`Tesla Motors
`August 16, 2006
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`By Gene Berdichevsky, Kurt Kelty, JB Straubel and Erik Toomre
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`Summary
`This paper provides details about the design of the Tesla Roadster’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 we’ve consulted, have seen.
`
`Background
`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.
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`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.
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`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 pack’s design and
`architecture.
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`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.
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`The Tesla Roadster battery pack is comprised of about 6800 of these 18650 cells, and the entire
`pack has a mass of about 450kg.
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`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 cell’s overall safety in the Tesla Roadster.
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`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.
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` 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.
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`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
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`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 cell’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.
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`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 we’ve consulted.
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`Architecturally, the battery pack is comprised of 11 battery modules (otherwise referred to as
`“Sheets”), 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.
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`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 cell’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.
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`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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`Figure 1 – Tesla Roadster battery pack
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`This cooling system design is especially effective because we have chosen to combine thousands
`of small cells rather than several large ones to build an ESS, dramatically increasing the surface
`to volume ratio. For example, with seven thousand 18650 cells the surface area is roughly 27
`square meters. If there were an imaginary set of 20 much larger cube-shaped cells that enclosed
`the same volume, the surface area would be only 3.5 square meters, more than seven times
`smaller. Surface area is essential to cooling batteries since the surface is where heat is removed;
`more is better. Also, because of their small size, each cell is able to quickly redistribute heat
`within and shed heat to the ambient environment making it essentially isothermal. This cooling
`architecture avoids “hot spots” which can lead to failures in large battery modules.
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`The multiple microprocessors within the ESS communicate via a CAN Bus, a robust automotive
`communication protocol. During normal vehicle operation and storage, the battery logic board
`communicates with the vehicle to initiate battery cooling, report state of charge, and signal
`battery faults. A fundamental element of the vehicle and battery pack safety design is the ability
`to electrically disconnect the high voltage of the pack from the rest of the car (by controlling two
`high voltage contactors) if any of a number of adverse conditions are detected.
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`The microprocessors, logic circuitry and sensors are continually monitoring voltages, currents
`and temperatures within the pack. These sensors also monitor inertia acceleration (e.g. to detect a
`crash) and vehicle orientation to the ground (e.g. to detect a rollover). Our battery packs also
`include smoke, humidity, and moisture sensors. If certain sensors exceed the specified range,
`then the high voltage contactors will immediately (within milliseconds) disconnect the high
`voltage of the battery pack from the car. In fact, the contactors are only closed (connected) when
`commanded and energized to do so. Without the proper commands these contactors will open.
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`In more severe fault conditions such as a vehicle collision, active protection systems including
`the logic board could fail due to damage. Therefore, the battery pack design incorporates an
`array of passive safety features as well. The passive design improves the robustness of the
`battery pack, particularly against mechanical damage and potential foreign object penetration of
`the battery pack.
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`None of the Tesla Roadster’s high voltage systems are accessible to accidental contact outside
`their protective enclosures and jacketed cables. Only with special tools can someone gain access
`to any high-voltage components. Our high-voltage systems are enclosed, labeled, and color-
`coded with markings that service technicians and emergency responders already understand.
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`Finally, the battery pack enclosure is designed to contain all the battery modules, fuses, bus bars,
`and safety circuitry of the system. The enclosure is electrically isolated from the battery pack and
`prevents users from directly accessing any high voltage connections. The enclosure is also
`designed to withstand substantial abuse in the vehicle, including collision, while maintaining the
`integrity of the battery modules and circuitry inside.
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`Testing
`Upon completion of our design, we collaborated with an outside firm known for expertise in
`lithium-ion batteries to perform hundreds of tests to validate the abuse tolerance and
`effectiveness of our design.
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`We have performed further tests including SAE (Society of Automotive Engineers) shock and
`vibration, crush, and vehicle collision testing. Additionally, the United Nations (UN) imposes
`strict rules regarding the transport of lithium-ion batteries. Tesla Motors will not be able to sell
`and deliver cars to its customers unless the production battery pack has met rigorous testing
`standards set by the UN or substitute testing agreed to by the United States Department of
`Transportation. Finally, we have passed all required tests from the Federal Motor Vehicle Safety
`Standards (FMVSS). This involves crashing of complete cars with functional battery packs in
`them.
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