regular temperatures of 95 degrees or more were more likely to lose capacity.

While capacity loss isn't significant for Roadster owners in hotter climates, there's still a positive correlation between very hot climates and reduced capacity.

The survey is relatively limited in its scope, with data from only around 4 percent of Roadsters worldwide, 126 cars, but still totals over 3 million miles of data--so other Roadster owners can be reasonably sure their vehicles will last to the same degree as those surveyed.

You can read the full survey data here (pdf file), which breaks down the results into several categories.

With PIA recently launching another survey--this time into the new Tesla Model S--it'll be interesting to see how Tesla's next electric vehicle fares as owners pile on the miles...


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