Tesla Roadster Battery Life Study: 85 Percent Capacity After 100,000 Miles
A new study from Plug In America predicts great things for Tesla Roadster battery life—at 100,000 miles, the pack should retain 80 to 85 percent of its original capacity.
The study is based on a survey of 126 Roadster owners, whose cars have collectively covered more than three million miles. It shows that owners of the cars are driving them an average of 16,000 miles per year (compared to 13,500 for the average conventional cars). So, on average, they're not garage queens.
Tesla itself said in a 2006 blog post that its Roadster packs would last for more than five years and be capable of covering 100,000 miles, with some decline in performance. The company estimated that a car that covered 50,000 miles over five years would have 70 percent capacity left.
So the survey’s results, which said that climate shouldn’t play a role in Tesla battery life, surpass what the company said to expect. “Roadster owners in hot climates are not seeing noticeably different battery capacity profiles than owners in moderate climates,” said Tom Saxton, Plug In America’s chief science officer and the author of the survey. “Of the factors considered—miles driven, vehicle age and climate—only mileage showed a significant correlation with battery pack capacity.”
Christine Rogala, a spokeswoman for the Electric Drive Transportation Association, said that Plug In America’s results are “compatible with other findings. It is important to add that battery technology advances are continuously extending life, capacity and performance, so the numbers don’t stand still for long.” She predicted that battery breakthroughs will be announced at the EVS27 trade show, which will be in Barcelona, Spain this fall.
Costs Coming Down
Tesla offers an eight-year, 125,000-mile warranty on the Model S 60 kilowatt-hour battery, and the same eight years but unlimited mileage on the 85-kilowatt-hour model. The company recently said it would replace Model S 85-kilowatt-hour packs for $12,000, but only after eight years of ownership if they sign up upon purchase. The company is betting that battery costs will have come down considerably by then. Right now, such a battery pack is probably a $34,000 item.
Plug In America is also studying the battery performance of the Toyota RAV4 EV and the Tesla Model S, but results aren’t available yet. The group’s earlier Nissan LEAF survey found more degradation in hot weather.
Doing the Math
Obviously, nobody knows exactly how long Tesla packs will last. The math is somewhat simple, though. The full capacity of a lithium-ion battery cell should be good for 300 to 500 cycles. So if you drive a Roadster through 300 194-mile standard-mode cycles, it translates to 58,200 miles. If it’s 500 cycles, how does 97,000 miles on one set of batteries sound?
Of course, as Battery University explains, it’s not as simple as that. After 300 to 500 cycles at 100 percent depth of discharge, a lithium-ion cell’s capacity will drop to 70 percent. But partial discharge “reduces stress and prolongs battery life.” Drain the batteries consistently to only 50 percent, as is often the case with electric cars that get plugged in frequently, and life expectancy of a healthy battery zooms up to 1,200 to 1,500 cycles. That, of course, translates to 366,000 miles, but don’t expect numbers like that on your odometer. Other wild cards such as frequency of fast recharge can also affect battery life.
Standard mode in the Roadster protects the battery by only allowing charging up to 90 percent; range mode uses the whole pack. In range mode, the Roadster should be capable of 244-mile runs, but longevity will be sacrificed.
On Tesla forums, some Roadster owners report generally good results with their batteries. “Donauker,” an owner of a Roadster (and also a Model S Signature Performance), reports, “At 3.5 years and 48,000 miles, I get 169 standard mode miles so about a 10 percent loss.” And “Cinergi” reports that driving 20,000 miles in two years cut range from 188 to 181 miles, a loss of about two percent per year.
Elon Musk tweeted: “A Tesla Roadster just passed the 100,000-mile mark for the first time and still has over 200 miles of range.” That’s more than 81 percent range retention, which isn’t bad at all.
One Bad Apple...
It's not all good. German owner Hansjörg von Gemmingen reported in 2012 that he had achieved a record 240,000 kilometers (149,000 miles) on his Roadster. He was a poster boy for effortless long-distance travel in a Tesla. But Die Welt said that, unfortunately, his car "is almost exclusively in the garage" because he now gets only 100 kilometers (62 miles) on a charge. "That annoys me," von Gemmingen said.
Finally, let’s check in with the American Chemical Society, which recently held a forum that dealt with battery longevity. “The battery pack [in an electric car] could be used during a quite reasonable period of time ranging from five to 20 years depending on many factors,” said Dr. Mikael G. Cugnet. In the lab, he subjected EV packs and cells to the equivalent of a lifetime of draining and recharging, and considered the pack to have used up its lifespan when it reached 80 percent capacity.
According to Cugnet, battery performance degrades as soon as the pack hits 86 degrees Fahrenheit. “The higher the temperature, the lower the battery service life,” he said. “A temperature above 86 degrees F affects the battery pack performance instantly and even permanently if it lasts many months like in Middle East countries.”
However, Plug In America’s study found that climate played no role in the Roadster’s battery life expectancy, so maybe Tesla has some kind of secret sauce.
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