Process and preventive measures of battery thermal runaway (2022)

Battery thermal runaway refers to a phenomenon in which the pressure of the battery cell increases and the temperature rises due to multiple reasons. If the excess heat of the battery is not dissipated in time, and the heat accumulates to a certain extent inside the battery, dangerous situations such as smoke and fire will occur.

The problem of battery thermal runaway must be taken seriously. While the battery itself is constantly releasing heat and has difficulty dissipating it, thermal runaway can be prevented with proper measures and careful maintenance.

Battery thermal runaway process

Battery thermal runaway is caused by the fact that the heat generation rate of the battery is much higher than the heat dissipation rate, and a large amount of heat is accumulated and not dissipated in time. Essentially, thermal runaway is an energy-positive feedback loop process: increased temperature causes the system to heat up, which in turn makes the system hotter. Without strict division, battery thermal runaway can be divided into three stages:

Thermal runaway phase inside the battery

Due to internal short circuit, external heating, or self-heating of the battery itself during high current charging and discharging, the internal temperature of the battery rises to about 90°C to 100°C, and the lithium salt LiPF6 begins to decompose. The chemical activity of the carbon anode in the charged state is very high, close to metallic lithium, and the SEI film on the surface decomposes at high temperature, and the lithium ions embedded in the graphite react with the electrolyte and the binder, further pushing the battery temperature to 150 °C.

At this temperature, a new violent exothermic reaction occurs, for example, a large amount of electrolyte is decomposed to generate PF5, and PF5 further catalyzes the decomposition reaction of organic solvents.

Process and preventive measures of battery thermal runaway (1)

Battery swollen stage

When the temperature of the battery reaches above 200°C, the cathode material decomposes, releasing a large amount of heat and gas, and the temperature continues to rise. That will causes swollen battery. At 250-350°C, the lithium intercalated anode begins to react with the electrolyte.

Battery thermal runaway, explosion failure stage

During the reaction process, the charged cathode material begins to undergo a violent decomposition reaction, and the electrolyte undergoes a violent oxidation reaction, releasing a large amount of heat, generating high temperature and a large amount of gas, and the battery will burn and explode.

Battery thermal runaway characteristic parameters and early warning

Regarding the early warning of battery thermal runaway, it is mainly to monitor and warn the battery for the critical condition of battery thermal runaway. By monitoring one or several characteristic parameters and characteristic gases, the battery thermal runaway can be effectively pre-warned, so as to avoid the large economic loss caused by thermal runaway.

Temperature

The battery early warning system uses temperature detectors to monitor the temperature. When the temperature exceeds the critical temperature, an early warning message is issued for early warning. For the law of heat production of 18650 batteries at different magnifications, a three-level early warning scheme was determined, with 50 °C as the first-level early warning temperature, 70 °C as the second-level early-warning temperature, and 80 °C as the third-level early warning temperature.

Internal resistance

Usually the battery is within the normal working temperature range, and the internal resistance of the battery decreases as the temperature increases, but when the battery exceeds the normal working range or even battery thermal runaway occurs, the internal resistance of the battery will increase significantly.

Process and preventive measures of battery thermal runaway (2)

Considering that the sudden change in the internal resistance of the battery is not necessarily caused by the battery thermal runaway, and the battery is subject to external disturbances resulting in poor contact, the internal resistance will also change.

Voltage

The process of voltage drop in different triggering methods is different. But in fact, the change of the voltage of the battery is very complex and the regularity is poor, and when the voltage drops sharply, the battery usually fails, and thermal runaway has occurred at this time. In addition to the battery thermal runaway, sometimes the battery has poor contact, which will also cause the battery voltage to suddenly change.

Characteristic gases and smoke

The battery produces CO, H2, HF, various hydrocarbons, etc. during the thermal runaway process. The appearance of these characteristic gases will lead to a rapid increase in the concentration of the gases in the surrounding environment. Using corresponding sensors to monitor these gases and smoke is also an important monitoring method for battery thermal runaway.

Causes of UPS battery thermal runaway

There are many reasons for UPS batteries thermal runaway. High charging voltage and large gas evolution will cause battery thermal runaway. If a single-cell battery in the battery pack suffers from severe battery thermal runaway, but the constant voltage value of charging remains unchanged, the charging voltage of other single-cell batteries will also be relatively high, resulting in thermal runaway problems. The specific analysis is as follows:

Difficulty in heat dissipation due to compact assembly

For compact batteries, the electrolyte is stored in porous separators, which makes it difficult to dissipate heat. Unlike ordinary lead-acid batteries, it can stir the electrolyte during charging and gas evolution, which is conducive to heat dissipation. When there is a partial short circuit inside the battery, the battery temperature will rise even higher.

The temperature is high and the float current will increase in summer

If the float voltage is not lowered in time, the temperature of the battery will rise rapidly. The charging voltage is too high, and the gas evolution is too large, resulting in excess oxygen circulation. If there is a micro-short circuit in one of the batteries in the battery pack, it is also equivalent to increasing the charging voltage.

Process and preventive measures of battery thermal runaway (3)

Safety valve is not tight or valve opening pressure is too low

The phenomenon of battery thermal runaway is very typical. The direct consequence of battery thermal runaway is that the internal electrolyte of the UPS battery dries up, the internal resistance of the UPS battery is abnormal, the battery case is deformed and expanded, or even ruptures, and a large amount of acid gas is emitted.

Battery fire and explosion are the most common thermal runaway hazards in power battery systems, and the impact is even more serious. It will not only cause property damage and environmental damage, but even cause personal injury or life danger.

How to prevent battery thermal runaway and prevention measures

UPS lithium battery thermal runaway means that when the battery is charged at constant voltage, the charging current and the temperature of the UPS battery have a cumulative enhancement effect, and gradually damage the battery. The battery thermal runaway of the battery will cause the battery to lose water rapidly, and the electrolyte in the diaphragm will dry up quickly, and eventually the battery will fail. The following steps can effectively prevent UPS battery thermal runaway:

● Charging equipment should have temperature compensation function or current limit;

● Strictly control the quality of the safety valve, so that the gas inside the battery can be discharged normally;

● The UPS battery should be set in a well-ventilated location, and the temperature of the UPS battery should be controlled.

Process and preventive measures of battery thermal runaway (4)

All in all, we can do the followings to prevent battery thermal runaway:

● Correct selection and timely adjustment of float voltage.

● The oxygen recombination reaction is carried out on the anode, so the temperature of the anode is the highest. Furthermore, the poles and grids are metal, which are good conductors of heat. They are definitely more sensitive to the temperature increase of the battery than the outer casing. Therefore, the temperature of the anode should be continuously monitored during charging. Especially in summer or noon, pay special attention to whether the battery temperature rises.

● The charging equipment should have temperature compensation and current limiting functions.

● Strictly control the quality of the safety valve to avoid failure and ensure the normal discharge of gas inside the battery.

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