This paper presents system architecture and control scheme of a photovoltaic (PV) string inverter allowing seamless battery integration with the dc-series integration method. The architecture uses the partial-power processing universal dc-dc optimizer to have flexible power control by regulating the T-node compensation current. The universal optimizer is configured with a dual active bridge dc-dc converter allowing bidirectional power flow and galvanic isolation. Three operation modes are proposed depending on the input sources connected. Based on the operation mode, the universal optimizer and a dc-ac inverter switch their control reference to deliver the maximum PV power and perform battery current control. The steady-state and transient operation of the closed-loop control scheme have been demonstrated through controller hardware-in-the-loop tests to verify feasibility and effectiveness of the PV-battery series inverter architecture.