Power lithium-ion batteries must pass a series of safety tests before they can be used in electric vehicles. These safety tests are to understand and identify the potential weakness and vulnerability of the battery under abnormal conditions, and determine the performance of the battery under severe conditions. Due to the limitation of space, the author only analyzes and summarizes the international standards and regulations for electrical safety and harsh environmental testing of lithium-ion batteries for electric vehicles, and discusses the problems existing in the standards.
1. Overview of domestic and foreign standards and regulations
Important international standards related to the safety and harsh environmental testing of lithium ion battery gas for electric vehicle power include:
IEC62660-2:2011 lithium ion power battery Part 2: reliability and abuse test;
ISO6469-1:2019 safety requirements for electrically driven road vehicles, Part 1: rechargeable energy storage system; (iso12405-3:2014 test specification for lithium ion traction battery pack and system of electric road vehicles, Part 3: safety performance requirements has been withdrawn and replaced by iso6469-1:2019);
SAEJ2464:2009 safety and abuse test of rechargeable energy storage system for electric and hybrid electric vehicles;
SAEJ2929:2013 safety standard for electric and hybrid lithium ion battery systems – lithium based rechargeable battery.
EU member states adopt the technical regulation ECER100.02 “Uniform provisions on vehicle certification with regard to special requirements for electric transmission system” issued by the United Nations Economic Commission for Europe on July 15, 2013 as a mandatory standard.
The first part is applicable to the electric drive system of class m and class n highway vehicles, with a design maximum speed of more than 25km/h and equipped with one or more electric traction motors; The second part applies to the safety requirements of rechargeable energy storage systems (REESS) for category m and category n road vehicles equipped with one or more electric traction motors and not permanently connected to the grid.
2. Electrical safety and severe environmental testing of lithium ion battery for automotive lithium ion power
This paper mainly analyzes and discusses the safety test standards for lithium-ion power lithium-ion batteries as auto parts, and does not involve the test standards and regulations at the vehicle level. Table 1 summarizes the most common test items specified in international and domestic standards and regulations related to lithium-ion power lithium-ion batteries for electric vehicles (this paper only discusses electrical safety performance test and harsh environmental test) . some standards stipulate that in some cases, the test can be carried out according to the agreement between the manufacturer and the customer. The test can be carried out separately at various levels and will be classified with reference to the equipment under test (DUT): battery unit (c), battery module (m), battery pack or system (P) and vehicle (V). The standards and regulations set pass and fail requirements for each test, which will be “no fire” “No explosion”, “no rupture” and “no leakage” are the acceptance criteria of the test, while the passing and failure criteria of fire prevention are only “no explosion”.
2.1 Electrical safety test
2.1.1 External short circuit test
The purpose of the test is to evaluate the safety performance of the DUT in case of external short circuit. This test is used to evaluate the activation status of overcurrent protection equipment or the battery withstand current without reaching dangerous situations (e.g. thermal runaway, explosion, fire) An important risk factor is thermal runaway due to the presence of a large amount of heat, arcing may damage the circuit or reduce the insulation resistance.
During the test, connect the positive and negative poles of the battery to a low resistance element (e.g. 5, 10 or 20m Ω) from the outside, short circuit it from the outside in less than 1s, and maintain it for a specified time (e.g. 10min) or until the overcurrent protection device (if any) is used. Generally, fuses, circuit breakers (passive components) and contactors (active components) Used to prevent overcurrent at the battery module or battery pack level.
The built-in current interruption device or positive thermal coefficient device is used for over-current protection at the battery cell level. If the internal pressure and / or temperature reach the limit, the connection between the internal circuit and its terminal can be disconnected or the passing current can be limited. The time characteristics of these protection devices determine the response time of disconnecting or limiting the current. The higher the current, the faster the interruption can be.
If the current is not high enough (e.g., low SOC) or the current drops rapidly, the current may not be interrupted, but these may lead to danger. Therefore, the standard requires that the short circuit resistance should be minimum in case of external hard short circuit or soft short circuit when the external resistance is equivalent to the internal resistance of the DUT.
As mentioned earlier, the standard or regulation requires a fixed external resistance, which is independent of the size of the DUT. However, the initial short-circuit current is affected by the size of the DUT and its connection type (i.e. parallel, serial or a combination thereof) Therefore, using the same external resistance connection for DUTs with different sizes and connection types may lead to the incompatibility of the initial short-circuit current of each battery cell. Therefore, some standards stipulate that the external resistance must be much less than the direct current impedance of the DUT for hard short circuit. For soft short circuit, the initial short-circuit current is heavy because the external short-circuit resistance is higher than the resistance of the DUT It is controlled by external resistance, so the initial short-circuit current is independent of the size of the battery energy storage system.
Temperature will affect the internal resistance of the battery, i.e. electrochemical reaction and transmission speed; therefore, the higher the initial current, the higher the temperature will lead to high temperature, resulting in more heat. Moreover, the higher the temperature, the closer the DUT temperature is to the temperature of thermal runaway. The standards and regulations in Table 2 do not require short-circuit test at a temperature higher than room temperature. However, it is appropriate to perform short-circuit test at a temperature higher than room temperature Yes, because the car is likely to reach a temperature higher than room temperature when parked outdoors, driving or when the cooling system fails.
Another parameter affecting the test results is the state of charge (SOC). The worst case is achieved at high SOC, because the initial short-circuit current is the largest, which is easy to lead to thermal runaway. Therefore, most standards require testing at 100% of the rated capacity. However, for UN / ECE-R100.02:2013, testing can be carried out at 50% SOC (or higher).