Compared to applications such as laptops, mobile phones, and fixed purposes such as energy storage and backup power, the usage environment of lithium-ion batteries for electric vehicles is more complex, varied, and demanding. For example, batteries need to be exposed to a wide temperature range for operation, battery packs need to withstand prolonged vibrations during vehicle operation, and require high rate charging and discharging. Among them, high rate charging and discharging can lead to an increase in heat generation inside the battery. If the thermal management system cannot heat up the battery in a timely manner, high temperatures can cause various side reactions inside the battery, such as SEI film decomposition, negative electrode and electrolyte reaction, electrolyte decomposition, etc., and ultimately lead to thermal runaway.
Once the battery enters the thermal runaway stage, it will face the risk of fire and explosion in a short period of time. In addition, unlike batteries used in consumer electronics, power batteries used in electric vehicles have lower fault tolerance. Taking the 18650 battery as an example, the probability of internal spontaneous failure (also known as field failure) can be controlled between 1 in 40 million and 1 in 10 million, which is relatively reliable for consumer electronics products. However, when used in electric vehicles, due to the number of battery cells in the battery pack usually being hundreds or even thousands, even such a low probability of spontaneous failure needs to be given sufficient attention.
In summary, as one of the core components of electric vehicles, improving the safety of power batteries is of utmost importance for the development of the electric vehicle industry. Therefore, it is particularly urgent to effectively carry out safety testing and evaluation of power batteries. This article combines the current standard system and related research results to analyze and summarize the current safety evaluation methods for power batteries, hoping to provide useful reference and guidance for establishing more scientific quantitative testing and evaluation methods.
1. Safety testing of individual power batteries
1.1. Test Standard
The possibility of fire, explosion, etc. occurring in the correct use of power batteries with high control level is extremely small. Only when the battery exceeds the state boundary in actual use, such as overcharging, short circuit, or overheating, can it possibly cause thermal runaway of the battery
Although thermal runaway of batteries is an abnormal situation, the working state and actual usage environment of power batteries in vehicles are complex and variable, so research on the thermal runaway behavior of batteries cannot be ignored. By studying the thermal runaway behavior of batteries, we can not only understand the characteristics of the battery thermal runaway process, detect safety hazards early in actual use, reduce safety risks, but also take effective measures to prevent accidents from further expanding when the battery experiences thermal runaway, providing strong technical support for rescue efforts
1.2. Thermal stability testing
The safety of power battery cells can be divided into intrinsic safety (thermal stability) and triggering safety (including thermal runaway caused by external factors such as overcharging, heating, puncture, short circuit, etc.) based on the amount of energy introduced or influencing factors. For the former, an accelerated adiabatic calorimeter is an effective characterization method. The temperature and temperature change rate curves during the thermal stability evolution of several lithium-ion battery products on the market (with samples A, C, and D being ternary carbon system batteries and sample B being lithium iron phosphate carbon system batteries). The intrinsic thermal runaway characteristics of power batteries mainly include six typical stages, namely capacity decay, self generated heat, membrane melting, internal short circuit, rapid internal temperature rise, and residual reaction.
In addition, for lithium-ion batteries with different material systems, the incubation time required for thermal runaway of lithium iron phosphate battery (sample B) is the longest, and the inflection point temperature for severe thermal runaway is the highest (using 10 ° min as the criterion for severe thermal runaway)
Compared to fresh batteries, thermal stability analysis of batteries throughout their entire life cycle is equally important. Comparison of thermal stability evolution curves of a certain lithium-ion battery at different cycle times. From the overall situation, there are significant differences in temperature nodes on the thermal runaway curves for different cycle cycles. As the number of cycles increases, the decomposition temperature of the SEI film gradually decreases, and the time for thermal runaway of the battery is advanced, making it more and more prone to thermal runaway. This requires that the design and use of power battery systems should fully consider the actual situation of the battery in the later stage of its life, to avoid safety hazards such as battery failure after a period of battery use.
1.3. Thermal runaway test
The research on the triggering method of thermal runaway in power batteries is as mentioned earlier. Power batteries will face various environments and working conditions during actual use, so it is necessary to study and verify the safety of their triggering. At present, the commonly used thermal runaway triggering methods in the industry mainly include overcharging, heating, and needling. The characteristics of three typical thermal runaway triggering methods are compared. Other triggering methods that are still in the exploratory stage include internal short circuits, mainly relying on embedding memory metals, phase change materials, etc. inside the battery to achieve controllable triggering of short circuits inside the battery. inside the battery to achieve controllable triggering of internal short circuits. The triggering probability, repeatability, and positional freedom of this method are relatively high, but due to the fact that it can only be modified by the battery factory in practical operation, its implementation is difficult and limited from an application perspective.
By selecting more than ten typical products commonly found in the market and conducting experimental research on the three typical triggering methods mentioned above, it was found that there are certain differences in the probability of sample thermal runaway triggering among the three triggering methods. That is, the heating method can trigger thermal runaway in all samples, acupuncture can almost trigger thermal runaway in all samples, and overcharging can only trigger thermal runaway in 46% of samples. The main reason for this is due to the structure of square and cylindrical batteries. Overcharging can trigger internal protection mechanisms to prevent thermal runaway.
1.4. Thermal runaway of power battery cells entire life cycle
As the number of battery cycles increases, there may be deterioration phenomena such as SEI film changes, lithium dendrite growth, and membrane micropores inside the battery, which can lead to a decrease in the safety of the battery. Therefore, studying the evolution characteristics of the safety of power batteries throughout their entire life cycle is of great guiding significance for the safe and reliable application of products. The evolution law of short-circuit safety of a certain lithium-ion battery with the number of cycles shows that when the number of cycles reaches 1000, the safety of the battery will deteriorate sharply.
Overall, through statistical analysis of the safety of a large number of samples under different cycles of puncture, heating, and overcharging, it was found that the safety evolution of some lithium-ion power batteries shows a clear pattern, that is, the safety will suddenly deteriorate after reaching a certain aging state. At the same time, a small number of samples exhibit specificity, and their safety does not significantly deteriorate with the increase of cycle times. Therefore, it is necessary to conduct evaluations for specific objects with specific material systems and structural designs, in order to provide necessary guidance for the design of battery management systems and safety protection measures throughout the entire life cycle.
2. Safety testing of power battery system
2.1. Test standard
The purpose of safety testing is to verify the safety of the power battery system in cases of abuse, and most importantly, to verify the ability of the power battery system to protect passengers in dangerous situations. This mainly includes mechanical and environmental simulation of different conditions, and to verify the reliability of the battery system in vibration, mechanical simulation of collisions, falls, squeezing, and other situations
Environmental safety testing is the process of simulating different environmental conditions to verify the safety of battery systems in environments such as high temperature, low temperature, high humidity, sudden temperature changes, salt spray, fire, and water immersion.
Protection reliability is to verify the protection function of the battery system by simulating possible unexpected situations that may occur during vehicle use, including overcharging protection, over discharge protection, over temperature protection, over current protection, short circuit protection, and other aspects. In the reliability testing of protection, the battery management system or protective device is the only qualified condition for it to function. Manufacturers can be classified into different levels in terms of protection conditions. Taking overcharging as an example, different levels of voltage thresholds can be specified to correspond to different actions – prompts, alarms, disconnecting relays, etc.
2.2. Thermal diffusion test
For battery system safety testing, a more important aspect is the testing of thermal diffusion. Its main principle is to ensure the safe escape of vehicle drivers and passengers, and to verify how to ensure the personal safety of passengers in the vehicle when the power battery system experiences thermal runaway.
The focus of research on the thermal diffusion safety of power battery systems mainly includes the selection of thermal runaway triggering methods for thermal runaway triggering objects (including the equivalence of different triggering methods and the comparability of test results for different objects using the same triggering method) and the determination of judgment conditions. At the same time, focus on the characteristics and propagation mechanisms of thermal diffusion behavior in power battery systems, in order to provide experimental data and technical support for the safety design of power battery systems.
At the level of vehicle application, it is also necessary to examine the safety and judgment conditions under vehicle operating conditions. If the analysis of thermal runaway and thermal diffusion occurs when the battery is installed in a bus, the experimental results show that after about 5 minutes of thermal runaway, the fire begins to spread and enter the car. At the same time, in order to determine the escape time of sudden battery thermal runaway when fully loaded, personnel of different age groups were selected as the test subjects for personnel escape tests, with the longest duration being 51 seconds. Based on the time it takes for the vehicle to detect an alarm and come to an emergency stop, along with a certain safety threshold time, a preliminary 5-minute escape time for personnel has been determined, which serves as the minimum requirement for evacuation of the entire vehicle.
3. Conclusion
This article summarizes and analyzes the current testing standard system and evaluation methods for the safety of power batteries. At the level of battery cells, the characterization methods of thermal stability and the current status and trends of the testing and evaluation standard system for triggering safety were mainly analyzed. At the system level, the focus was on discussing the standard system for battery system safety testing and the evaluation method for thermal diffusion testing.
With the promotion and application of new energy vehicles and the increasing number of vehicles, many vehicles have been driving for several years or tens of thousands of kilometers, and there have been some safety accidents of electric vehicles in recent times. Mastering the evolution law of the safety of power batteries throughout their entire life cycle, as well as effectively detecting and managing the safety of batteries throughout their entire life cycle, is of great significance for the cascading utilization of electric vehicle safety batteries. Specifically, establishing a testing and evaluation system for power batteries from individual to system, covering the entire life cycle, and further considering the actual vehicle application conditions, in order to form a comprehensive testing and evaluation system for the safety of power batteries, will be beneficial for upgrading the safety level of the power battery industry.
