The energy efficiency for electric vehicle battery is affected by many factors. Through the definition of energy efficiency we find the relationship between energy efficiency, voltage efficiency and coulomb efficiency. The factors such as current, internal resistance, SOC and temperature which affect coulomb efficiency and voltage efficiency, will affect energy efficiency as well. An equation is given to show how internal resistance and current influence the energy efficiency. The relationship between these factors and energy efficiency was analyzed through theory and experimental data. This will show ways to increase battery energy efficiency and improve the battery performance.