As power battery safety standards are further improved, how will automakers develop battery thermal safety technology?

Battery thermal runaway has become a hot topic, and battery safety has become a research and development focus for automakers.

In recent years, with the rapid development of the new energy vehicle industry, electric vehicles have become increasingly accepted by the public.

However, a problem that cannot be ignored has also arisen - the frequent spontaneous combustion of electric vehicles has attracted widespread attention from all walks of life.

Spontaneous combustion not only causes property losses to car owners, but also poses a serious threat to public safety.

Against this background, battery safety issues have become the key to the development of the electric vehicle industry.

In this regard, the national standard safety requirements for new energy batteries are becoming increasingly higher.

In 2020, my country issued the first three mandatory national standards for new energy vehicles, including GB 38031-2020 "Safety Requirements for Power Batteries for Electric Vehicles", GB 18384-2020 "Safety Requirements for Electric Vehicles", and GB 38032-2020 "Safety Requirements for Electric Buses".

Among them, GB 38031-2020 "Safety Requirements for Power Batteries for Electric Vehicles" specifically stipulates the safety requirements and test methods for power batteries (including cells, battery packs or systems) for electric vehicles.

Specifically, the regulations have deleted the battery cell puncture test and only used it as a trigger condition for thermal runaway of the cell. A battery system thermal diffusion test has been added, requiring that after thermal runaway of the battery cell occurs, the battery system will not catch fire or explode within 5 minutes, leaving safe escape time for the occupants.

The standard was implemented on January 1, 2021, marking a shift in the understanding of vehicle safety from single-unit safety to system safety. In other words, whether the single-unit puncture passes is no longer important. The key lies in whether the spread of the single-unit thermal runaway can be controlled in time. The concepts of single-unit thermal runaway and system thermal diffusion are distinguished.

▲(GB38031-2020) Safety test items, methods, requirements (part)

On May 27, 2024, in order to further improve the safety of power batteries, the Ministry of Industry and Information Technology issued GB 38031 "Safety Requirements for Power Batteries for Electric Vehicles (Draft for Comments)".

The "Draft for Comments" further tightens the requirements for thermal diffusion testing of power batteries based on existing standards, mainly including improving thermal diffusion requirements, adding thermal event warning requirements, and improving thermal diffusion testing methods.

The Draft for Comments emphasizes the protection capability of the power battery system in the case of thermal runaway of battery cells, and raises the thermal diffusion requirement from "alarm 5 minutes before fire or explosion" to "no fire or explosion". In addition, internal heating trigger is added as one of the recommended methods for thermal runaway triggers.

▲The Draft for Comments on the Determination Process of Thermal Diffusion Test

According to industry standard definitions, thermal runaway refers to the phenomenon in which the battery temperature rises uncontrollably due to a chain reaction of heat release from battery cells.

Thermal diffusion refers to the phenomenon that thermal runaway of one battery cell in a battery pack or system causes thermal runaway of the remaining battery cells one after another.

In response to these new national standards, BYD also has its own unique thinking and research on battery thermal safety.

1. Blade battery needle puncture test sparked heated discussion and detailed BYD technical solution

In early 2020, BYD launched its blade battery and announced its battery puncture test, sparking heated discussions in the industry. According to the results of the single-cell puncture test, the ternary lithium battery burned violently, with the surface temperature exceeding 500°C;

BYD's lithium iron phosphate blade battery has no open flame or smoke, and its surface temperature is 30℃-60℃. It is said that the core purpose of BYD's blade battery puncture test is to cause a short circuit in the battery in order to observe the thermal runaway of the battery and formulate a better safety protection strategy.

Afterwards, according to BYD's public thermal diffusion test data, the battery in the middle was forced to thermally run away, with the highest temperature being about 350°C. At this time, the temperature of the battery in the adjacent position was only 80°C, and the blade battery only emitted smoke, without catching fire or exploding.

Based on the lithium iron phosphate material system (low heat generation) and the long and thin blade battery design (fast heat dissipation), the blade battery already had a good ability to control heat diffusion when it was first released.

▲BYD releases acupuncture test comparison

Blade batteries use lithium iron phosphate material, and theoretically their low-temperature performance is relatively weak.

In order to improve the low-temperature performance of blade batteries, many improvements were first made in the thermal management system. For example, thermal insulation materials were added to the structural design of blade batteries, and a layer of thermal insulation cotton was arranged on the outside of the thermal management system, forming a complete thermal management system for blade batteries.

At the same time, BYD pioneered the intelligent dual-circulation battery direct cooling and heating technology to improve the performance of blade batteries in winter.

▲BYD Blade Battery Thermal Management Solution

2. BYD improves the thermal insulation level of battery cells to further improve the overall safety of batteries

In February 2023, BYD focused on the patent layout of super heat-absorbing materials between battery cells, such as "Cooling component and battery pack and vehicle with it" (application date: 2023.02.28), "Heat-absorbing component, battery module, battery pack and vehicle" (application date: 2023.02.28), "Package, phase change components set between battery cells" (application date: 2023.02.27) and other patents, introducing high heat-absorbing materials and phase change components into the battery system, suppressing heat diffusion through a combination of heat absorption + insulation, further reducing the risk of heat diffusion in the power battery pack, and improving the safety and reliability of the power battery pack.

Traditional phase change materials are solid-liquid phase change. BYD has taken a different approach this time and adopted liquid-gas phase change, which has a heat absorption capacity ten times higher than solid-liquid phase change. Therefore, it can completely solve the extreme heat diffusion problem caused by the simultaneous loss of control of multiple battery cells through heat absorption.

According to patent information, the phase change filling material is mainly a lyophilic polymer material and a liquid phase change medium. The cooling component composed of the skeleton has the characteristics of not being easy to deform, high heat dissipation efficiency, fast heat dissipation response and good exhaust effect.

The key point of this technology is to use high heat absorption materials and utilize the latent heat of vaporization of the material to greatly enhance the heat absorption capacity between battery cells. It can completely absorb all the heat transferred from one battery cell to the next due to thermal runaway, thereby making it possible to solve the heat diffusion problem caused by thermal runaway of multiple batteries and the destruction of multiple batteries under extreme working conditions. The technical solution can cover all battery cell sizes and different battery cell systems.

The blade battery has become famous for passing the needle penetration test. The monomer itself has good thermal stability, coupled with the added effect of high heat absorption materials between battery cells, it is believed that the thermal safety capability of the blade battery system is sufficient to meet the "no fire, no explosion" requirement of the draft for comments. The key lies in the cost of the high heat absorption material.

▲Filling phase change materials between battery cells (patent screenshot)

Under the guidance of the "Safety Requirements for Power Batteries for Electric Vehicles (Draft for Comments)", "no fire, no explosion" will become one of the basic requirements for power battery systems.

The "no fire, no explosion" of the entire vehicle or battery system is divided into two parts: one is to control the thermal runaway at the battery cell level, and the other is to control the thermal runaway of single or multiple battery cells without causing heat diffusion at the system level.

Conclusion: Battery safety becomes the focus of car companies' research and development

In response to the requirements of the new national standard "Draft for Comments", BYD continues to "increase its investment" in battery safety.

Through the overall design of blade batteries, dual-circulation battery direct cooling and heating technology, and super heat-absorbing materials between battery cells, the overall safety performance of BYD batteries has been improved in all dimensions.

As new energy vehicles are developing rapidly, battery safety is of paramount importance.

Only when car companies attach enough importance to it and "stick to" safety can they be responsible to consumers to the greatest extent.