The zinc-nickel single-flow battery is a new and special type of flow battery with a number of promising features, such as membrane free and high scalability, and thus has attracted substantial interests in recent years. However, the cyclability of alkaline zinc cells is rather poor, with sharpened capacity degradation resulted from undesirable zinc deposition formation. Yet, little has been done so far to investigate how to effectively and reliably manage this new type of battery. In this article, an open-circuit-voltage estimator based online joint estimation of both the state of charge (SoC) and the state of health (SoH) is proposed. A second-order equivalent circuit model is applied to improve the accuracy. Meanwhile, an extended Kalman filter with reduced state dimension is formulated to estimate the SoC assisted with the proposed estimator, which solves the increased complexity issue in a higher order model. An SoH indicator is then derived from capacity estimate and employed to determine the time for reconditioning maintenance, which is a key stage to update capacity and prolong service life. The method is finally applied to a bench-scale cell demonstrator, and the experimental results confirm the efficacy of the proposed method.