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Abstract: Lithium batteries are widely used in new energy electric vehicles and energy storage due to their superior performance. However, the micro short circuit problem of lithium batteries is a safety hazard related to both vehicle and energy storage battery packs during use. In order to diagnose whether the battery pack has a micro short circuit and determine the individual of the micro short circuit, this paper proposes a method for diagnosing micro short circuits based on the relative charging time changes of the individual. This method takes the voltage curve of the first monomer to reach the charging cutoff voltage as the benchmark at the end of battery pack charging, analyzes the charging time of other monomers to reach the charging cutoff voltage under the condition of being able to continue charging, and characterizes it with relative charging time. Due to the continuous consumption of electrical energy in micro short circuited batteries, their relative charging time increases with the number of charges. Based on this characteristic, the relative charging time of each individual in the battery pack is analyzed, and abnormal detection is performed through a box diagram. Among the abnormal units in the detection results, the unit with the highest number of repeated occurrences is the micro short circuit unit. While analyzing the relative charging time, it is necessary to analyze the impact of DC internal resistance on the diagnostic results, in order to improve the accuracy of the diagnostic results. After comparative analysis, the diagnostic results and actual results are highly consistent. The implementation of this method does not require special testing of the battery pack, and the operation is convenient. It can provide method guidance for battery pack safety detection.

Keywords battery pack; Micro short circuit; Relative charging time; Box diagram

Lithium batteries gradually age during long-term use, and micro short circuits are one of the aging characteristics. If they cannot be detected in a timely manner, they can lead to internal short circuits. The deterioration of internal short circuits is usually accompanied by an increase in self discharge rate and heat, leading to thermal runaway and safety accidents. Internal micro short circuits in lithium batteries are more likely to be triggered, even if not due to manufacturing reasons. Overcharging, discharging, and excessive vibration during use can lead to the formation of lithium dendrites on the negative electrode surface, piercing the separator, and causing micro short circuits inside the lithium battery. Therefore, the diagnosis of micro short circuits in lithium batteries is still an urgent problem to be solved in battery management.

The existing methods for detecting micro short circuits in lithium batteries are important through specific devices and equivalent circuit models. Li Kefeng et al. applied pressure evenly in the direction perpendicular to the battery electrode group using a micro short circuit detection device. The hidden danger of a short circuit occurred when the positive and negative poles were connected under pressure, and then applied a DC voltage to test the insulation resistance between the two poles. If the insulation resistance value of the micro short circuit electrode was lower than the threshold, it indicates the existence of a micro short circuit. However, this method is applicable to individual batteries and requires specific devices, making it difficult to implement. Zheng Yuejiu et al. quantified micro short circuits by calculating the self discharge current between the end of two battery charges, calculated the remaining rechargeable capacity after two charges to obtain the leakage current, and then obtained the self discharge current. Combined with the equivalent circuit model of the micro short circuit unit, calculated the internal short circuit resistance value, and determined whether the unit had micro short circuits based on its threshold. However, this method relies on the setting of the internal short-circuit resistance threshold and is only applicable to the diagnosis of micro short circuits in new battery packs, resulting in significant diagnostic errors for aging battery packs.

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