Characteristics and Firefighting Challenges of Battery Fires
Lithium-ion battery fires are characterized by their suddenness, intense combustion, and high re-ignition rate. Thermal runaway propagation is a major challenge in lithium battery fires; the thermal runaway of one cell can trigger a chain reaction throughout the entire battery pack or system within a short period.
Thermal runaway refers to an irreversible, self-exothermic chain chemical reaction.
Lithium iron phosphate battery packs can reach thermal runaway temperatures of 600-800℃, but the reaction is relatively mild and the temperature rise rate is slow;
Ternary lithium battery packs can reach temperatures above 800-1000℃, and are more prone to sudden onset and spread faster.
1. Triggering Stage: The battery experiences an abnormally high localized temperature due to internal short circuits, overcharging, over-discharging, mechanical damage, or external heating.
2. Chain-like Exothermic Stage: The temperature rise triggers a series of exothermic side reactions within the battery, such as SEI film decomposition, negative electrode-electrolyte reaction, positive electrode oxygen release, and electrolyte decomposition. These reactions release a large amount of heat, causing the battery temperature to rise exponentially.
3. Ejection and Combustion Stage: When the temperature exceeds the critical point (typically around 150-200℃), the electrolyte vaporizes violently, causing a sudden increase in internal battery pressure. This leads to the rupture of the safety valve or the explosion of the casing, ejecting flammable electrolyte vapor, smoke, and solid particles. These flammable gases mix with air and ignite upon contact with high temperatures or sparks, forming a powerful jet flame.
Unlike traditional fires, lithium battery fires exhibit characteristics of both solid and gaseous fires. Thermal runaway from the battery releases flammable gases, posing an explosion risk, while the ejected electrolyte further intensifies the fire.
This complexity necessitates a multi-layered, multi-strategy approach to battery fire suppression; single firefighting methods are often insufficient to effectively control battery fires.
Fire Safety Requirements for Power Battery Packs
Due to different application scenarios, the design objectives of fire protection systems for power battery packs and energy storage battery packs differ:
* Power Battery Packs (e.g., electric vehicles): Space is limited, so the focus is on quickly suppressing initial open flames to buy time for evacuation. Perfluorohexanone (PFH), due to its insulating and rapid vaporization properties, is often considered for active fire suppression systems in such spaces.
* Energy Storage Systems (e.g., energy storage power stations): Large-scale and high-energy, the core objective is to prevent the spread of thermal runaway and eliminate reignition. Fine water mist, due to its excellent and continuous cooling capabilities, is often used as the primary fire protection method for large energy storage power stations, but an effective drainage system must be designed.
Fire Safety Configuration for Bus Battery Packs
For buses with a length of 6 meters or more, national standards have clear fire safety requirements. The “Technical Conditions for Safe Operation of Motor Vehicles” (GB 7258-2017) stipulates that the battery system of such buses must be able to monitor the working status of the power battery, alarm in case of abnormality, and the battery box must not catch fire or explode within 5 minutes after the alarm.
More specific requirements can be found in the “Configuration Requirements for Fire Prevention Devices in Lithium-ion Power Battery Boxes of Buses” (JT/T 1461-2023). This standard requires that the fire prevention device have an automatic start function; when the detection device meets the start conditions, the fire suppression device should automatically start within 2 seconds and spray a suppressing medium into the battery box.
Safety Concepts for Passenger Vehicle Battery Packs
Compared to the mandatory fire-fighting requirements for buses, the safety concept for passenger vehicles places greater emphasis on the safety performance of the battery itself. The national standard “Safety Requirements for Power Batteries for Electric Vehicles” (GB 38031-2025) stipulates that the battery pack system must provide occupants with at least 5 minutes of escape time in the event of thermal runaway.
The above differences mainly stem from different application scenarios:
Buses have a large passenger capacity, involve public safety, and typically have larger battery capacities; passenger cars, on the other hand, are more constrained by space, weight, and cost limitations. Passenger cars primarily prevent thermal runaway or its spread by improving the battery’s own heat resistance and short-circuit protection capabilities, rather than relying on mandatory external fire suppression systems.
Fire Safety Requirements for Energy Storage Battery Packs
Energy storage battery packs are often densely integrated in containers or outdoor cabinets, representing a massive scale and investment (MWh level). If a single battery cell experiences thermal runaway, the fire can easily spread to adjacent battery packs, triggering a catastrophic chain reaction.
Mandatory Fire Extinguishing System Requirements
According to relevant standards such as the “Safety Regulations for Electrochemical Energy Storage Power Stations” (GB/T 42288—2022), energy storage power stations must be equipped with automatic fire alarm systems and fixed fire extinguishing systems. Especially for lithium-ion battery energy storage power stations, the fire extinguishing system must not only be able to extinguish open flames at the battery module level, but also ensure that reignition does not occur within 24 hours.
Development of Fire Monitoring and Early Warning Systems
Given the unique characteristics of energy storage battery fires, traditional building fire detectors often cannot meet the early warning requirements. Currently, the “General Technical Requirements for Fire Monitoring and Early Warning Systems for Electrochemical Energy Storage Power Stations” is being developed, which will specify more specialized monitoring and early warning systems.
PACK Firefighting Technology
Passive Defense:
Main Objective: To prevent thermal runaway, or to buy time and control the scope of impact when it occurs.
Insulation Design: Install aerogel, mica, or other insulation layers between battery cells to prevent rapid heat transfer to adjacent cells.
Flame Retardant Design: Use flame-retardant plastics, ceramicized silicone, or other materials, generally requiring a flame retardant rating of UL94-V0 or higher.
Pressure Relief and Flow Diversion: Design explosion-proof valves to guide high-temperature ejected materials to safe passages.
Active Firefighting
Thermal Runaway Detection and Early Warning
If the passive defense system fails to prevent thermal runaway, the early warning system will activate, promptly detecting early thermal runaway signals, issuing alarm signals, and taking corresponding measures.
Gas Detection: Monitors changes in the concentrations of carbon monoxide (CO), hydrogen (H₂), and volatile organic compounds (VOCs).
Temperature Rise Alarm: Deploys multiple temperature sensors to monitor point temperatures and abnormal temperature rises (e.g., increases of a few degrees per minute).
Smoke Detection: Monitors abnormal increases in smoke particle concentration or internal air pressure within the battery pack.
Upon receiving a thermal runaway signal, the BMS actively controls valves (e.g., solenoid valves, puncture valves) to open, and the extinguishing medium in the fire extinguishing pipeline is sprayed into the battery pack through nozzles, achieving precise fire suppression at the battery pack level.
Selection of Extinguishing Media
The selection of extinguishing media is crucial to fire suppression effectiveness. Commonly used extinguishing media include perfluorohexanone and fine water mist.
Perfluorohexanone primarily utilizes chemical inhibition and suffocation to quickly extinguish open flames in the early stages of a fire; fine water mist provides sustained, high-flow-rate cooling over a long period, inhibiting thermal runaway.
An ideal extinguishing media should possess the following characteristics:
Highly efficient cooling capacity: Insulates against the propagation of thermal runaway
Electrical insulation: Prevents the risk of short circuits
Non-corrosive: Does not affect components inside the battery compartment
Environmentally friendly and non-toxic: Friendly to personnel and the environment.
