Tesla Motors Inc, which surprised the world with the idea of using notebook PCs' lithium-ion (Li-ion) rechargeable batteries for electric vehicles (EVs), is commanding attention while being invested by Toyota Motor Corp, Panasonic Corp and Daimler AG.
Tesla released the "Roadster" electric sports car, which is mounted with a Li-ion rechargeable battery with a capacity as high as 56kWh, in 2008 and has sold more than 1,300 units since then. The cell of the battery is the "18650," which is 18mm in diameter and 65mm in length and was developed for PCs. Because the supply of the cell is high, it is less expensive than battery cells dedicated for use in vehicles.
However, much higher safety and reliability are required for automotive batteries than for PC batteries. How could Tesla launch the EV using the 18650? We interviewed Kurt Kelty, who is responsible for battery technologies as a director, Battery Technology, Tesla.
Q: How did you develop the battery system of the Roadster?
Kelty: At an early stage of the development, It was difficult for us to find battery cell suppliers. At that time, Tesla was an unknown EV startup. Battery makers probably had concern about us because, if they supplied their batteries to us and problems occurred to our EVs, they might have been blamed for the problems. In fact, when battery troubles occurred to notebook PCs, battery makers were blamed for them several times.
We visited many battery makers only to receive negative responses. But one engineer showed interest in our project. He was an engineer of a Japan-based maker. He not only understood our system but also clearly explained it to us. Thanks to him, we could start developing the Roadster. Now that our company is well-known, many battery makers are offering their products to us.
Q: How did you ensure the safety and reliability of the vehicle while using batteries for PCs?
Kelty: We cannot reveal the details, but we developed a physical (passive) structure in which, even if one battery cell gets broken and causes thermal runaway, other cells will not be affected. The battery pack of the Roadster contains 11 battery modules, each of which consists of 621 cells. So, it is mounted with a total of 6,831 battery cells.
The mass of the battery pack is about 450kg, and its volume is 300L. It might sound heavy and large. But the battery pack features a system to ensure safety, and its energy density per mass is over 120Wh/kg.
There is an advantage in loading many battery cells. Even when one of them gets broken, the entire performance will hardly be affected. The battery pack will continue to function, and the user probably will not notice anything. However, the Roadster will show an error message to the user so that the user can bring the car to a dealer and we can collect the broken battery cell and determine the cause.
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