Battery safety is a critical aspect of energy storage technology, as improper operation or failure can lead to hazardous conditions. Understanding key terms related to battery safety helps in identifying risks and implementing appropriate measures. Below are definitions and explanations of fundamental battery safety concepts, focusing on failure modes without delving into protective mechanisms or materials.

**Thermal Runaway**
Thermal runaway is a self-sustaining, uncontrolled increase in temperature within a battery cell, often leading to catastrophic failure. This phenomenon occurs when heat generation exceeds the cell's ability to dissipate it, creating a positive feedback loop. Common triggers include internal short circuits, overcharging, or mechanical damage. During thermal runaway, exothermic reactions within the cell accelerate, releasing gases, flames, or even causing explosions. Lithium-ion batteries are particularly susceptible due to their high energy density and flammable electrolytes. The process can propagate to adjacent cells in a battery pack, amplifying the hazard.

**Venting**
Venting refers to the release of gases from a battery cell due to internal pressure buildup. This occurs when electrochemical reactions or thermal decomposition produce gases faster than they can be reabsorbed. In some designs, safety vents open at a predetermined pressure to prevent rupture. However, venting may release flammable or toxic gases, posing fire and health risks. In severe cases, venting precedes thermal runaway if the released gases ignite. The composition of vented gases depends on battery chemistry, with lithium-ion cells often emitting carbon dioxide, carbon monoxide, and hydrogen fluoride.

**Short Circuit**
A short circuit happens when an unintended low-resistance path forms between a battery's positive and negative terminals, bypassing the normal load. Internal short circuits occur due to separator failure, dendrite growth, or manufacturing defects, while external short circuits result from physical damage or improper handling. The high current flow generates excessive heat, potentially leading to thermal runaway. Short circuits in high-capacity batteries can produce dangerous arc flashes or rapid energy discharge, damaging the cell and surrounding components.

**Overcharge**
Overcharge occurs when a battery continues to receive current after reaching full charge, driving undesirable side reactions. These reactions may include electrolyte decomposition, electrode degradation, and gas generation. In lithium-ion batteries, overcharging can cause lithium plating on the anode, reducing cycle life and increasing the risk of internal shorts. Excessive voltage also accelerates heat generation, potentially triggering thermal runaway. While most modern batteries include protection circuits to prevent overcharge, understanding the failure mode remains essential for safety assessments.

**Over-Discharge**
Over-discharge happens when a battery is drained beyond its safe voltage threshold, leading to irreversible damage. In lithium-ion cells, over-discharge can cause copper dissolution from the anode current collector, creating conductive pathways that increase short-circuit risk upon recharge. It also degrades electrode materials, reducing capacity and lifespan. In multi-cell packs, over-discharge of individual cells can create imbalances, further compromising performance and safety. Unlike overcharge, over-discharge may not produce immediate hazards but contributes to long-term reliability issues.

**Additional Safety Terms**

**Dendrite Formation**
Dendrites are needle-like metallic growths that can form on battery electrodes during cycling, particularly in lithium-metal or aged lithium-ion cells. These structures penetrate the separator, creating internal short circuits. Dendrite formation is influenced by factors such as charging rate, temperature, and electrolyte composition. While not an immediate failure mode, progressive dendrite growth increases the likelihood of catastrophic events.

**State of Charge (SOC) and State of Health (SOH)**
SOC indicates a battery's remaining energy capacity relative to its full charge, while SOH reflects its condition compared to a new cell. Operating outside recommended SOC ranges (either too high or too low) accelerates degradation and raises safety risks. SOH monitoring helps predict end-of-life behavior, as aged batteries are more prone to failures like increased internal resistance or gas generation.

**Thermal Management**
Though not a failure mode itself, inadequate thermal management exacerbates safety risks. Batteries generate heat during operation, and poor dissipation leads to localized hot spots. Elevated temperatures accelerate side reactions, reduce efficiency, and increase the likelihood of thermal runaway. Passive cooling (e.g., heat sinks) or active systems (e.g., liquid cooling) mitigate these effects but are beyond the scope of this discussion.

**Conclusion**
Battery safety terminology provides a framework for understanding how and why failures occur. Thermal runaway, venting, short circuits, overcharge, and over-discharge represent critical risks that stem from chemical and physical processes within cells. Recognizing these concepts aids in designing safer systems and responding to incidents effectively. While protection mechanisms exist to prevent such failures, the underlying principles remain essential knowledge for engineers, researchers, and users handling energy storage technologies.