This paper presents the design, implementation, and validation of a low-cost embedded system for energy management and State-of-Charge estimation in modular mobile robotic platforms. The system integrates voltage, current, and temperature sensors, wireless relay control, and a web-based monitoring dashboard, all managed via an ESP8266 microcontroller. Two SoC estimation methods were experimentally implemented: one based on open-circuit voltage regression using a fifth-degree polynomial model, and another relying on current integration. Experimental validation was performed using both lithium-ion (3S, 2200 mAh) and lead-acid (12 V/7 Ah) batteries, under idle, nominal, and high-load conditions. A comparative error analysis confirmed consistent estimation accuracy, with relative errors remaining below 5% in critical ranges. The system’s real-time performance was assessed through stepwise discharge protocols, complemented by asynchronous web visualization and relay switching. The web interface was developed in two versions, resulting in a modular architecture based on the LittleFS file system and dynamic JavaScript rendering. Results confirm the platform’s suitability for integration in service robotics, where energy autonomy and transparent system supervision are essential. Due to its low power footprint and flexible structure, the solution is well-suited for smart indoor environments and robotics prototyping logistics, clinical, and Smart City applications.