118 Chapter 10 cooled pack or an uncooled pack, this concept must be examined to evaluate its impact. Heat capacity may also come into effect if the application is being used in regions of high ambient temperatures. This is because in these regions the battery pack may absorb some of this ambient heat, which will cause the thermal system to have to work much harder in order to remove this heat. An example of this may be seen in regions such as the state of Arizona in the United States.
Here, with summer ambient temperatures that may be in range of 38–44 °C (100–110 °F) or higher, the battery pack will absorb that heat and therefore will have to work to cool down the pack from these initial high temperatures back down to 23 °C in order to preserve the life and performance of the battery system.
At temperatures above about 90 °C, a polymer-based separator may begin to melt and break- down, and between 90 and 130 °C the separator will continue to breakdown until a series of internal short circuits between the anode and the cathode are experienced; at this point, the cell will begin to move toward what is known as “thermal runaway.” In effect, thermal runaway means that the cell becomes hot enough to create self-sustaining heat generation and failure is imminent in the form of cells venting and/or explosion (often referred to as “rapid disassembly”). There is no way to stop a cell in thermal runaway once the threshold has been surpassed.
The exact temperature that a cell reaches the thermal runaway threshold is differ- ent for different chemistries. Some may reach it as low as 120 °C, while others may be able to exceed 140 °C before reaching this event. The other factor that occurs during a thermal runaway is that as the chemistry inside the cells begins to breakdown, side reactions begin occurring rapidly generating oxygen. This oxygen generation may continue to fuel the burning of the cell. In this instance, a sealed pack design may offer some benefit due to the fact that no new oxygen is allowed to enter the pack. Therefore, once the oxygen inside the pack is consumed, there may not be enough oxygen to sustain a flame.
One final comment on thermal runaway is that the temperatures that can be experienced during these events may be able to exceed 600–800 °C depending on the size of the cell and how many cell fail. There are few solutions that can prevent a cascading failure in the event of a thermal runaway with current chemistries. However, system design should take these safety factors in account as much as possible and attempt to either manage to thermally isolate the failing cell and/or manage the venting of the cell. In any case managing cascading cell failures needs to be looked at as a complete system solution. There is no “silver” bullet that can by itself prevent cascading cell failures.
There are a couple other terms that we should briefly discuss in relation to thermal manage- ment systems design and testing, since they require a bit of explaining to clearly understand. The first one is the term adiabatic. The adiabatic process is related to the first law of thermo- dynamics that basically means that there is no transfer of heat from the battery to the outside environment. In relation to lithium-ion batteries, this may be used in relation to thermal characterization testing on a lithium-ion cell. For example, when the cell is being tested, it is done in a thermal chamber that has the capability to operate at varying temperatures. So when