In this graduate thesis, the SOC of two 75 Ah lead-acid batteries of different ages was evaluated at the simulation level. The SOC was estimated using the no-load voltage method and the Coulomb counter method. The equivalent circuit-based battery models were used in the simulations, namely the standard GNL model and the proposed modified GNL battery model. First, the various battery models and SOC estimation methods found in the literature were described. Then, on the basis of experimental measurements, the parameters of resistors and capacitors in the used models were determined, as well as the characteristics describing the dependence of the SOC on the no-load voltage. The model used for the simulations was built in the MATLAB-Simulink environment. All used components, their purpose, and associated parameter values are listed and described. Then, the voltage responses of the used battery models obtained through simulations were compared with the experimentally recorded responses. In addition, the SOC values obtained using the no-load voltage method were compared with the values obtained using the Coulomb counter method, with the latter values being considered as reference. The need for modification of the standard GNL model was explained along with the newly introduced components, followed by the determination and comparison of the integral absolute error values of the battery terminal voltage and SOC obtained with the considered models. Furthermore, the percentage error of the estimated SOC value obtained based on these models was determined and compared. Finally, a discrete SOC estimator was created, which can be experimentally implemented for real-time SOC estimation.