A battery stores chemical energy and converts it into electrical energy through controlled electrochemical reactions. Subjecting any battery to external heat or fire introduces extreme danger due to the volatile nature of the stored energy and components. Fire causes a rapid breakdown of the internal structure, which is designed to safely contain highly reactive materials. This external thermal abuse sets off a chain reaction that quickly turns the power source into a serious hazard.
Internal Pressure and Thermal Runaway
The core danger when a battery is exposed to fire comes from the rapid increase in internal temperature. This external heat causes the liquid electrolyte, a flammable organic solvent, to vaporize into gas. Since the battery casing is a sealed unit, this vaporization leads to a rapid buildup of internal pressure within the cell.
Thermal Runaway
This escalating heat can trigger thermal runaway, a self-sustaining process. Thermal runaway begins when the internal temperature causes components to break down and react exothermically, generating their own heat. This internal heat quickly exceeds the battery’s ability to dissipate it, creating a positive feedback loop. This uncontrolled temperature rise leads to the decomposition of electrode materials and rapid structural failure.
Immediate Physical Reactions
As internal pressure from the vaporized electrolyte and decomposition gases increases, the battery casing eventually fails. This structural failure typically begins with venting, where pressurized, flammable gases and electrolyte are forcefully released through a designated or compromised point. The released gases are volatile and can immediately ignite upon contact with the external heat source or oxygen, resulting in rapid flame production.
The energy released during this process is intense, often resulting in a violent rupture of the casing, commonly described as an explosion. This event can eject burning debris and molten internal components, spreading the fire to surrounding materials. The entire physical reaction, from venting to full conflagration, can happen in seconds once thermal runaway is initiated.
Release of Hazardous Materials
Battery combustion releases a complex mixture of hazardous chemical compounds beyond the immediate danger of fire and explosion. The smoke produced is a toxic plume containing metal oxides and other dangerous substances, not typical wood or paper smoke. Inhaling this smoke presents a severe health risk, even after the flames have been extinguished.
For batteries containing fluorine-based components, such as lithium hexafluorophosphate (LiPF6), decomposition releases extremely dangerous gases. One concerning gas is hydrogen fluoride (HF), which is highly corrosive and forms hydrofluoric acid upon contact with moisture in the lungs or on the skin. Other toxic gases, including carbon monoxide (CO) and hydrogen cyanide (HCN), are also released. Immediate evacuation and professional ventilation are necessary due to the nature of these invisible and often odorless gases.
How Battery Chemistry Changes the Outcome
The severity of the reaction is determined by the specific chemistry of the battery involved. Common household alkaline batteries, which use zinc and manganese dioxide chemistry with a potassium hydroxide electrolyte, present a lower fire risk. When exposed to fire, alkaline batteries are more likely to rupture, vent, and leak their corrosive electrolyte, but they are less prone to explosive thermal runaway.
In contrast, lithium-ion and lithium-polymer batteries, used in most modern electronics, have a higher energy density and a flammable organic electrolyte. This increased energy storage means they undergo a far more violent thermal runaway, producing hotter fires and a greater volume of toxic gases like hydrogen fluoride. Therefore, a lithium coin cell or a lithium-ion pack represents a significantly greater danger than a standard alkaline battery when subjected to fire.