What is a sign that thermal runaway is occurring?

The correct answer regarding a sign that thermal runaway is occurring is the rise in temperature above 100 degrees. Thermal runaway is a condition that occurs in batteries, particularly lithium-ion batteries, when the internal temperature of the cell rises uncontrollably due to exothermic reactions. This process is often initiated by factors such as overcharging or manufacturing defects and can result in catastrophic failure, fire, or explosion.

When the temperature exceeds 100 degrees Fahrenheit, it indicates that the battery is reaching dangerous levels, which is a clear warning sign of thermal runaway. The rise in temperature can cause the electrolyte within the battery to break down, leading to further heat generation and potential ignition of flammable components.

In contrast, a decrease in charging current does not indicate thermal runaway; it might suggest that the battery is reaching full charge or is experiencing internal resistance changes. An increase in battery voltage might reflect normal charging behavior but does not specifically signify thermal runaway. A sudden drop in battery size is not a recognized indicator of thermal runaway and may refer to physical damage or battery swelling, which is an unrelated symptom.

Understanding the critical temperature thresholds and the behavior of batteries during charging and discharging cycles can help in identifying and mitigating the risks associated with thermal runaway.