A new method to calculate energy efficiency for rechargeable batteries is proposed successfully and described in the study. The energy efficiency is divided into three categories, the energy efficiency under charge, the energy efficiency under discharge and the energy efficiency under charge–discharge. A key factor in calculating the energy efficiency is attributed to resolve the chemical energy stored in batteries (referred to as net energy hereafter). The net energy is stated with an equation of a function of the open circuit voltage (OCV) and the state of charge (SOC). Therefore, the relationship of OCV and SOC must be sought to solve the function. The OCV curve is computed by average of the charge curve and the discharge curve at a low rate. The quantitative relation of OCV to SOC is acquired by nonlinear fit. According to the method, LiFePO4 power batteries’ energy efficiencies are measured under the moderate current charging and discharging condition. The quantitative relations of the efficiencies are also analyzed for the batteries.