Performance Evaluation of Retired Lithium-ion Batteries for Echelon Utilization

With the rapid growth of the electric vehicle (EV) market, the number of Lithium-ion (Li-ion) batteries that reach their end of life (EOL) is increasing rapidly. Given the stringent capacity fading threshold of EV batteries, tools are required for better understanding and evaluating the health condition of large volume EV batteries that have reached EOL. In this paper, four modules from the same battery pack of a hybrid electric vehicle have been evaluated in terms of their current capacity and performance of the cells within each module. The results have been analyzed to find an affordable method for performance assessment of the retired batteries for echelon utilization. Electrochemical impedance spectroscopy (EIS) as an accurate and powerful technique has been used as a benchmark for the measurement to show the reliability of the tests. Experimental results are obtained from different test approaches on both modules and cell levels and show a different ageing degradation pattern for the cells inside the modules. The result shows that even for the modules with the same range of state of health, any non-uniformity of the cells inside the modules will affect the reliability of the modules for a second life. We also show there is a meaningful dependency between the voltage monitoring of the cells and other test approaches to determine the uniformity of a module.