Increasing energy density means increasing range.
The battery pack in the Tesla Roadster is the result of innovative systems engineering and 20 years of advances in Lithium-ion cell technology. Tesla's ingenious battery pack architecture enables world-class acceleration, safety, range, and reliability. The non-toxic pack is built at Tesla’s Headquarters in Northern California.
The pack weighs 990 pounds, stores 56 kWh of electric energy, and delivers up to 215 kW of electric power. Tesla battery packs have the highest energy density in the industry. To achieve this energy density, Tesla starts with thousands of best-in-class Lithium-ion cells and assembles them into a liquid-cooled battery pack, wrapped in a strong metal enclosure. The battery is optimized for performance, safety, longevity, and cost.
The Most Energy Dense Packs in the Industry
The cells used in a Roadster employ an ideal chemistry for electric vehicles.
Nickel Metal Hydride (NiMH) batteries are commonly used in hybrid cars. However, a 56 kWh NiMH battery pack would weigh over twice as much as the Roadster battery. Instead, Tesla uses Li-ion battery cells which dramatically decrease the weight of the Roadster pack and improve acceleration, handling, and range.
With Lithium-ion chemistry, there is no need to drain the battery before recharging - there is no “memory effect”. Roadster owners simply "top-off" each night.
Start with the Best Cells
The cells used in a Roadster battery pack are referred to as "18650 form-factor" because of their measurements: 18mm in diameter by 65mm length. This form factor is a commodity in the consumer electronics market - over a billion 18650 cells are produced each year. Tesla uses versions of this form factor modified for use in EVs.
In order to assure the highest reliability and safety in the battery pack, Tesla only uses cells from the top worldwide producers. These companies not only have the most experience in making cells but have also been instrumental in providing guidance in designing a safe and reliable pack. Although Tesla does not identify which cells are used in which vehicles (similar to laptop companies), some of Tesla's battery supplier relationships have been publicized.
The small cell size enables efficient heat transfer, allows for precise charge management, improves reliability, and extends battery pack life. Each cell is enclosed in a steel case which effectively transfers heat away from the cell. The small size makes the cell essentially isothermal, and its large surface area allows it to shed heat to the ambient environment.
Architect a Smart, Safe Pack System
Sixty-nine cells are wired in parallel to create bricks. Ninety-nine bricks are connected in series to create sheets, and 11 sheets are inserted into the pack casing. In total, this creates a pack made up of 6,831 cells. Appropriate cell temperature levels are maintained by a proprietary liquid-cooling system which includes sensors within the pack monitored by the car's firmware. Liquid coolant is pumped through the pack to enable effective heat transfer to and from each cell. The cooling system is so effective that the cells on opposite sides of the battery pack stay within a few degrees of each other. This is important for maximizing battery life, optimizing performance and safety.
Tesla has focused a great deal of effort on the safety of the battery pack through both its engineering as well as its industry involvement. Tesla assists the industry as an active member of SAE 2464 and SAE 2929, two major battery pack safety committees. Active involvement in standards not only helps Tesla adopt best practices, but also provides a forum for Tesla engineers to educate others about the high level of safety they can expect in EVs.
The Tesla Roadster’s high voltage systems are protected against accidental contact outside their protective enclosures and jacketed cables. Only with special tools may one gain access to the high-voltage components. In the event of significant impact or rollover, the high voltage supply is automatically disconnected inside the pack to reduce risk of exposure to high voltage. Air bag deployment causes the high-voltage circuits in the vehicle to immediately shut down. The high-voltage systems are enclosed, labeled, and color-coded with markings that service technicians and emergency responders are trained to recognize.
The pack enclosure is designed to withstand substantial abuse in the vehicle, while maintaining the integrity of the internal components. The pack is a stressed member of the chassis and helps provide rigidity to the rear of the car. The Roadster has been tested in frontal, rear and side impact crash tests of the FMVSS as specified by the United States National Highway Transportation and Safety Administration. Reports from actual collisions in the field confirm that the battery pack is well-protected in such scenarios.
Out-Perform in Any Conditions
In general, Lithium-ion cells cannot be charged below 0 degrees Celsius, which would theoretically prevent charging in cold environments. To overcome this cold weather charging obstacle, the Roadster is designed with a heater to warm the cells (when plugged in) to an appropriate charging temperature. If there were no battery pack heater, drivers living in cold environments would have difficulty charging and experience stunted driving performance.
Likewise, the cells are designed to operate in high-temperature environments. High-performance driving is possible in even the hottest environments of the world. If the temperature rises above a set threshold, the air conditioning unit sends chilled coolant through the pack. Similar to the radiator fan of ICE-powered cars, the chilled coolant continues to circulate after the motor has been turned off to keep the temperature at an appropriate level. Cooling the pack enables a driver to quickly charge immediately after hard driving in hot climates. Without such a cooling system, recharging in hot weather would be delayed after each drive.
At the end its usable life, the cells and pack could, by law, be disposed in a landfill. The pack contains neither heavy metals nor toxic materials. While the components could be thrown away, Tesla has implemented a recycling strategy which reuses or recycles over 60% of the battery. Once pack production volumes increase, further recycling steps become profitable and the recycling percentage increases to 90%. The components recovered in the recycling process are valuable which creates a financial incentive for recycling.