One drawback to cars that run on batteries is that over time and multiple charge/discharge cycles, batteries will lose capacity. Individual cells start to fail to meet specifications and when enough cells go bad, it’s time for another battery pack. Since capacity is directly related to range and since battery packs are expensive to replace, how quickly batteries deteriorate is an important factor in the overall cost and practicality of EVs.

When Tesla first announced their Roadster EV in 2006, the company said that due the company’s proprietary battery management system and design of their lithium-iom battery packs, would ensure that after five years or 50,000 miles, the Roadster’s battery pack would still have 70 percent of it’s rated capacity when new, 53 kWh, enough electrons for a 244 mile range ( 2006 statement on battery age by Tesla founder Martin Eberhard here). The Tesla Roadster went on sale in 2008, which means there are now roadsters that have been on the road for as long as five years, and I’m sure many that have reached or exceeded 50,000 miles of use. It’s now possible to test Tesla’s claims regarding battery durability. A standard from the laptop industry is that lithium-ion battery packs are still serviceable above 80% capacity.

The independent EV advocacy group, Plug In America (PIA) decided to do just that and their chief science officer, Tom Saxton has reported the results of an owner-reported survey of Tesla battery packs, based on a sample size of 4% of all 2,500 Roadsters made. Plug In America discovered that the Tesla battery packs are performing much better than advertised. After 100,000 miles, double the advertised 70% capacity life, the battery packs have an average capacity of 80-85%.

PIA also tested for climate differences because PIA’s earlier first ever survey of EV battery life involving owners of Nissan Leafs showed measurable declines in battery capacity in hot climates. The Leaf has a much simpler battery heat management system than used by Tesla. A surveys of first generation Toyota RAV4 EV owners, which was on sale from 1997 to 2003, in order to measure performance in batteries at least 10 years old, is also underway, as is a survey of Tesla Model S owners but it’s too early for any real meaningful data to be obtained on that car.