Safety requirements for power batteries used in electric vehicles

On May 27th, in accordance with the Standardization Law of the People's Republic of China and the Management Measures for Compulsory National Standards, the Ministry of Industry and Information Technology publicly solicited opinions on the mandatory national standard "Safety Requirements for Power Batteries for Electric Vehicles" (hereinafter referred to as the "Draft for Soliciting Opinions"). The deadline for soliciting opinions is July 27th, 2024.

GB 38031-2020 "Safety Requirements for Power Batteries for Electric Vehicles" is one of the first mandatory national standards in the field of electric vehicles in China. Since its release in May 2020, it has played an important role in regulating product production, guiding technological progress, and supporting government management. Starting from practical application scenarios, GB 38031-2020 has strengthened the safety requirements for battery packs and systems, and proposed thermal diffusion safety requirements, which has increased the attention of enterprises to the dangers caused by thermal runaway of battery cells and played a positive role in reducing product thermal runaway accidents. With the rapid increase in the number of new energy vehicles, electric vehicle fires still occur from time to time. Through summarizing the experience of electric vehicle safety accidents in recent years, the industry has further understood the failure mechanism of power batteries in practical application scenarios. Based on this, it is necessary to revise and improve GB 38031 "Safety Requirements for Power Batteries for Electric Vehicles", further enhance safety requirements, strengthen the safety bottom line of power batteries, and maintain the safety of consumer life and property.

The draft for soliciting opinions shows that the new national standard will replace GB 38031-2020 "Safety Requirements for Power Batteries for Electric Vehicles". The main technical changes are as follows:

(1) Scope

This standard specifies the safety requirements and test methods for power batteries used in electric vehicles, and the scope of application should also be clearly defined as power batteries, excluding batteries that do not provide power for electric vehicles, such as 12V low-voltage auxiliary power sources. In addition, considering the development of industry technology, the standard scope should be able to cover new types of power batteries such as sodium ion batteries and lithium metal batteries. Therefore, the original text should be changed from "This standard applies to rechargeable energy storage devices such as lithium-ion batteries and nickel hydrogen batteries for electric vehicles" to "This document applies to power batteries for electric vehicles".

(2) Requirements for abnormal termination conditions

In the high-altitude safety requirements (5.2.10) and test methods (8.2.10), in order to protect the safety of test operators and laboratories, it is stipulated that manufacturers need to provide abnormal termination conditions, and it is required that abnormal termination conditions cannot be triggered. In order to maintain consistency among the test items, the same requirements were applied to environmental safety tests such as wet heat cycle (5.2.5, 8.2.5), temperature shock (5.2.8, 8.2.8), and salt spray (5.2.9, 8.2.9).

(3) Temperature shock test

In the temperature shock test (8.2.8), it is not specified whether to start with low temperature or high temperature, and the process cannot be unified during actual testing. Therefore, a temperature shock test temperature schematic diagram has been added for R&D and testing personnel to execute.

(4) Salt spray test

In the salt spray test (8.2.9), the testing method in the original standard referred to the testing method in 5.5.2 of GB/T 28046.4-2011, which stipulated the low-voltage power on monitoring between the 4th and 5th hours of a cycle. However, the focus of the salt spray test in GB/T 28046.4-2011 is to examine the functional status of the device/system under the specified working mode between the 4th and 5th hours, while the focus of GB 38031 is to examine the safety status of the product after testing. Therefore, the low-voltage power on monitoring between the 4th and 5th hours has no substantive significance. After discussion by the working group, this condition has been decided to be deleted.

(5) Battery system protection test

The five protection testing methods in the original standard have been converted from UN GTR 20. The safety requirements stipulate that the insulation resistance after the test should not be less than 100 Ω/V. However, in ISO 6469-1: 2019, it is stipulated that if the battery system includes AC circuits and does not have additional AC protection in accordance with ISO 6469-3, the insulation resistance should not be less than 500 Ω/V; GB 18384-2020 also stipulates that the insulation resistance of DC circuits should not be less than 100 Ω/V, and that of AC circuits should not be less than 500 Ω/V. Therefore, the requirement that "if there is an AC circuit, the insulation resistance should not be less than 500 Ω/V" should be added to the protection type test. In addition, in the over temperature protection (8.2.11), the test object SOC is not specified. Therefore, before the test, the sample SOC is adjusted by default according to the highest working state of charge specified in 6.1.10. However, due to the requirement in the over temperature protection test conditions that the temperature of the test object should be raised as quickly as possible through continuous charging and discharging, the necessity of SOC adjustment before the test is relatively low. After discussion and confirmation by the working group, the SOC of the over temperature protection test object is not limited, as long as it meets the normal working range.

(6) Battery pack or system extrusion test

For battery packs or systems installed in carriages, such as HEV batteries, the structural strength of the vehicle body can to some extent protect the battery pack or system from collisions or reduce the impact on the battery pack or system. In EVS GTR and UN R100, it is specified that battery packs or vehicles can be selected for testing. Therefore, in section 8.4.2.1 of the test object, it has been added that "for battery packs or systems installed in carriages, testing with vehicle structural components is allowed". In addition, for the case of testing with body structure, due to the irregularity of the body structure, it is difficult to use 30% deformation as the cutoff condition for testing execution. Therefore, for the case of squeezing with body structure components, it is clear that the extrusion force should reach 100 kN as the cutoff condition. In addition, regarding the statement in the original standard that the current displacement should be maintained for 10 minutes after reaching the cut-off condition, it was not specified whether to ensure force or displacement. After discussion by the working group, it was confirmed that the current displacement should be maintained for 10 minutes, and the hierarchical expression of the battery cell should also be kept synchronized.

(7) External fire test

The original standard defined direct combustion for 70s and indirect combustion for 60s. However, during the testing process, there was a misunderstanding of the timing for the start and end of combustion. Some companies believed that the timing should start/end when the sample first came into contact/left the flame, resulting in the actual combustion time not meeting the standard requirements. In response to this, it is added in section 8.2.7.1.3 that "the combustion time should start or end when both the test object and the oil pan are stationary.".

(8) Thermal diffusion analysis and verification, including safety requirements, triggering methods, judgment logic, etc. (Details can be found in the attachment)

(9) Safe after charging individual batteries

Since 2022, the research group has conducted multiple thematic discussions on post charging safety. For overcharging and overcurrent caused by the failure of charging stations and battery management systems, individual overcharging, system overcharging protection, and overcurrent protection tests have been conducted in the current GB 38031. There is some industry disagreement on whether long-term cycling leads to additional safety risks for batteries. Some companies have provided test data on the narrowing of battery safety boundaries and reduced safety after fast charging cycles. Some companies believe that the design of batteries has already taken into account the charging and discharging conditions in the usage scenario, and release the product application range after reserving sufficient protection intervals. Under aging conditions, some active substances in the battery are consumed, and the battery energy decreases. Therefore, it is believed that the battery after charging cycle has no additional safety risks compared to conventional batteries. The drafting group has comprehensively considered the potential risks of long-term fast charging for power batteries, and based on existing research data, has drafted a draft proposal for soliciting opinions:

Based on the requirements of the Energy Conservation and New Energy Vehicle Technology Roadmap 2.0 for the charging rate of fast charging power batteries, the test object is set as battery cells with a charging time of less than 15 minutes for 20% SOC to 80% SOC (excluding battery cells used for non externally rechargeable hybrid electric vehicles); The number of cycles is 120000 kilometers, and the corresponding mileage for fast charging is 400 kilometers, set to 300 times. Considering the intensification of internal side reactions and even lithium deposition issues after fast charging cycles, there is local performance degradation. This local performance degradation is manifested as significant temperature rise in the high impedance part (i.e. the area with severe side reactions) during high current discharge. During external short circuit testing, the internal side reaction aggregation area or lithium evolution area of the battery during long-term fast charging cycles will intensify the reaction rate under high temperature rise, leading to battery ignition. Therefore, external short circuit testing is set after the fast charging cycle, and industry opinions are further collected through the solicitation stage, and verification testing is carried out to further confirm or optimize the solicitation draft plan.

(10) Battery pack or system bottom protection

In recent years, the proportion of power battery fires caused by bottom impacts of new energy vehicles has been relatively high, and there are no testing items for this scenario in the current standards. At present, there are two widely recognized bottom collision conditions in the industry, namely bottom scraping condition (X-direction) and bottom support condition (Z-direction). The bottom scraping condition corresponds to the scene where the vehicle collides with obstacles from the front, while the bottom supporting condition mainly corresponds to the scene where foreign objects such as flying stones and ground obstacles collide from below the vehicle. Most enterprises agree on the necessity of the above two working condition tests, and the main difference in the early stage is whether the test object is the battery or the entire vehicle. The whole vehicle testing is more in line with practical scenarios, but the testing cost and cycle are longer. Battery testing is more convenient, but it is difficult to reflect the actual chassis layout, mounting stiffness, and mass distribution characteristics of the entire vehicle. After multiple discussions in the industry, it is unanimously agreed that a battery bottom collision test plan needs to be developed based on the actual working conditions of the vehicle. The results of the bottom scraping condition test are strongly related to factors such as the vehicle bottom guard, suspension, ground clearance, and collision beam. It is recommended to implement it through vehicle level testing and further study it in the subsequent revision of relevant vehicle standards. The bottom impact test based on the bottom support condition allows manufacturers to choose to implement it at the battery pack or vehicle level.

In the preliminary discussion of the bottom impact test plan, there were certain differences in the industry regarding the assessment objectives (safety testing/reliability testing), impact energy, etc. The drafting group has drafted a draft proposal for soliciting opinions based on the collected data on actual vehicles and battery packs, as well as the verification and testing situation, in response to the previous industry discussions. The plan is to further collect actual vehicle data and industry opinions during the solicitation stage, and conduct verification and testing to further confirm or optimize the draft proposal for soliciting opinions.