The widespread adoption of electric vehicles has led to an increasing number of lithium-ion (Li-ion) and lithium iron phosphate (LiFePO4) batteries reaching the end of their automotive service life while still retaining between 70% and 80% of their nominal capacity. Giving these batteries a second life in stationary storage extends their usefulness and supports circular-economy goals, but large-scale reuse is held back by the lack of affordable monitoring solutions. We present a low-cost IoT-based sensor network architecture for remote monitoring and SoH/SoC estimation in second-life batteries. The proposed system employs ESP32-based acquisition nodes integrated with low-cost voltage, current, and temperature sensors, with an estimated hardware cost below USD 15 per node. Measurement data are transmitted through the HTTPS protocol to an edge-server platform for storage, visualization, and analysis. SoC estimation is performed using Coulomb Counting combined with Open-Circuit Voltage (OCV) correction, while SoH assessment is based on capacity estimation derived from operational measurements. The architecture is designed to support the simultaneous monitoring of multiple battery cells. In this study, the hardware architecture is validated using a DC power supply emulating cell behavior, while the SoC and SoH estimation algorithms are evaluated via Monte Carlo simulations on a synthetic dataset. Future experimental validation will integrate cells recovered from electric vehicles to assess long-term monitoring stability. Our study seeks to assess the technical viability of the proposed architecture as a cost-effective alternative for monitoring second-life battery systems, particularly in applications where commercial battery management solutions may be economically impractical.