This paper presents state-of-charge (SOC) control between battery modules in an active battery management system (BMS) with series output connected DC-DC modules. To increase both the pack and system-level modularity, the BMS regulates the battery pack average SOC by controlling a common input current from all series-connected modules. A balancing loop introduces input current offsets to regulate all module SOCs. The BMS functions are divided between a central controller for battery pack SOC regulation and individual module SOC control; a module level controller regulates the input current. Experimental results are presented on a 1.5 kW prototype consisting of five series modules that are formed by 15 DC/DC series and parallel connected converters in an active BMS.