In order to increase system inertia, a virtual DC generator (VDCG) control scheme is often used for lithium battery controllers in DC microgrids, but this scheme cannot achieve self-balancing of the state of charge (SOC) of the lithium battery. The existing SOC equalization scheme based on VDCG for lithium batteries can only achieve SOC equalization for non-capacity lithium batteries with the same voltage level, while it is difficult to maintain the same capacity and voltage level of retired lithium battery energy storage systems (RLBESS). In response to this issue, this paper proposes a SOC equalization scheme for RLBESS with different voltage levels and capacities based on VDCG. This scheme establishes a U-Pm relationship based on traditional VDCG and introduces an SOC balance factor, which can automatically adjust the power distribution of lithium batteries based on the initial SOC state, while maintaining good voltage quality. A small-signal modeling of the proposed scheme is established, and the influence of key control parameters on the system stability is analyzed. Finally, the effectiveness of the proposed scheme is verified in different working conditions by using Matlab/Simulink simulation software. The simulation results show that the proposed scheme can achieve SOC balancing of RLBESS under inconsistent voltage levels of lithium batteries, with good scalability.