A Brief Explanation of Thermal Runaway

What is Thermal Runaway

Thermal runaway is a cyclic, self-propagating process of temperature increase on electrical component(s) that occurs when the component is unable to dissipate heat quickly enough to keep up with the rate of heat generation. The cycle occurs in an electrical circuit in conditions of undissipated high temperature which leads to a reduction in the component’s electrical resistance which further leads to an increase in current which again leads to an increase in temperature (see figure).

Thermal runaway is of particular concern in aviation due to the limited space available to EWIS components on aircraft. In tight spaces, the generated heat cannot dissipate as freely as in open air, thus extra consideration for heat dissipation must be made to avoid the potential for thermal runaway.

Batteries

With the industry push toward electric and hybrid aircraft, extra consideration must be made for batteries used on aircraft. This includes, most notably, the thermal runaway of lithium batteries. Thermal runaway may occur in lithium batteries on aircraft whether or not they are in active use due to temperature, pressure, and charge conditions; even lithium batteries merely being transported by aircraft are capable of thermal runaway during flight. An FAA report stated that “As of April 1, 2022, there have been 357 aviation related incidents involving Lithium batteries carried as cargo or baggage recorded since January 23, 2006”

The cycle can occur when the batteries are subject to temperature conditions higher than their maximum rated storage temperature. These elevated temperatures can trigger internal exothermic chemical reactions within a battery cell, thus releasing more heat and increasing the rate of the chemical reactions within the cell. The excess heat generated in this process may affect the adjacent cells of the battery, resulting in thermal runaway in multiple cells throughout the battery.

There have been several studies conducted to understand this process specifically for batteries on aircraft. One such study, DOT/FAA/TC-20/12: Thermal Runaway Initiation Methods for Lithium Batteries, performed by the FAA sought to inform the development of standard tests for lithium battery thermal runaway. Lectromec plans to release a more detailed article discussing this study in the near future.

Thermal runaway is of particular concern in aviation due to the limited space available to EWIS components on aircraft. In tight spaces, the generated heat cannot dissipate as freely as in open air, thus extra consideration for heat dissipation must be made to avoid the potential for thermal runaway.

Potential Hazards

Thermal runaway poses a very serious threat to the affected component’s surroundings. In severe cases, the excessive heat generated by thermal runaway may ignite nearby flammable materials resulting in fire or explosions. Even less severe cases can lead to serious problems; overheating caused by thermal runaway can heat components surrounding the affected component and can lead to loss of functionality. Properties of many EWIS components may change when exposed to temperatures above their rated temperature; for example, the electrical resistance of most conductive and insulative materials decreases with an increase in heat. This poses issues such as reducing the efficacy of wire and cable insulation and creating overcurrent conditions on conductive components which may damage connected equipment or result in degraded signals on data lines.

Preventative Measures

It is clear that thermal runaway is problematic and thus preventative measures must be implemented to avoid the phenomenon. The most straightforward method of preventing thermal runaway in any system is to ensure that the only components used in the system be capable of withstanding the anticipated conditions of the application. In order to choose the right components, design considerations regarding spacing and ventilation must be taken into account.

On aircraft, space is limited for the placement and routing of EWIS components; many such electrical components are known to generate heat when in use. In tighter spaces, generated heat is less capable of dissipating and causes an increase in the ambient temperature of the operational environment. This affects the criteria for determining which components are appropriate in application and informs the need for adequate spacing/ ventilation for components known to generate heat.

Realistically, unanticipated conditions may arise in any application. In addition to taking appropriate design precautions, it is important to prepare for unexpected incidents. Some circuit protection devices can help to protect systems by interrupting the flow of current when the temperature of the circuit exceeds a specified value, mitigating the cascading effects of thermal runaway in applicable situations. Heat sinks (typically large pieces of thermally conductive material, such as metal) are also commonly used to draw excessive heat away from more vulnerable areas to areas where it can be more easily dissipated.

Conclusion

Thermal runaway is an important and current problem facing the aerospace industry. As the industry moves in the direction of more electric and hybrid aircraft, thermal runaway considerations for batteries become more and more relevant. Though existing measures do exist for mitigating the dangers of thermal runaway, future developments in testing and technology will surely seek to address today’s concerns.

Contact Lectromec to learn how we can help to protect your electrical systems from thermal hazards.