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In this article, the modeling of an optimum fast charging profile for lead-acid batteries (LABs) is proposed. The proposed profile is a multi-step constant current (MSCC) where various current magnitudes in a descending manner are applied to the battery; therefore, it prevents the over-voltage and gassing phenomenon at the end of charging process, and shortens the charging time at the same time. Using an electro-thermal model of a LAB, an optimization problem containing charging time, battery temperature during the process, and battery's terminal voltage as objective functions is defined. The magnitude and duration of all charging steps are taken into account as control variables to be optimized for minimizing the objective functions using a particle swarm optimization (PSO) algorithm. A comparison between different conventional charging profiles for LABs is also provided in terms of charging time and battery's temperature and voltage. The simulation results carried out by MATLAB/Simulink show that the proposed profile reduces the charging time, while keeps the battery's temperature and voltage during the charging process within allowable range. The impacts of model's uncertainties on the proposed approach is evaluated. A comprehensive comparison is also provided to confirm the benefits of the proposed method over MSCC profiles reported in the literature. The effectiveness of the proposed approach is finally validated using the experimental implementation for a 12 V 200Ah lead-acid battery.