Development of a converterless energy management system for reusing automotive lithium-ion battery applied in smart-grid balancing

Abstract

To reuse lithium-ion battery scrapped from electric vehicles in accumulating off-peak electricity at night and unstable renewable energies is regarded as an effective way of extracting the residual capacity in scrapped automotive battery pack. However, decay of life cycle in such reused lithium-ion battery is an issue. In this study, a simple converterless energy management system is developed for controlling the power flow. One physical battery of ultracapacitor is used to couple with energy management system. It functions in the supply of peak power so that to ease the deviation of voltage drop. Two types of lithium-ion batteries (LiFePO4 and LiMnNiCoO2) in electric vehicles are utilized and imposed US federal driving pattern for simulating random use in urban area. Assessment of energy management system is conducted through the bench test of monitoring the voltage drop of battery pack. In addition, a real-time simulator is developed for optimizing current sharing ratio between battery pack and ultracapacitor for minimizing voltage drop, and then compared with bench-test results. In conclusion, energy management system improves up to 50% reduction of voltage drop in the case of LiMnNiCoO2, and much effective than passive control. A rough estimation by calculating the relative effect of elongating life cycle is up to 145% than original usage without energy management system. This study enables us to confirm the performance of converterless energy management system used in scrapped lithium-ion battery pack for reusing in smart-grid energy balancing.