In this research, we study the relation between charge voltage and charge time of a Lithium-Ion battery. It combines an Exponential and a Gauss function to describe the charge behavior of a Lithium-Ion battery. The exponential is used to characterize the earlier part of the charge time, and the Gaussian is for the later part. From our simulation, the differences between our model and actual observation are less than 0.3% or 0.03V. Therefore, we believe our model is effective to describe the charge behavior. It provides a reliable way of predicting the charge voltage of a battery at any given time. In the future, we should study the effect of temperature on the charge of Li-ion batteries for better understanding their charge behavior.