The automotive industry has been rapidly transforming and moving further from internal combustion engines, towards hybrid or electric vehicles. A key component for the successful adoption of the aforementioned approach is their Energy Management Systems (EMSs). In the proposed work, we describe in detail a custom EMS, with unique characteristics, which was developed and installed in a hydrogen-powered prototype vehicle. The development of the EMS was based on off-the-shelf components and the adoption of a Robot Operating System (ROS), a meta-operating system developed for robotic-oriented applications. Our approach offers soft real-time control and the ability to organize the controller of the EMS as a straightforward and comprehensive message system that provides the necessary inter-process communication at the core of the EMS control procedure. We describe in detail the software-based implementation and validate our approach through experimental results obtained while the prototype was racing in a low-energy consumption competition.