The efficiency of the 60QNY6.5 battery is studied by testing and theoretical analysis in order to improve the control precision of hybrid electric vehicles (HEVs) and the battery useful life. A rig for testing the NiMH batteries of HEVs is established. The data from experiments performed on the NiMH battery are analyzed. Based on the experimental results and the temperature of the batteries, internal resistance and electromotance models of NiMH batteries are proposed by functionfitting methods. The theoretic formula for battery efficiency is deduced by the operational principle of the battery and motor, which provides theoretical basis for dynamic optimizing technology of HEV system controlling strategy.