A novel fast capacity estimation method based on current curves of parallel-connected cells for retired lithium-ion batteries in second-use applications

Retired lithium-ion batteries from electric vehicles provide considerable social, economic and environmental benefits in second-use applications especially for energy storage. However, the effective capacity estimation is a key technique and difficulty for second-life of large-scale retired cells. To address the issue, this paper provides a fast novel capacity estimation method from a new perspective of current distribution within parallel-connected cells. Firstly, using the parallel-connected cells analytical model, the correlation between capacity and current distribution is deduced. According to the correlation, the capacity of each cell is simultaneously estimated for the n parallel-connected cells by measured the current curves in only one complete discharge cycle, which is more efficient than one after another. Then an experimental device and procedure are designed to verify the method. Results demonstrate that the method is feasible and accurate. Meanwhile, through comparing the results of packs with three different battery types, the main factors affecting accuracy are found including open circuit voltage, parameter difference of cell and loads. Finally, comparison of different methods shows that the method has the advantages of high efficiency, low calculation, low accumulative errors and good consistency of state-of-charge, which is helpful for regrouping of retired cells and promoting second-life battery applications.