The influence of the content of SiO2 (in the form of a soot) on the capacity and the self-discharge of lead–acid batteries, as well as on the structure of gelled-electrolyte, is studied in detail by means of cyclic voltammetry (CV), electrochemical impedance spectroscopy and scanning electron microscopy. The content of SiO2 and the viscosity of the gelled-electrolyte are important factors which affect the capacity of batteries. A comparison of cyclic voltammograms indicates a change in the electrochemical behaviour of the lead electrode in the presence of SiO2. The reaction resistance increases as a function of the SiO2 content. The mechanism of the effect of the content of silica soot on electrode capacity and on the performance of gelled-electrolyte is discussed.