Battery cells generate heat during normal operation. Charging, discharging, and short-circuit conditions all produce heat within the cell as chemical reactions occur to provide stored electrical energy.
When heat generation exceeds the battery's ability to dissipate it, temperatures can rise to the point where the cell undergoes thermal runaway. Because thermal runaway can lead to severe battery damage and safety hazards, it is a critical consideration in battery pack design.
What is Thermal Runaway?
Thermal runaway occurs when a battery cell reaches a temperature that triggers a self-sustaining chain reaction within the battery chemistry.
Once the reaction begins, rising temperatures accelerate additional chemical reactions inside the cell. These reactions generate even more heat, causing temperatures to increase rapidly. Temperatures may reach up to 752°F.
As temperatures continue to rise, battery materials can transition into gaseous forms. If those gases cannot vent effectively, the cell may become damaged, melt, catch fire, or explode.
One of the most important characteristics of thermal runaway is that once the process starts, it can be extremely difficult and dangerous to stop. As a result, battery pack design generally focuses on reducing the conditions that can initiate the event.
Common Causes of Thermal Runaway
The specific causes of thermal runaway can vary depending on battery chemistry. However, there are several common contributors that apply across battery technologies. Although thermal runaway is frequently associated with lithium-based batteries, all battery chemistries may experience thermal runaway under certain conditions, including lead-acid batteries.
A battery cell's temperature, if it reaches a certain point, will undergo thermal runaway.
Excessive Charging and Discharging Conditions
Rapid charging and discharging cycles can create excessive current levels within the battery cell. These conditions increase heat generation and can contribute to temperatures reaching unsafe levels.
Overcharging
Charging a battery cell beyond its maximum voltage can create conditions that increase the likelihood of thermal runaway. Monitoring and controlling charging behavior is therefore an important element of battery pack design.
Short Circuits
Short circuits represent one of the most common paths to thermal runaway. Lithium-ion cells may experience short circuits when physical deformation damages the cell. These faults can occur internally or externally and may result from impact events or other forms of physical damage.
Physical Damage and Mishandling
Cell damage caused by dropping, crushing, or mishandling a battery can create failure conditions that increase the risk of thermal runaway. Mechanical damage can alter the internal structure of the cell and create pathways for short-circuit events.
Designing Battery Packs to Reduce Thermal Runaway Risk
Efforts to reduce thermal runaway begin during battery pack design and continue throughout the product's operational life.
Three primary design areas that can help manage temperature and improve safety:
- Cell arrangement
- Venting and cooling methods
- Battery Management Systems (BMS)
How Cell Arrangement Affects Heat Management
Battery pack configuration plays an important role in controlling temperature distribution.
The objective is to reduce heat accumulation around individual cells and avoid creating localized areas of elevated temperature.
Clustering many cells together may create a heat sink that limits heat dissipation. Arranging cells in rows when possible, can help distribute temperatures more evenly throughout the battery pack.
Proper cell arrangement therefore becomes both an electrical and thermal design consideration, particularly as battery pack size and cell count increase.
Cooling Technologies and Thermal Protection Methods
Additional thermal management features can be incorporated into the battery pack to help regulate temperature.
Depending on the design requirements, cooling solutions may include:
- Fans
- Liquid cooling systems
- Thermal conducting films
- Thermal conducting plates
Thermal Conducting Plates
For some lithium-ion battery applications, thermal conducting plates can be incorporated to draw heat away from cells and transfer it elsewhere within the design.
Thermal Conducting Films
Thermal conducting films may be placed along battery cells to reduce the formation of hot spots as an additional protective function. If thermal runaway occurs in one cell, the thermal film can help limit heat transfer to adjacent cells, helping contain the event and reduce propagation throughout the battery pack.
The Role of a Battery Management System
A Battery Management System (BMS) serves as one of the primary monitoring and protection mechanisms within a battery pack.
The BMS monitors and manages functions that include:
- Voltage
- Current
- Charge control
- Charge balancing
- Cell temperature
The system is also designed to monitor battery state of health (SoH) and state of charge (SoC).
How a BMS Responds to Rising Temperatures
As temperatures increase, the BMS may activate available cooling mechanisms and help maintain balance across the cells within the battery pack.
If the temperatures continue to rise and cooling measures are unable to control conditions, the BMS can shut down other cells to help reduce the likelihood of additional cells undergoing thermal runaway.
While a BMS is an important safety feature, it should not be viewed as a complete safeguard. Thermal runaway may still occur under certain circumstances, even when a Battery Management System is present.
Why Ventilation Matters in Battery Enclosures
Battery cells are commonly installed within an enclosure along with supporting electronics.
Vents can improve heat transfer by allowing heat to leave the enclosure while bringing in cooler outside air. This airflow can help reduce operating temperatures within both the battery pack and associated electronics.
Ventilation for Different Battery Chemistries
Many battery chemistries incorporate ventilation requirements. For example, lead-acid batteries require ventilation because hydrogen gas is generated during charging.
Lithium-based cells do not generate gases during normal operation in the same manner. However, ventilation may still provide thermal benefits by allowing heat to escape the enclosure and reducing overall cell temperatures.
Summary
Thermal runaway is a serious battery safety concern that can affect multiple battery chemistries. It occurs when rising temperatures trigger a self-sustaining chemical reaction that generates increasing amounts of heat, potentially resulting in battery damage, fire, or explosion.
Reducing thermal runaway risk begins during the battery pack design process. Cell arrangement, temperature management strategies, ventilation, cooling technologies, and Battery Management Systems can all contribute to controlling operating temperatures and improving overall battery safety.
Because thermal runaway becomes difficult to control once initiated, designing systems that minimize heat buildup and continuously monitor cell conditions remains a fundamental aspect of battery pack development.
Key Takeaways:
- Thermal Runaway is a Dangerous Chain Reaction: When a battery cell overheats beyond a critical point, it triggers an uncontrollable reaction that rapidly increases temperature, potentially leading to fire, explosion, or battery failure.
- Common Causes Include Overcharging, Short Circuits, and Mishandling: Factors such as excessive charging/discharging, internal/external short circuits, and physical damage can initiate thermal runaway, impacting all battery chemistries, not just lithium-based cells.
- Proper Cell Arrangement Helps Dissipate Heat: Aligning battery cells in a row rather than clustering them together prevents heat buildup and reduces the risk of localized overheating.
- Battery Management Systems (BMS) Enhance Safety: A BMS monitors voltage, temperature, and current, helping regulate charge balance and prevent overcharging, overheating, or cell failure.
- Venting and Cooling Systems Reduce Heat Risks: Cooling plates, thermal conductive films, and proper ventilation allow heat to escape the battery enclosure, preventing dangerous temperature spikes.
