Multi-timescale electricity cost optimization for commercial buildings using EV second-life battery as energy storage systems

The rapid growth of electric vehicles creates significant opportunities for stationary energy storage through second-life battery utilization. This paper proposes a multi-timescale electricity cost optimization framework for second-life battery energy storage systems (SLBESS) in commercial buildings and validates it on a real deployed system. To address the complex challenge of commercial tariffs that include both energy and demand charges, our approach decomposes the problem by timescale. An upper layer uses hourly model predictive control (MPC) with a rolling horizon for long-term energy arbitrage, while a lower layer employs real-time control to mitigate short-term power peaks. Critically, the framework integrates empirically validated, health-preserving constraints for second-life batteries, including a restricted 15%–85% state-of-charge window and a 0.25 C-rate current limit, directly linking battery longevity to economic optimization. Comprehensive validation using 12 months of real-world operational data from a deployed SLBESS demonstrates a 28.6% electricity cost reduction compared to no-storage operation, outperforming baseline rule-based and Lyapunov optimization methods by 6% and 16.1%, respectively. The framework ensures sub-500 ms computation times, achieves a modest annual battery degradation rate of 1.20%, and delivers a 5.0-year payback period, highlighting its practical viability and performance in real-world commercial applications. • A novel hierarchical multi-timescale framework decomposes dual-objective cost optimization into tractable sub-problems. • The upper layer MPC optimizes hourly energy arbitrage while the lower layer manages short-term demand charges. • Comprehensive validation using 12 months of real operational data from the deployed second-life battery energy storage system. • The proposed strategy achieves 28.6% electricity cost reduction with sub-500ms computation times suitable for real-time deployment, and modest 1.20% annual battery degradation. • The proposed strategy promotes the circular economy by enabling practical second-life applications for EV batteries in commercial buildings.