In the energy storage application of retired batteries, the large inconsistency between the retired batteries makes the battery pack more prone to over-charge and over-discharge during the charging and discharging process, which reduces the overall available capacity of the battery pack and even causes safety issues. In order to solve the above problems, an active equalization circuit based on an isolated dual half-bridge DC-DC converter is proposed in this paper. The equalizer circuit consists of a switch array of N+5 switches (N is the number of cells) and an isolated dual half-bridge DC-DC converter, which ensures the flexibility of the circuit. Based on the analysis of the working principle of the main circuit, a sub-state equalization control strategy based on SOC is proposed to achieve balancing of the battery pack by using the corresponding equalization strategy under three different states of battery pack: charging, discharging and resting. Finally, an equalization experiment was carried out on 5 series lithium-ion batteries. The experimental results show that the available capacity of this method is increased by 12%, 9.9%, and 17.5% in the resting, charging, and discharging states, respectively, compared with the battery pack without the equalizer, which proves the feasibility and effectiveness of this method.