To simulate lead-acid battery (LAB) charging has never been an easy task due to the influences of: (1) secondary reactions that involve gas evolution and recombination and grid corrosion, (2) prior end-of-discharge (EOD) and rest conditions; and (3) complexity caused by charging algorithm. In this work, successful results have been obtained with considerations of internal oxygen cycle and gas phase in the valve-regulated lead-acid (VRLA) cells. The success is first attributed to the satisfactory validation of a mathematical model that has been able to simulate discharge regimes with various rates consistently. The model has been subsequently used to simulate a galvanostatic charge regime performed at C/10. The results give a better understanding of the role each electrode played in the polarization, the nature of the polarization (constituted by reaction kinetics and mass transport), and the charging efficiency. We were able to extrapolate the simulation results to rates beyond what the model has been validated for, and the results are still consistent, confirming some experimental observations, notably the maximum charging rate specified by most LAB manufacturers.