A method for deriving battery one-way efficiencies

Batteries are becoming an important decarbonization technology because they can act as convenient energy storage in various applications. They are often part of larger, complex systems and, in order to preserve computational tractability, battery models are usually oversimplified. Majority of such battery models ignore dependency of the charging/discharging efficiency on the charging/discharging power rate and instead use a constant efficiency over the entire range of power rates. This paper presents a method for obtaining individual one-way charging and discharging efficiencies dependent on the charging/discharging power. The method consists of two parts. First, the roundtrip cycle efficiency is experimentally obtained for different pairs of charging and discharging power rates. Second, an optimization problem is solved to decompose the roundtrip efficiency into the charging and the discharging efficiency for different power rates, resulting in one-way (charging/discharging) efficiency characteristics. As a demonstration, we apply the obtained efficiency characteristics to an electric vehicle driving profile consisting of both charging and discharging stages under different power rates and experimentally validate the accuracy of the proposed method.