ETD

• balancing systems
• lithium-ion batteries
• second-life batteries
• supercapacitor

Second-life lithium-ion batteries retrieved from electric vehicles represent an attractive resource for stationary energy applications. They address the need for end-of-life treatment of retired electric vehicle batteries by repurposing them in less demanding applications. However, the use of second-life batteries pose a significant challenge due to the substantial capacity mismatch among cells within the same battery pack, which limits its usable capacity when not properly addressed.
Dynamic balancing is a promising approach to increase the usable capacity of
second-life battery, by transferring charge from the best-performing cells to the
least-performing ones during battery operation. This approach is similar to the
active balancing used in first-life batteries, but requires higher current DC/DC converters to compensate for the larger cell mismatches. Therefore, optimal converter
design is crucial for achieving high efficiency in the dynamic balancing system.
Among the dynamic balancing topology reported in the literature, the Cell-to-
Auxiliary topology has been identified as a cost-effective and high-performing solution. This topology uses a synchronous rectifying buck DC/DC converter and a
supercapacitor as temporary storage element to transfer charge between the cells.
In this thesis a novel closed-form analytical approach is proposed to determine
the architecture efficiency. This method significantly reduces computation time compared to existing approaches while maintaining comparable accuracy.
An optimization procedure based on genetic algorithm is then applied to identify
the optimal component set for the DC/DC converter design. Based on the optimization results, an hardware prototype is designed and manufactured. Experimental
test results highlight non ideal-effect not captured by the initial model, enabling
further refinement of the developed closed-form analysis. The prototype achieves an
average efficiency of 75.75 % at an average DC/DC current of 5.2 A.
This result demonstrates that supercapacitor-based balancing architectures represent a viable and cost-effective solution for second-life battery applications.