To prevent a continuous discharge until the capacity of a battery module 11 becomes almost zero when the charging rate of each battery module 11 is adjusted for equalizing in a set battery 1 which is constituted by connecting the battery module 11 made of one or multiple simple batteries in series.

The charging rate adjustment circuit of the set battery comprises a paired discharge wires 12, 13 drawn from both poles of the each battery module 11, a discharge resistance 2 and a switch 3 which exist between both discharge wires 12, 13, a time limit holding circuit 4 which consists of a CR circuit to supply a control signal to the gate of the switch 3 and a photo coupler 5 which makes the time limit holding circuit 4 start a holding operation in response to a discharge order.