Stand-alone power systems based on renewable energy sources are widely used for energy generation in remote locations and for distributed generation in urban environments. The DC bus is an essential component of these systems since it enables power transmission between the sources, loads and batteries. The batteries are interfaced with the bus using a charger/discharger DC/DC converter, which is controlled to regulate the DC bus voltage under any operating conditions. This is an important task because unsafe over-voltages and under-voltages in the bus could damage the sources, loads and power converters. This paper proposes a sliding-mode controller for a charger/discharger DC/DC converter with improved disturbance rejection to provide a tight bus voltage regulation for safe operation. The main novelty of this solution is the inclusion of the bus current in the sliding surface, which accelerates the controller response. Moreover, a formal proof of the system global stability is provided, and a detailed process is developed to calculate the controller and implementation parameters. Finally, the proposed solution is validated through simulations and experiments.