Lithium battery fires are extreme incidents, but they receive a lot of media attention and are often viewed with alarm. From large-scale energy storage power stations to small electric bicycles, the cause of a battery fire may not be the battery itself, or rather, it may not be the battery itself, but rather a malfunction in the battery system. The main cause of lithium battery fires is thermal runaway. Today, when we discuss fire extinguishing, we first need to understand the true cause of thermal runaway.
I. Causes of Lithium Battery Fires
The essence of lithium battery fire is that the heat inside the battery fails to be released as designed, causing the internal and external combustibles to ignite and catch fire. The main causes are external short circuit, external high temperature and internal short circuit.
1. Internal short circuit: Due to the misuse of the battery, such as overcharging and over-discharging leading to dendrite formation, impurities and dust in the battery production process, etc., the battery will deteriorate and puncture the separator, resulting in a micro short circuit. The release of electrical energy leads to a temperature rise, and the chemical reaction of the material caused by the temperature rise further expands the short circuit path, forming a larger short circuit current. This mutually cumulative and mutually reinforcing damage leads to thermal runaway.
2. External Short Circuit: Taking electric vehicles as an example. The probability of a dangerous situation occurring during actual vehicle operation is very low. Firstly, the vehicle system is equipped with fuses and a Battery Management System (BMS). Secondly, the battery can withstand short-term high-current surges. In extreme cases, the short circuit point may extend beyond the vehicle's fuses, and the BMS may fail. A prolonged external short circuit generally only causes weak points in the circuit to burn out, rarely leading to thermal runaway of the battery. Currently, many PACK manufacturers use fuses in the circuit, which more effectively avoids the hazards caused by external short circuits.
3. External High Temperatures: Due to the structural characteristics of lithium batteries, the SEI film, electrolyte, and EC will decompose under high temperatures. The decomposition products of the electrolyte will also react with the positive and negative electrodes, causing the cell separator to melt and decompose. These multiple reactions generate a large amount of heat. The melting of the separator leads to an internal short circuit, and the release of electrical energy further increases heat generation. This cumulative, mutually reinforcing destructive effect results in the rupture of the cell's explosion-proof membrane, electrolyte leakage, and subsequent combustion and fire.
II. Lithium Battery Fire Extinguishing Plan
Having analyzed the causes of battery fires, let's look at Tesla's recommended approach when a lithium battery fire requires extinguishing:
1. In case of a small fire that has not spread to the high-voltage battery section, carbon dioxide or ABC dry powder fire extinguishers can be used to extinguish the fire.
2. When thoroughly inspecting for fire, do not come into contact with any high-voltage components, and always use insulated tools for inspection.
3. If a high-voltage battery is bent, twisted, or damaged during a fire—in short, becomes unrecognizable—or if a battery problem is suspected, then the amount of water used for firefighting must be sufficient.
4. A battery fire may take up to 24 hours to be completely extinguished. Using a thermal imaging camera can ensure that the high-voltage battery is completely cooled before the incident ends. If a thermal imaging camera is not available, the battery must be monitored for reignition. Smoke indicates that the battery is still hot; monitoring should continue for at least one hour after the battery stops smoking.
The emergency response manual for the GM Volt in the US provides the following guidance for firefighting of electric vehicles: If the battery reaches a sufficiently high temperature, leaking and releasing electrolyte, the electrolyte is definitely flammable. This necessitates using a large amount of water to cool the battery and extinguish the fire. Because DC and AC systems are not grounded, firefighters can safely use water as the primary extinguishing agent without the risk of electric shock. ABC dry powder fire extinguishers will not extinguish battery flames. Firefighters should avoid direct contact with the interior of any high-voltage components during firefighting or unloading operations, as this could potentially result in electric shock.
In summary, extinguishing a lithium battery fire requires a large amount of water, a considerable amount of time and patience, and cooling down is crucial. Lithium batteries are not explosive and will not explode under normal circumstances.
