Lead-acid batteries have the advantages of wide temperature adaptability, large discharge power, and high safety factor. It is still widely used in electrochemical energy storage systems. In order to ensure the application of batteries under extreme working conditions, it is necessary to explore the degradation mechanism. In this study, the experimental battery is the same type of 2 V-500 Ah lead-acid battery produced by different manufacturers. First, the three batteries were subjected to the same high temperature and high current cycle thermal shock test (50 °C, 0.2 C current) combined with quantitative analysis of plate active material and microscopic morphology observation. In addition, numerical studies are used to simulate the distribution of electrical parameters on the positive plate and grid. The above three parts are combined to study the causes of accelerated battery decay under high temperature and high current conditions. The results showed that the extreme conditions aggravated the non-uniformity of the potential distribution of the positive plate and the grid, which increased by 10.62% and 51.59%, respectively. The battery with higher remaining capacity has more α-PbO2 in the active material, and has a considerable amount of β-PbO2. The battery with the smallest remaining capacity has the largest volume of active material. The volume of the material affects the electrochemical reaction surface area. The larger the volume of the material, the higher the resistance of that part, which will lead to an increase in the overall impedance of the battery.