Tesla revealed more details of the lithium-ion battery pack of the Tesla Roadster. This battery pack has been under development and refinement for over three years and is the cornerstone of the Tesla Roadster. This is actually one of the largest technically most advanced Li-ion battery packs in the world. It is capable of delivering enough power to accelerate the Tesla Roadster from 0 to 60 mph in around 4 seconds. Also, the battery powers enough energy for the vehicle to travel more than 200 miles without recharging. The battery pack is comprised of 11 battery modules, a main control and logic PCB and a 12V DC-DC power supply. Because of the weight, size and cost of the Tesla Roadster battery pack, the engineers found a way to add more safety features than can be contained in a laptop battery pack; some are active, some are passive, some are mechanical and some are electrical.
This paper provides details about the design of the Tesla Roadsterâ€™s lithium-ion (Li-ion) battery pack (otherwise known as the ESS, or Energy Storage System) with a particular focus on the multiple safety systems, both passive and active, that are incorporated into the pack. This battery pack has been under development and refinement for over three years and is the cornerstone of the Tesla Roadster. The high level of redundancy and multiple layers of protection in the Tesla Roadster battery pack have culminated in the safest large Li-ion battery that we or many of the experts in the field, with whom weâ€™ve consulted, have seen.
The battery pack of the Tesla Roadster electric vehicle is one of the largest and technically most advanced Li-ion battery packs in the world. It is capable of delivering enough power to accelerate the Tesla Roadster from 0 to 60 mph in about 4 seconds. Meanwhile, the battery stores enough energy for the vehicle to travel more than 200 miles (based on EPA city/highway cycle) without recharging, something no production electric vehicle in history can claim.
Designed to use commodity, 18650 form-factor, Li-ion cells, the Tesla Roadster battery draws on the progress made in Li-ion batteries over the past 15 years. Under the market pull of consumer electronics products, energy and power densities have increased while cost has dropped making Li-ion the choice for an electric vehicle. In the past, to achieve such tremendous range for an electric vehicle it would need to carry more than a thousand kilograms of nickel metal hydride batteries. Physically large and heavy, such a car could never achieve the acceleration and handling performance that the Tesla Roadster has achieved.
Due to their high energy density, Li-ion batteries have become the technology of choice for laptops, cell phones, and many other portable applications. Precisely because they have all this energy stored in a small space, Li-ion batteries can be dangerous if not handled properly. In fact, there have been several cases of Li-ion batteries going into thermal runaway in laptop applications leading to recalls by Dell, Apple, IBM, and other manufacturers. However, even with this high energy density, the Li-ion batteries in the Tesla Roadster only store the energy equivalent of about 8 liters of gasoline; a very small amount of energy for a typical vehicle. The pack operates at a nominal 375 volts, stores about 53 kilowatt hours of electric energy, and delivers up to 200 kilowatts of electric power. The power and energy capabilities of the pack make it essential that safety be considered a primary criterion in the packâ€™s design and architecture.
Fundamentally, cells within the pack need to be protected from adverse situations that could be electrical, mechanical, or thermal in nature. The entire design must also be fault tolerant to reasonably expected manufacturing defects in the cells and in the pack itself. In the body of the paper that follows, we discuss aspects of the Tesla Roadster battery pack design that address these concerns. However, this is not a complete summary of all the battery pack safety features, since some aspects of our design and implementation are proprietary and/or patent pending.
Picking a Cell Design and Supplier
We started our design by purposely picking a small form factor battery cell. This cell is called the 18650 because of its measurements of 18mm diameter by 65mm length (i.e., just a bit larger than a AA battery). Due to its small size, the cell contains a limited amount of energy. If a failure event occurs with this cell, the effect will be much less than that expected from a cell many times larger. Billions of 18650 cells are made each year. Though the chance of a safety event in a laptop is small, the number of safety incidents involving Li-ion batteries is rising each year because there are so many more devices using small and powerful power sources.
The Tesla Roadster battery pack is comprised of about 6800 of these 18650 cells, and the entire pack has a mass of about 450kg.
The engineers at Tesla Motors selected cells from reputable Fortune 500 battery suppliers that have each produced billions of safe, reliable, Li-ion batteries. All the cell manufacturers that Tesla Motors has considered invest a great deal of money and engineering resources to minimize manufacturing defects within their cells. Overall, the selection criteria used by Tesla Motors included multiple factors, confirmed by extensive internal and external testing, that directly relate to the cellâ€™s overall safety in the Tesla Roadster.
Design Safety Features: Cell Level
Since the 18650 cell is the fundamental building block of the battery pack, it is important that it be fault tolerant. The cells used in the Tesla Roadster all have an internal positive temperature coefficient (PTC) current limiting device. The primary role of this PTC is to limit short circuit current on an individual cell level. It is important to note that this device is completely passive and functions without any inputs from the rest of the battery pack systems.
A second level of protection is provided by the Current Interrupt Device (CID). Each battery cell used in the Tesla Roadster has an internal CID. These devices serve to protect the cell from excessive internal pressure. In such a case the CID will break and electrically disconnect the cell. High internal pressure is generally caused by over-temperature or other failures that then result in over-temperature.
The cells also incorporate numerous mechanical, thermal, and chemical factors that contribute to their safety in the Tesla Roadster. For example, cells used in the Tesla Roadster battery pack are all packaged in steel cans. This feature offers multiple safety benefits. From a mechanical standpoint, the steel case of each cell provides structural rigidity and strength. This helps
dissipate extreme mechanical loading as well as providing protection against objects penetrating or compressing a cell and thereby shorting it. From a thermal standpoint, the steel case also offers good thermal conductivity. The dissipation of heat from a cell both extends battery life and helps maintain the pack at an even temperature. From a chemical and materials standpoint, the materials used in the cellâ€™s construction can greatly impact the flammability and initiation temperature of thermal runaway. Tesla Motors has chosen a very safe cell with great attention paid to both these factors.
Design Safety Features: Battery Pack Level
Due to the size, weight, and cost of the Tesla Roadster battery pack, we have the opportunity to add many more safety features than can be contained in a laptop battery pack. Overall, some of these battery pack safety features are active and others are passive. Some are mechanical and others are electrical. For example, the battery pack is controlled internally by several embedded microprocessors that operate both when the battery pack is installed in the car, and when the pack is being transported. An example of a passive safety feature is the selection of Aluminum for our battery enclosure instead of plastic as in all laptop packs. The Aluminum provides greater structural strength in case of mechanical abuse tolerance and does not easily melt or burn. Collectively, the high levels of redundancy and layers of protection culminate in the safest large battery seen by the experts in the field with whom weâ€™ve consulted.
Architecturally, the battery pack is comprised of 11 battery modules (otherwise referred to as â€śSheetsâ€ť), a main control and logic PCB (printed circuit board), and a 12V DC-DC power supply. Each of the 11 modules carries a monitoring PCB (with its own microprocessor) that communicates with the rest of the vehicle microcontrollers, broadcasting the voltage and temperature measurements of its module over a standard CAN bus.
The method by which the cells are electrically connected together can have a huge impact (positive or adverse) on the overall pack safety. In the Tesla Roadster battery pack, each of the thousands of cells has two fuses (one each for the cellâ€™s anode and cathode). This results in tremendous safety benefits since a cell becomes electrically separated from the rest of the pack if either of its fuses blow (generally by a short circuit). In addition to cell fuses, each of the 11 battery modules has its own main fuse that guards against a short circuit across the complete module.
The picture below (Figure 1) shows the complete battery pack on a cart. Note the tubes and manifold extending out of the battery pack at its lower long edge. These are used to circulate cooling fluid (a 50/50 mix of water and glycol) throughout the pack via sealed fluid paths. This enables us to keep the cells thermally balanced. This extends the life of the battery pack and also has numerous safety benefits.
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