The production of expanders involves three main processes: grinding, mixing, and sorting. The grinding process creates a uniform mixture from different particle sizes of the components and the optimal particle size of the final mixture is a critical parameter that affects the performance of the NAM. In this study, we investigate the effect of expander particle size on the performance of the NAM and determine the optimum particle size for SLI applications. We prepared six expanders with various particle sizes using a prototype ball mill and evaluated them using a technique based on cyclic voltammetry. For this, we prepare an electrolyte by dissolving 3g of the expanders in 100cc H2SO4(5M) and cycle a lead electrode immersed in the electrolyte within the potential range of -0.325 to -0.725 for 15 cycles. The expander prevented the passivation of the surface in the Pb│PbSO4 electrode, which increases the current passed through the negative plate by approximately 164% upon decreasing the expander material particle size. The expander also impeded the process of PbSO4 reduction in the PbSO4│Pb electrode, leading to a decrease in cathodic Imax compared to the no-expander condition and negatively affecting the charge acceptance and a suitable expander could reduce this effect. We found that decreasing the expander particle size also increases the current during the charging process, resulting in better charge acceptance by about 53.5%. The results showed that the particle size decreased with the continuation of the grinding process, thus significantly improving the charging, and discharging performance. However, this effect was not linear, and the particle size increased again with the continuation of the grinding process, negatively affecting the NAM’s performance. Therefore, we determined the optimum point in this investigation.