Demonstration of reusing electric vehicle battery for solar energy storage and demand side management

Abstract

This paper demonstrated reusing electric vehicle traction lithium ion batteries for solar energy time shifting and demand side management in a single family house. Batteries retired from electric vehicle usage retain 70% to 80% of their capacity and can be re-purposed as stationary storage system at reduced cost. However, they have mismatched aging conditions and unbalanced state-of-charge levels. Under typical series-parallel connection, the cells in a pack are prone to over charging or discharging due to deviated cycling conditions and misestimated states. The demonstrated battery management system included an extended Kalman filter based states estimator, enhanced current shunting, and protective circuitry to ensure system safety. One novel contribution was the introduction of a worst-difference state-of-charge estimation scheme for the battery pack, which places more computational resources on the battery cell of the worst health. The scheme provided satisfactory overall estimation accuracy and offered a method to optimize computational cost when large number of battery cells was integrated. In addition, a function was proposed to indicate the overall states of the entire energy storage system by aggregating the states of the battery cells within. Therefore, the energy management unit was able to dispatch the battery assembly as a unified pack. Three decision-table-based control strategies were demonstrated with objectives to maximize economic benefits, minimize grid energy consumption, or a balance of both. The data obtained from the demonstrating system located in Davis, CA showed that the battery energy storage system was able to successfully mitigate solar intermittency and energy demand fluctuation by charging from excess solar energy and discharging during the period of peak demand. It reduced daily grid energy consumption by 64%–100% and significantly improved solar penetration.