The world has made great progress in providing everyone with access to energy; however, recently, the number of unelectrified people on Earth has increased. Providing access to energy is a critical and complex challenge as most unelectrified people live in extremely remote and hard-to-reach areas, often having limited mobile network connectivity, unknown (future) demand patterns, and extreme weather conditions combined with very low purchasing power of the affected population. The power of Open Source is still completely underrepresented in technological innovations in this sector. We are working towards improving this situation and will showcase one of our flagship examples: An Open Source Battery Management System (BMS) specifically developed for off-grid energy application. Energy storage and its related technologies are key to any off-grid energy application and the Libre Solar BMS has been specially developed for this use case. We will deep dive into the design decisions and features, covering hardware, firmware and an app. The BMS was developed leveraging solely open source tools: The PCB is designed in KiCad, the firmware runs on Zephyr RTOS and the communication interfaces use the ThingSet protocol over various lower layers like Serial, CAN, WebSocket or MQTT. The hardware went through three design iterations and has been lab and field-tested by several organizations. We will conclude and show why this kind of Open Source technology is of such great importance to bring power to the people in both the sense of bringing electricity literally and also creating the grounds for local value creation in the affected geographies. We invite the whole Energy Open Source community to contribute with their efforts to where the contributions can be most impactful on various levels. The Open Source BMS is one great piece, but we need much more of those.