[Interview] How Did Tesla Develop EV Battery?

Dec 20, 2010
Naoshige Shimizu & Kouji Kariatsumari, Nikkei Electronics
Kurt Kelty, a battery engineer at Tesla
Kurt Kelty, a battery engineer at Tesla
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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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Q: Why did you use a battery with a capacity as high as 56kWh? The Leaf EV, which Nissan Motor will release in December 2010, will come with a battery with a capacity of 24kWh, which is less than half that of the Roadster's battery.

Kelty: We are developing EVs under the concept of "no compromise." This means that we are offering the same convenience as that of gasoline vehicles to EV users.

We did not want to say, "Its drive range is shorter because it's an EV," to our users. That's why we equipped the Roadster with a battery with a capacity of 56kWh and ensured a drive range of 394km under the US LA4 mode, which is equivalent to the drive range of a gasoline vehicle.

Another reason is that there was an extra space in the vehicle body. If not, we would have employed a battery with a lower capacity though it would not have been half of the current capacity.

The Roadster has the 56kWh capacity battery, but the EV is normally used in a mode in which the battery is charged up to about 90% of its capacity to reduce the load on the battery. To charge the battery to 100%, the mode has to be switched to the "Extended Range" mode. We consider it sufficient because the capacity of the battery will not be fully used in normal driving conditions.

Q: For the "Roadster 2.5," which will be released in Japan, you will offer a fee-based service of exchanging batteries. The 56kWh capacity battery can be replaced only for ¥1.26 million (approx US$15,005). Why is it so inexpensive?

Kelty: It is a forward-looking price. If the vehicle is purchased now, its user will replace the battery after about seven years. Depending on how the vehicle will be used, it might be ten years. At that point, this price will be reasonable.

Q: In addition to the Roadster, you are planning to develop the "Model S" electric sedan and co-develop the "RAV4 EV" with Toyota Motor and another EV with Daimler. Do you intend to use the 18650 battery for them, too?

Kelty: If the prices of automotive batteries will drop, we do not have to stick to the 18650. However, they are still expensive at this point.

Currently, we are evaluating battery cells of various battery makers. We do not intend to have an exclusive relationship with one battery maker. For the vehicles that we will develop from now, we will use batteries of several makers.

However, we will not bother to disclose the names of those battery suppliers. When general consumers purchase notebook PCs, they do not care about the names of battery makers, do they? I believe that the same thing can be said to EVs.

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Q: Finally, I would like to ask you a personal question. Why did you join Tesla?

Kelty: I was engaged in research at a Japan-based battery maker. At first, I was researching nickel-metal-hydride batteries and had been engaged in the development of lithium-ion rechargeable batteries for more than 10 years since about 1993.

At that time, many battery engineers used to say that it was difficult to mount lithium-ion batteries on vehicles from the perspectives of safety and reliability. But I did not think so. Then, I got to know a venture company that was trying to make an EV by using lithium-ion batteries. That was Tesla.

When I was talking with JB Straubel, one of Tesla's founders, I realized that he was seriously trying to develop EVs and he had excellent ideas. I thought I might be able to develop an EV using lithium-ion batteries at this company. It was about 2006, but, at that time, I did not think that Tesla will become a company that can attract this much attention.