Microsoft word - the tesla roadster battery system_121807.doc

The Tesla Roadster Battery System
Tesla Motors
By Gene Berdichevsky, Kurt Kelty, JB Straubel and Erik Toomre
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.
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 pack’s design and
architecture.
1 of 5 Copyright 2007 Tesla Motors Updated: December 19, 2007 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 cell’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
2 of 5 Copyright 2007 Tesla Motors Updated: December 19, 2007 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.
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.
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.
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.
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.
3 of 5 Copyright 2007 Tesla Motors Updated: December 19, 2007 Figure 1 – Tesla Roadster battery pack 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. 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. 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. 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. 4 of 5 Copyright 2007 Tesla Motors Updated: December 19, 2007
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.
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.
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.
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.
5 of 5 Copyright 2007 Tesla Motors Updated: December 19, 2007

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