2.1 Comparison of Structural Safety Requirements
UL1973 puts forward more detailed and specific requirements for product shell. For example, there are specific requirements for non-metallic shell, metal shell, wall hanging support handle, cable and terminal. In these aspects, IEC and GB standards have no detailed specification requirements, but only require that the shell shall not be broken in relevant experiments, resulting in internal material exposure or liquid leakage. UL standard also regulates the reliable protective grounding of the shell, while IEC and GB have no protective grounding specifications for the battery system, which are implemented according to relevant electrical standards, which is easy to be neglected in the process of inspection.
2.2 Comparison of Safety Requirements for Battery
The battery body safety here refers to the test requirements directly related to the battery, such as overcharge and discharge test. The national standard specifies the battery test requirements and test methods according to the three levels of battery unit, battery module and battery cluster (system). UL and IEC indicate the test object in each test. In terms of battery body safety, the difference between the three is not too great. In some places, the national standard requirements are even more stringent. For example, the internal resistance of the line with external short circuit is required to be less than 5m Ω, and the maximum temperature in the temperature rise test is required to reach 130 ℃ and keep it for 30min. An important difference is that the UL standard is more strict in judging the test qualification. It not only requires that there should be no fire, explosion and liquid leakage, but also stipulates that there should be no accumulation of toxic gas, combustible gas, exposed electric shock hazard and loss of protection control after the test.
The safety requirements related to temperature differ greatly from each standard. UL’s charge discharge limit temperature test: under the maximum charge discharge condition, it shall be kept within the specified limit working conditions. The key safety devices sensitive to temperature shall be kept within their rated temperature range. At the same time, the accessible surface temperature shall not exceed the safety limit. IEC puts forward requirements from the aspect of heat abuse, and requires BMS protection for overheat charge and discharge. GB standard is similar to IEC, but the test temperature is increased to 130 ℃.
Considering the reality, UL’s battery charge discharge limit temperature test requirements are closer to the actual engineering application. In addition, UL also stipulates the unbalanced charging test of battery modules, which is not specified in IEC and GB. In terms of drop test, UL and IEC carry out grading test according to the weight of the test object, while GB standard uniformly adopts the test method of free falling from the height of 1.2m to the cement surface with the positive or negative pole facing down. This one size fits all approach is easy to implement, but whether it meets the needs of product development remains to be discussed. The three standards all have requirements for thermal runaway diffusion, among which IEC standard can select the short circuit test in the core as the alternative test.
In addition, in the United States, UL9540A is used to evaluate the characteristics of thermal runaway of battery energy storage system and select appropriate fire and explosion protection mechanism through test data, which is the standard of some test methods. The purpose is to help the supplier clarify the isolation requirements between the system and the wall, the heating capacity of the system, combustible parts, the type of gas generated by combustion and the selection of fire extinguishers. IEC and GB have not formulated similar standards.
2.3 Comparison of Environmental Impact Requirements
UL standard specifies environmental tests such as salt spray test, moisture-proof test and external fire test, while GB standard specifies salt spray test, high temperature and high humidity test. It should be noted that these two environmental tests are determined according to the actual application environment of the product, which is not a necessary project. IEC62619 has no provisions on environmental test, which may be due to the fact that the standard is a general standard, while IEC62485-5 has not been released. It is unknown whether there is any content in this regard. In terms of electromagnetic compatibility, the national standard specifies the test items and required levels in BMS technical specification GB / T34131. In UL and IEC, “functional safety analysis shall be carried out for control systems that play a key role in safety and depend on electricity, circuit and software” includes tests on BMS. One of the reference standards IEC60730-1 appendix h includes electromagnetic compatibility test items and test methods. The number of items is more than the national standard. The test method emphasizes testing various modes of controller (BMS belongs to the category of electronic controller), The impact of inspection on the safety of controlled equipment.
2.4 Comparison of System Requirements
In terms of system safety analysis, both UL and IEC require functional safety assessment of electronic circuit software. UL also requires risk analysis of the system and provide FMEA or fault tree analysis report, which is conducive to the investigation of various risks of the system, such as electric shock risk, fire risk, mechanical risk, etc., and control the risk at an appropriate low probability. The GB standard does not specify in this regard. In terms of system component safety, UL standard indicates the standard specifications to be met for key components, while IEC and GB standards have no direct requirements.
In this paper, the characteristics of the safety standards of lithium-ion batteries in the main energy storage systems at home and abroad are analyzed in detail, and the similarities, differences, advantages and disadvantages of the relevant safety standards are summarized and compared. Generally speaking, the IEC standard system is relatively perfect, but the preparation progress is relatively slow. UL standard specifications are comprehensive and detailed, and the compliance of inspection methods and inspection equipment needs to be verified during implementation. The domestic standards are guided by the national standards. The standards of various industry groups are prepared in combination with the national standards and IEC standards. There are great differences in organizational structure and more difficulties in implementation. Therefore, the manufacturing of energy storage system in the domestic market is mainly implemented with reference to the national standards. For China’s lithium battery energy storage system, there are the following problems or suggestions.
(1) The system level safety assessment in GB is not perfect, which is mainly reflected in the lack of system risk identification and assessment requirements, the lack of BMS functional safety assessment, and the failure of BMS to conduct overall assessment in combination with the system.
(2) There is a lack of detailed specifications or clear guidelines on the requirements of housing, protective grounding, terminals and cables, documentation and other key components.
(3) It is suggested to separate the safety related standards of energy storage system from the functional performance standards, carry out relevant certification according to the safety standards, and make mandatory provisions on product access, which is conducive to the fair competition and development of participants in China’s energy storage industry.