Battery stacks based on lithium-ion (Li-ion) cells are used in many applications on a wide scale. Battery Management System (BMS) is description of the state of the Battery according to a specific input to get multiple outputs which could be used in system control. This paper introduces the application, and control of a DC/DC buck converter in charging the Li-ion batteries. The main parameter that can be estimated to describe the state of battery charging capacity is State of Charge (SoC). SoC is estimated based on the battery open circuit voltage (OCV), and the Coulomb's counting method. SoC limits are controlled to avoid overcharging the battery. Pulse Adjustment (PA) control technique is applied to control the switching of the of the DC/DC buck converter used to charge the Li-ion battery. So ensures constant charging current at the beginning of the charging process, within permissible limits. Hence avoid excessive heating that may degrade or damage the battery. MATLAB/Simulink tool is used for design verification. Simulation & Practical results were encouraging. A mobile application is designed to monitor the battery charging/discharging parameters.