When a converter is composed of cascaded system with another unknown converter, the optimal design of the converter is especially important owing to stability of the cascaded systems. Battery 's model is introduced to analysis. Battery converter's different work conditions are considered. Battery converter is the front stage when it discharges, and it is the rear one when it charges in a cascade DC system. From the view of impedance, it's good for the cascaded system stability to reduce output

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... mpedance of battery converter when the converter is the first stage. Another, increasing the converter's input impedance could improve the cascade system stability when the converter becomes the rear one. The battery equivalent circuit model is introduced as a key component of system. Battery bidirectional converter should act as the front or rear stage of cascaded system. Selection rules of LC filter are put forward according to analyzing the inductor and capacitor's effects on two conditions. The PI controller parameters' effects on impedance are analyzed to furnish the basis for the PI controller parameters selection. Finally, experiment is carried out to verify the correctness of the analyses. INDEX TERMS Battery equivalent model, bidirectional DC-DC converter, cascade system, impedance analysis, small-signal model.