Applications
The primary use for this module is as part of the battery in the Model S, and they are not sold separately nor authorized for use for any other purpose.
Nevertheless, there is a significant secondary market for these modules due to their excellent energy density; they have been recovered from used or damaged vehicles and combined to form custom batteries for home photovoltaic installations, electric conversions of ICE vehicles, and other applications. However, this has not been without issues - see Safety below.
In Depth
Problems Solved
Conventional batteries generate significant heat from their cores. Exposing a battery to heat can significantly shorten its life, and thus, it is desirable to dissipate the heat from a battery.
To obtain higher current, voltage, or both, from a battery, the battery can be made larger. However, as batteries become larger in size, the ratio of surface area to volume decreases, causing the battery to retain more heat, decreasing its life. The 18650 battery has been developed with these constraints in mind, and because it is a standard size and is produced in large quantities, the costs of such batteries can be lower than other batteries on a cost per milliwatt-hour basis.
Banks of batteries may be connected in parallel to provide higher currents, in series to provide higher voltages, or both. Conventional banks of batteries connect in parallel multiple batteries in each bank, and then connect the banks in series or parallel to achieve the current and voltage desired. However, other configurations may be used to obtain any desired current and voltage.
Some conventional banks of batteries are mounted into a plastic housing. If there are numerous batteries to be mounted, larger banks of batteries may be assembled. Assembling banks of batteries in a plastic housing can be cumbersome and bulky, so an earlier design used banks of conventional batteries in an alternative fashion by gluing batteries together in a side-by-side stack, like stacked firewood, and then connecting the terminals in each battery in the stack using a flexible nickel reed. The positive terminals on one end of the batteries are welded to the reed, and the case, which forms the negative connection, is welded to another reed at the opposite end of the row. All of the reeds at one of each of the ends of the stack are electrically connected and an external means of connection is provided at either end of the stack. The stacks are then used as building blocks to build the desired voltage and current.
However, there are significant problems with this technique. One such problem is that each of the stacks is not physically stable, because the form factor of each battery is not perfectly cylindrical. Instead, each battery is slightly conical, and so the ends of each of the batteries in a stack can shift slightly, causing the joints between the batteries to fail. This makes the glued stacks approach particularly unsuitable for environments in which significant vibration can occur, such as automotive applications. The narrower ends of the batteries can be wrapped with tape to even out the diameter of each end of the batteries, but such wrapping is labor intensive, prone to error and subject to failure.
Another problem with stacks of glued batteries is mechanical strength. A stack is only as mechanically strong as the weakest battery in the stack. If the end of a single battery is crushed, the chemicals in the battery can be compressed, causing a short circuit or other reaction that can heat the battery to an extent that a thermal runaway occurs, in which the heat from the initial reaction causes a thermal reaction to become self sustaining and propagate until the battery fails. The heat from the battery can cause the adjacent batteries to incur the same thermal reaction until many or all of the batteries in the stack have failed. The heat from multiple such reactions can ignite adjacent materials. Turning the batteries on their sides like stacks of firewood can make the problem worse in certain environments, such as when the stack has a large number of batteries in a vibration-prone environment. The force from the vibrations can cause upper batteries to crush the lower batteries in the stack, causing the lower batteries to fail.
Additionally, conventional banks of batteries suffer frosts the problem that the conductors running across, and connected to, the positive “button” on top of the batteries can be pressed into the case of the battery during a significant impact, causing a short circuit between the positive button terminal and the electrically negative case. This makes banks of batteries particularly unsuitable for applications such as an electric or hybrid automobile, or other applications in which the batteries are likely to be vibrated or crushed. The case of the battery is insulated by a thin plastic or other material, such as Mylar. However, during an impact, the conductors that draw the positive current from the battery button terminal can burst through this insulating material to the metallic battery case, which is electrically connected to the negative terminal, thereby shorting the battery. Such an occurrence could also happen simply due to vibrations occurring over an extended period of time.
Still another problem with batteries that are arranged with their edges contacting is that the heat from the batteries can cause the batteries in the center of the stack to become hotter than the batteries at the edges. As noted above, hotter batteries fail sooner than batteries that remain cool.
When multiple banks of batteries are interconnected, the connections between each bank must be manually made, increasing the costs of manufacturing. Wiring for voltage and temperature sensors at various points in the stack to allow for optimum performance further increase the costs of manufacturing.
Furthermore, the space in which the banks of batteries will be placed may not fit the banks exactly, requiring extra space to be allocated for the batteries, wasting space, and such space may be valuable in certain applications. The banks can each be made relatively smaller to reduce any wasted space, but this approach increases the need for interconnection, adding additional cost and potential points of failure.
What is needed is a system and method for providing multiple batteries in a manner that is physically strong and stable, can resist accidental crushing of one or more batteries, when handled or in an environment of vibration, that can resist crushing a conductor attached to the positive terminal into the negative body of the battery, that operates cooler and at a more even temperature than a glued stack or one in which the edges are otherwise touching, that does not require manual connection of each stack, and that can be shaped to more closely fit the available space.
Cells, Substrate, and Connectors
This battery module contains 444 18650 cells arranged in a 74p6s configuration (74 cells in parallel, 6 cells in series).
The cells are arranged in seven double rows and sandwiched between two clear plastic sheets. The number of cells in each double row varies slightly. Each plastic sheet acts as an insulator and has holes which are used to align the cells and hold them in place.
Thermal Management
A flat serpentine fluid tube is threaded through the rows of cells. The fluid in the tube can be used both to cool the modules (e.g. during high power output) or to warm them up (e.g. to ensure they are warm enough to be safely charged).
In total, the tube makes eight lengthwise passes from end to end, and terminates via two 5/16" coolant nipples at the end of the module (opposite from the electrical terminals). The U-bends in the tube are covered in a gold-colored Kapton material which can be seen at each end of the module.
External Connectivity
Like most batteries, the module has two main terminals, negative (left) and positive (right). Each terminal has a 37 mm x 37 mm landing area with an M8 x 27 mm center bolt, and is normally covered by the orange rubber terminal protector (see image).
Safety
Lithium cells are inherently dangerous and overcharging or overdischarging them can lead to thermal runaway, fires, and/or an explosion. It is also critical that the batteries are never charged below freezing (0°C), which leads to lithium dendrite formation on the anode of the cells and eventually catastrophic failure, short circuit, and/or fire. Because the cathode materials produce their own oxygen, any lithium battery fire can be extremely difficult to extinguish.
Tesla has gone to great lengths to create a safe battery system, with many sensors to monitor the module's status, a thermal management system to keep the cells at an appropriate temperature, and the Battery Management System to regulate charging.
However, many of the key safety features depend on both the Battery Management System on board each module and the active monitoring and thermal management systems of the overall battery pack. When a module is removed from the Tesla battery pack, the Battery Management System will not be active, and the module is unsafe to use. There have been reports of fires and explosions from do-it-yourself builds that attempt to use these modules for custom projects without an appropriate battery management system (example). Because of this, there are aftermarket battery controllers available specifically for use with these modules that can emulate some of the functionality of the Battery Management System.
Manufacturing
At present, there is little public information about how these battery modules are assembled, but a general overview of the process is given in patent US20140178722.
Lifecycle
Once a battery module is no longer useful in its original battery pack (e.g. if the vehicle has been damaged or the cells have reached the end of their useful life), then it can either be remanufactured to become part of a new battery pack (the preferred and most economical option) or recycled into raw materials. Tesla states on their website that “None of our scrapped lithium-ion batteries go to landfilling, and 100% are recycled.”