PURPOSE: To easily conduct charge and discharge without providing a DC stabilized power source and a number of switches by on-controlling a first semiconductor element on the basis of the detection signal of a current detector at the time of charge of a zinc-bromine battery, and on-controlling a second semiconductor element on the basis of the detection signal of the current detector at the time of discharge of zinc- bromine battery.

CONSTITUTION: A single-phase AC power source 12 alternately outputs positive and negative half-wave currents. At the time of charge of a zinc-bromine battery 5, a first semiconductor element 13a is on-controlled only when the detection signal of a current detector 14 shows the positive half wave, whereby a positive current is sent through the element 13a, and the zinc-bromine battery 5 is charged. At the time of discharge of the zinc-bromine battery 5, the first semiconductor element 13a is off-controlled, and a second semiconductor element 13b is on-controlled only when the detection signal of the current detector 14 shows the negative half wave, whereby a negative current is sent through the second semiconductor element 13b, and the zinc-bromine battery 5 is discharged. Hence, the charge and discharge can be easily performed without providing a DC stabilized power source and a number of switches.