The investigation of design parameters is very helpful for optimizing the capacity of an electrochemical cell, which can be done by both experimental and numerical methods. In this study, a lead-acid battery has been simulated numerically using the CFD commercial software package FLUENT. The governing equations, including conservation of charge in solid and liquid phases and conservation of species, are solved by developing several user defined functions (UDF). The effect of some basic parameters such as electrode porosity, discharge current density, and width of the electrodes and separator on the cell voltage behavior of a lead-acid battery is investigated. It has been shown that increasing the width and porosity of separator has both positive and negative effects on the performance parameters of battery. Furthermore, a thicker $PbO_2$ electrode has a more pronounced effect than a thicker Pb one.