The impedance parameters of Pb/PbSO4 electrode in 4.8 M sulfuric acid have been measured potentiostatically under open-circuit conditions over a wide range of frequencies (10−2 to 104 Hz) under both deep-discharged (state-of-charge, SoC=0) and fully charged (SoC=1) conditions. Immobilization of the electrolyte by adding sodium silicate (5 g/l) on the impedance of Pb/PbSO4 electrode shows significant difference in comparison with the flooded system. Four equivalent circuits have been formulated for overcharged and deep-discharged electrodes in gelled and flooded electrolytes, respectively. Impedance parameters are calculated using a phenomenological model, which indicates that in the case of gelled electrolytes, the electrochemical reactions are mass-transfer controlled, in contrast to the flooded electrolyte cells, where the process is known to be charge-transfer controlled. The study provides a non-destructive investigation of the effect of gelling on the electrical performance of sealed maintenance-free lead-acid batteries.