Using　retired　power　batteries　in　battery　energy　storage　systems　(BESSs)　is　beneficial　for　environmental　protection　and　cost　reduction.　Modular　multilevel　converter　(MMC)　is　the　most　promising　structure　in　power　conversion　systems　integrated　with　retired　power　batteries.　However,　in　MMC-based　BESS,　intersubmodule　active　power　disparity　is　inevitable.　It　will　lead　to　an　increase　in　fundamental　circulation　and　capacitor　voltage　ripple,　limiting　the　usable　capacity　of　the　battery.　This　article　proposes　a　bidirectional　three-port　converter　(BTPC)　configuration　suitable　for　MMC-based　BESS.　It　ensures　that　retired　batteries　can　set　up　charging/discharging　strategies　according　to　their　states-of-health　and　state-of-charge,　while　maintaining　power　balance　at　each　port　of　the　MMC.　This　configuration　facilitates　the　integration　of　a　large　number　of　retired　batteries　into　the　grid　via　the　MMC　converter.　In　light　of　the　BTPC　configuration,　the　battery　charge/discharge　control　strategy　and　power　balance　strategy　are　proposed.　Simulation　and　experimental　results　are　obtained　on　a　single-phase　system,　verifying　the　effectiveness　of　the　proposed　converter　and　the　corresponding　control　strategy.