Most thermal runaway events trace back to one of a handful of well understood root causes.
What causes thermal runaway in batteries is one of the most frequently asked questions among engineers, safety teams, and operators working with lithium-ion technology. While the visible result, a rapidly escalating chain reaction leading to extreme heat and potentially fire, can look dramatic and unpredictable, the underlying causes are well understood and generally fall into a small number of recognised categories.
Mechanical Damage
Physical damage to a cell is one of the most common root causes of thermal runaway. A collision, drop, puncture, or even sustained vibration over time can damage the internal separator that keeps a cell’s positive and negative electrodes apart. Once that separator is compromised, an internal short circuit can develop, generating localised heat that may be enough to initiate the thermal runaway process. This is one of the reasons shock and impact monitoring has become an increasingly important consideration in battery pack design, not only to prevent immediate failure but to support post-incident assessment of whether a pack remains safe to use.
Productiefouten
Even with rigorous quality control, cell manufacturing defects do occur. Contamination introduced during production, inconsistent electrode coating, or microscopic flaws in the separator material can all create conditions where an internal short circuit develops gradually, sometimes months or years after a cell first enters service. These defects are notoriously difficult to detect through conventional inspection, which is part of why ongoing environmental and chemical monitoring throughout a battery’s working life has become so valuable.
Overcharging and Overdischarging
Charging a lithium-ion cell beyond its designed voltage limit, or discharging it below its minimum safe threshold, places significant stress on the cell’s internal chemistry. Overcharging in particular can cause lithium plating on the anode and the breakdown of the electrolyte, both of which generate heat and increase the risk of an internal short circuit. Well designed battery management systems are intended to prevent this, but software faults, sensor calibration errors, or unusual operating conditions can occasionally allow it to occur.
Excessive Heat Exposure
Lithium-ion cells operate within a relatively narrow optimal temperature range. Prolonged exposure to high ambient temperatures, whether from direct sunlight, proximity to other heat generating components, or inadequate thermal management, can accelerate degradation and increase the likelihood of thermal runaway being triggered by a comparatively minor additional stress. This is particularly relevant in densely packed battery enclosures or installations in hot climates, where passive cooling alone may not be sufficient.
Internal Short Circuits from Ageing
Even without an obvious external cause, cells naturally degrade over their operational lifetime. Repeated charge and discharge cycles, particularly at high rates or extreme states of charge, can cause the gradual formation of dendrites, needle-like lithium structures that can eventually pierce the separator and create an internal short circuit. This is one of the reasons second-life battery applications, where cells from retired electric vehicles are repurposed for stationary energy storage, place such emphasis on rigorous health testing and ongoing monitoring before and during deployment.
Why Identifying the Cause Matters for Detection Strategy
Understanding what causes thermal runaway in batteries is not simply an academic exercise; it directly informs how monitoring systems should be designed. Mechanical damage points to the value of shock and acceleration sensing. Manufacturing defects and ageing related short circuits point to the importance of continuous chemical and environmental monitoring throughout a battery’s working life, not just during initial testing. Overcharging and heat exposure point to the need for accurate temperature and voltage tracking integrated closely with charge control systems.
Regardless of root cause, nearly all of these failure pathways converge on the same observable early symptom: cell venting, accompanied by the release of volatile organic compounds, before more severe consequences develop. This is why VOC detection, as used in Metis Engineering’s Celbewaking sensor, has proven effective across such a wide range of failure scenarios. It does not need to know the specific root cause in order to detect that something has gone wrong.
A Multi-Parameter Approach to Detection
Because the causes of thermal runaway are varied, the most effective detection strategies tend to monitor multiple parameters simultaneously rather than relying on a single signal. Cell Guard, for example, combines VOC detection with absolute pressure, humidity, dew point, and optional hydrogen and accelerometer monitoring, giving engineers a fuller picture of pack health and a greater chance of catching a developing fault regardless of its origin.
Speak to Our Engineering Team
If you are designing a battery system and want to discuss the right detection strategy for your specific risk profile, get in touch with our team or explore the Cell Guard product page for full technical detail.
