PURPOSE: To stabilize outputs by making the use of a voltage converter possible both for step-down and step-up when constructing a battery module of a storage battery whose discharge characteristic slopes and the voltage converter for converting the output voltage of the battery into a predetermined constant DC voltage and for outputting the same.

CONSTITUTION: A lithium battery 1 comprises a plurality of lithium ion cells connected in series with one another, and a terminal voltage VS generated therein is converted into a voltage Vd using a voltage converter 2 adapted both for step-down and step-up, and the voltage Vd is applied to a load RL via an output terminal 3 and a common terminal 5 and one end of the voltage VS is applied to a charging terminal 4 via a diode 28. In this constitution, the converter 2 comprises a coil 21 and a check diode 22 which are connected in series with each other, and the terminal of this parallel circuit is connected to the output terminal 3 and a circuit connected in parallel to the parallel circuit is connected to the common terminal 5. Capacitors 23, 24 and a switching transistor 25 are provided between the circuits connected in parallel to each other, and a modulating circuit 26 and a voltage detection circuit 27 are coupled to the respective portions of the transistor 25.