This document describes a proposed constant current/constant voltage charger for lithium-ion batteries using a buck converter topology. The charger uses a multi-power source control system to switch between three different power sources based on priority. It includes comparators to check the voltage levels of each source and a logical circuit to select the active source. A buck converter circuit regulates the battery charging in both constant current and constant voltage modes. Simulation results showed the charger can successfully charge the battery without reducing its lifespan while smoothly transitioning between charging modes. The design aims to increase efficiency and use fewer components compared to traditional chargers.