Lithium-ion battery is an energy storage component with high power density. It is widely used in various spacecraft. Lithium-ion battery charging strategy is mature in present applications. But there is no accurate theoretical model and mathematical tools to describe and analyze the curves of certain-voltage (CV) stage, which causes many errors when evaluating the battery condition of its whole life stage during the design process of power system on the spacecraft. In this paper, the principle of automatic control theory is used in modeling and analyzing the CV charging process of the battery, and the expression of the battery charging current with time is given to obtain an accurate method to describe the charging process of the battery.