An apparatus is provided for measuring the power of electrolytes at different positions of a flow battery by switching six-way valves without reconnecting channels. With the measurements at the positions, weighting is processed to obtain power corresponding to charging statuses for determining accurate power. The charging and discharging of voltage and current of the battery are controlled for constant operations with high efficiency. Consequently, the efficiency of power conversion is improved; energy consumption is reduced; and the battery is always run within a safe power-range for avoiding accidents or damages to the battery. In addition, the present invention is further applicable to a device monitoring the features of a battery unit. The six-way valves online monitor the power at center positions by switching. The values measured at different positions are aimed at the abnormality of the battery unit for processing adjustment or offline replacement to maintain best operation performance.