The second use of a retired battery is an urgent problem needing to be solved for sustainable development of electric vehicles. To ensure the economy of a second-use battery energy storage system, considering the capacity configuration during the planning stages, a method is proposed for predicting the second-use battery service life based on equivalent cycling life and the rain flow counting method. Considering the impact of changes in capacity retention rate on energy storage system life and economy, an optimization model is built for the capacity configuration. This aims at the maximum net present value of total service life. The optimization results are obtained by a firefly algorithm. The results of numerical examples show that the capacity configuration method of the second-use battery storage system considering economic optimization within service life can increase the net present value and extend the life of the energy storage system. This work is supported by the National Key Research and Development Program of China (No. 2018YFB0905305) “Key Technologies for Large-Scale Engineering Application of Second-Use Battery”.