Reusing batteries from vehicular applications, known as 2nd-life batteries, for stationary applications is a concept gaining traction due to the exponential increase in electric vehicles. The advantages of 2nd-life batteries are both economic and environmental. However, to enable 2nd-life applications, it is necessary to address issues related to the diagnostics and management of used batteries. 2nd-life batteries are already partially aged and often exhibit inconsistencies between cells. To create a 2nd-life Energy Storage System (ESS), it would be necessary to conduct costly testing on the cells to reassess their performance. The research conducted has led to the development and validation of alternative solutions that could reduce the cost of enabling 2nd-life batteries. To monitor the battery without performing characterization tests, the Voltage Dynamic-Based SoC Estimation (VDB-SE) algorithm was developed, which estimates the battery’s equivalent parameters using machine learning techniques. Simulation results showed an average error in state-of-charge estimation lower than current state-of-the-art methods, specifically less than 1% for individual cells and under 2.5% for a battery pack. In the coming year, the VDB-SE algorithm will be implemented and validated on a real ESS.

Regarding the management of a 2nd-life battery, two activities were carried out: one involving the integration of an active Battery Management System (BMS) into a 2nd-life ESS prototype, and another concerning the design of a multi-level converter (MMC) that integrates the functionalities of the inverter and the active BMS.

In the first activity, a 2nd-life ESS prototype was developed using used electric vehicle cells without characterization tests. The cells were assembled and managed using an active BMS. The prototype was tested, demonstrating that the use of an active BMS effectively manages 2nd-life cells and addresses cell inconsistencies. The 2nd-life ESS will be further tested in the next research phase to assess cell aging due to the active BMS usage.

In the second activity, a model for a 30 kW-60 kWh ESS with an MMC was designed and developed. A technical-economic analysis revealed an MMC efficiency greater than 98% and a cost lower than that of a traditional converter with an equivalent BMS. Additionally, life tests on three lithium-ion cells showed that degradation due to high-frequency disturbances (50 and 100 Hz), typical of MMC operation, is negligible after 1500 cycles. The results of the life tests and technical-economic analysis indicate that using an MMC is advantageous compared to traditional converters, leading to the continuation of research to develop a prototype.