The design and simulation of a battery charger is presented, for medical applications using three-state mode charge, containing a Buck converter and a lead-acid battery. Implementing Proportional Integral (PI) controllers, to modify the duty cycle of the Buck converter, in such a way that the battery is subjected to the 3 states during the charging process, the PI controller reference changes for each of the 3 states. In this way the battery is subjected to the 3 states with a single PI controller. In order to predict the current and voltage values to which it would be subjected in the real system in a charging process with this algorithm, it was possible to obtain the battery charge graphs, which describe all the voltage and current values. that the actual battery will undergo in a 3-state charge. Concluding that the charging current is maintained at adequate values, however there is a possibility that in state 2, the voltage exceeds the tolerated overvoltage value, so it is recommended to reduce the voltage reference in state 2.