In this paper, an original online technique for the state of health monitoring of lithium-ion batteries energy storage systems is proposed. The aging of lithium-ion batteries is represented by the degradation of their internal parameters as the decrease of the capacitance and the increase of the Equivalent Series Resistance (ESR). This degradation is continuous over aging until reaching the storage system failure criteria. Thus, the original online technique is based on tracking the ESR representing the battery aging. The proposed approach uses a suitable balancing circuit connected to the terminal of the storage element to extract its actual parameter. The information about this actual parameter makes the state of health monitoring possible. The analysis is performed using simulation with realistic electrical model to demonstrate and validate the effectiveness and the accuracy of the proposed new approach.