This paper presents the results of the investigations of the thermal behavior of a model valve-regulated lead-acid cell at overcharge and saturation level around 90%. The heat capacity and the heat transfer coefficient are experimentally obtained. The quasi-steady state characteristics of the cell at constant charging current are measured. The experimental data show that, when operating in recombination regime, the stationary cell temperature is a linear function of the charging current. When the cell is gassing, part of the heat is dissipated with the released gases, as a result of which the cell temperature is lower than predicted from the linear temperature/current dependence. Introduced dimensionless slope of the temperature/current dependence is a characteristic of the VRLA battery, which could be used for fast and easy estimation of the temperature increase of the battery. The kinetic parameters of the oxygen evolution reaction (OER) proceeding at the positive plates of VRLA cells are experimentally determined. The current/overpotential dependence of the OER obeys the Tafel relation. The temperature/current dependence and the kinetics of the OER are used to estimate the critical charging voltage for safe operation of the battery.