A brief discussion on the safety test of lithium batteries. Is the battery puncture test necessary?

What is the focus of public attention on new energy vehicles? There is no doubt that it is safety. Since the popularization of new energy vehicles, the word "spontaneous combustion" seems to have never stopped on the Internet. Battery safety issues have become a pain point for everyone, and even many consumers who are interested in new energy vehicles are discouraged. It is said that the battery puncture test is a very strict test in battery safety tests. Although the old national standard GB/T 31486-2015 "Electrical Performance Requirements and Test Methods for Power Batteries for Electric Vehicles" has requirements, after the steel needle vertically pierces the single battery cell, it must stay in the battery cell for observation for 1 hour, and it will pass the test if there is no fire or explosion. However, in 2020, the new national standard cancelled the single battery cell puncture test; the newly promulgated GB38031-2020 "Safety Requirements for Power Batteries for Electric Vehicles" stipulates that the battery pack or system should provide a thermal event alarm signal 5 minutes before the passenger compartment is in danger due to thermal runaway of a single battery causing heat diffusion (serving the thermal event alarm of the whole vehicle to remind the evacuation of passengers). If the heat diffusion does not cause danger to the vehicle occupants, the requirement is considered to be met. So is the battery puncture test still necessary? Especially after BYD's blade battery passed the puncture test, this topic has caused a lot of controversy.

Battery protection and safety measures are very necessary, but over-reliance on external protection measures will not improve the safety performance of the battery itself. And protection measures are not foolproof and there may be a risk of failure. So when the protection measures fail, how can safety performance be guaranteed? Therefore, only when the foundation of the battery is safe, coupled with protection and safety measures, can the risk of battery spontaneous combustion be eliminated. BYD always adheres to the principle of user safety first and insists on using the single cell needle penetration test as the company's standard requirement for power batteries. Because of this, many well-known battery companies have also conducted public battery needle penetration tests and discussions:

The puncture test of ternary lithium battery, ordinary lithium iron phosphate battery and blade battery showed obvious differences in test results. The puncture test removes all the promotional packaging and exposes the true safety of the battery to consumers.

After the steel needle was inserted, the shell of the ternary lithium battery expanded rapidly, then caught fire and exploded instantly, with the surface temperature exceeding 500°C. Although the ordinary lithium iron phosphate battery did not catch fire or explode, it still smoked, with a surface temperature of 200°C-400°C. The best performance was the blade battery, which did not catch fire or smoke, and the surface temperature was only 30-60°C.

In fact, spontaneous combustion of batteries is essentially thermal runaway of the battery. Thermal runaway means that the battery temperature rises uncontrollably for some reason, and then exceeds the critical point, causing fire or even explosion. That is, the battery is in an overheated state and the battery temperature is out of control. There are several important reasons for thermal runaway:

When an electric vehicle is driving, a collision may cause foreign objects to invade the power battery, causing an internal short circuit in the battery. Heat accumulation at the location of the internal short circuit will bring the risk of spontaneous combustion.

During the charging process of electric vehicles, the internal temperature of the battery may become too high due to overcharging or failure of the cooling system, which may bring the risk of spontaneous combustion.

The puncture test uses a tungsten steel needle to pierce the battery perpendicular to the battery. The energy of the entire battery will be released in a short time through the puncture point, simulating the internal short circuit of the battery in a bad situation. The blade battery passed the puncture test safely, demonstrating its performance in the bad situation of internal short circuit of the battery. Regarding the safety issues of electric vehicle batteries that consumers are concerned about, the blade battery has delivered a perfect answer, and has become famous and recognized by consumers; but it does not mean that other batteries are unsafe.

The safety of power batteries is mainly reflected in the system thermal safety, mechanical safety, electrical safety and functional safety, not just the safety of the battery cells. In reality, the battery cells will not be punctured by needles. The "blunt puncture" and squeezing of the bottom of the battery pack are more realistic testing methods.

In general, lithium iron phosphate batteries are more likely to pass the puncture test not because of advanced technology, but because the material itself has good thermal stability; and ternary lithium batteries have high energy density, which does pose challenges to puncture, but it is not impossible to complete. In other words, batteries that pass the puncture test are not necessarily safe, and not doing a puncture test does not mean that the battery is unsafe. The core purpose of the puncture test is to short-circuit the battery in order to observe the thermal runaway of the battery and formulate better safety protection strategies. So is it necessary to conduct a puncture test on the battery? "Although the new standard has deleted the puncture test for battery cells, it does not mean that the battery does not need to be punctured. It is worth noting that the "Safety Requirements for Power Batteries for Electric Vehicles" gives two recommended methods for triggering thermal runaway for thermal diffusion tests. For battery cells in battery packs or systems, one is heating to trigger thermal runaway, and the other is puncture to trigger thermal runaway. Manufacturers can choose one of them, or they can choose other methods to trigger thermal runaway.