About EV Lithium Battery Testing

 1. Basic Introduction

The research and application of power batteries for electric vehicles have gone through the development process from aluminum acid batteries, nickel hydrogen batteries to lithium batteries, and lithium batteries have also gone through the development process from metallic lithium to lithium compounds and now to lithium-ion batteries.

 

Lithium ion power batteries are mainly composed of battery cells, battery packs, or battery systems. Due to the activity of lithium ions on metal surfaces in lithium-ion batteries, there may be safety and stability issues in the development, use, and testing of lithium-ion batteries, which need to be prevented and eliminated.

 

2 Dangerous reasons

The battery cells of lithium-ion batteries are the main components that cause fires and explosions during use and testing. The reasons for their hazards can be summarized as follows:

 

2.1 Short circuit

(1) External short circuit

The short circuit between the two poles of a battery is mainly caused by external structural faults or damage, usually due to mechanical or physical reasons.

 

(2) Internal short circuit

Except for physical short circuits between battery terminals (plates) inside the battery body. Lithium ion batteries may also be affected by the rupture of polymer separators, causing short circuits. Under laboratory conditions, internal short circuits typically occur in various overcharge tests, environmental tests, and life cycle tests.

 

The mainstream separator thickness used in power lithium-ion batteries is generally above 30 μ m, which is 16-20 μ m higher than that of general lithium-ion batteries. After being subjected to mechanical forces such as external forces and thermal deformation, it will directly cause internal short circuits. In addition, overheating can also damage the diaphragm and cause internal short circuits.

 

Meanwhile, defects in the diaphragm raw materials may cause minor damage to the diaphragm during the production process. These reasons may all lead to local temperature increase in power lithium-ion batteries under small short circuit conditions. Moreover, these small short circuits will gradually expand during use or experimental loading, forming influential internal short circuits.

 

2.2 Temperature rise

Battery temperature rise refers to the difference between the internal temperature of the battery and the ambient temperature. The above safety factors are accompanied by a temperature rise process. Therefore, the power lithium-ion battery separator used for electric road vehicles should have physical properties of automatic shutdown protection to improve the safety performance of electric vehicle use.

 

2.3 Direct mechanical damage

During mechanical environment testing and abnormal operation bath testing, it can directly cause damage to the battery cells or result in performance degradation after damaging the internal structure of the battery cells. Direct combustion or explosion has also been recorded in mechanical impact testing. After the vibration test, the internal structure of the battery cells is damaged, and there is a possibility of delayed ignition or explosion hazards.

 

2.4 Temperature and humidity environment

Temperature and humidity environment tests are important testing conditions. Due to conducting experiments in the laboratory, safety issues and secondary disasters have become important factors that need to be prevented.

 

3. Key points of testing

(1) In laboratory experiments, fire and explosion are two situations that require careful prevention. The results of the first few incidents may cause pollution or secondary hazards, and may not necessarily result in destructive direct hazards, but they should also be recorded in graphic and textual form as much as possible.

 

(2) Overcharging generated in various experiments is one of the main causes of laboratory safety issues. The sample battery generates heat due to various overcharging reasons, causing the diaphragm to melt, resulting in short circuit heating and electrolyte vaporization and explosion.

Obviously, the temperature and heating rate during this period are measurable factors.

 

The measurement of temperature elements will be repeated at all stages of the experiment. As long as appropriate temperature sampling, measurement, recording, and comparison are arranged at the experimental nodes, the degree of sample degradation can be detected and evaluated early, and corresponding preventive measures can be taken. Appropriate preventive measures should be taken for the misuse of temperature testing.

 

(3) Mechanical environmental testing. Mechanical shock and vibration are the effects of driving environmental conditions on battery samples during simulated loading and actual use, and the rigor of the test is not high.

 

Even if the adverse effects are caused by direct detection, the reaction is generally mild or delayed. However, mechanical testing that cannot function properly (abuse) may cause serious reactions and directly affect the laboratory environment. After the mechanical test of the sample, temperature sampling, measurement, and inspection should be continuously arranged to predict and prevent the occurrence of dangerous situations, in order to cope with potential hysteresis reactions.

 

(4) There are also tests with severe temperature reactions and short-circuit tests with abnormal operations (abuse).

 

(5) When the temperature of the battery sample rises sharply, attention should be paid to whether all external controls of the battery pack/system have failed at this time.