Series-connected　lithium-ion　battery　packs　have　inherent　divergence　of　cell　state　of　charge　(SOC)　and　the　differences　will　be　further　enlarged　with　repeated　charging　and　discharging　cycles,　which　will　reduce　the　available　capacity　and　lifetime.　In　this　paper,　an　improved　equalization　circuit　is　proposed　based　on　the　buck-boost　principle　and　the　control　simulation　is　implemented.　The　lithium-ion　battery　model　is　established　and　a　series-connected　lithium-ion　battery　pack　balancing　system　with　SOC　as　the　balancing　target　is　built.　Simulation　verification　and　comparative　analysis　show　that　comparing　with　the　traditional　buck-boost　circuit,　the　improved　design　without　adding　any　components　is　faster　and　much　efficient:　the　time　to　realize　the　equalization　is　reduced　and　the　energy　utilization　rate　is　improved.　©　2021　IEEE.