In this study we examined the use of diatomites to improve the discharge capacity and utilization of the positive electrode of the lead-acid battery. A large fraction of the positive electrode performance of this battery system (half-reaction shown below) is based on the ionic conduction of sulfuric acid through the plate. PbO2(s)+HSO4 − +3H+ +2e− →PbSO4(s)+2H2O The porous diatomites improve the ionic conduction by providing channels in addition to the natural ones formed within the paste. The choice of diatomite is based on its stability in the oxidative and corrosive environment of the lead-acid battery, as well as its naturally high porosity. The diatomite particles were size sorted into 20–30μm, 30–53μm, 53–74μm, and 74–90μm fractions, and the discharge performances of each were measured at 1%, 3%, and 5% weight ratios. The best performance was observed with 53–74μm particles with an optimum weight ratio of 3%. At this size fraction and weight ratio, diatomites show a 12.7% increase in active material utilization and a 9.3% increase in specific capacity at high rate discharge (50mAcm−2) relative to control samples without diatomites.