Equalization is one of the core functions of battery management system for electric vehicles. Several kinds of common active and passive equalization strategies have been systematically compared by using capacity-quantity scatter plots. The results show that the equalization strategies based on the state-of-charge (SOC) are the simplest and most efficient. Furthermore, an online equalization strategy for lithium-ion battery packs based on SOC is proposed. Compared with other strategies, this strategy has less computation cost and higher efficiency. At the same time, uncertainty coefficients and optimal interval for equalization are designed to solve the problems of mis-equalization and over-equalization. Finally, a hardware-in-loop (HIL) testing platform is built and the results show that the initial 10% SOC difference is almost completely eliminated in about 7500 s.