Battery pack, power source apparatus, and charging and discharging method

A rectifying circuit (a) comprising a diode Da and a capacitor Ca is provided for a winding Ta on the secondary side of a transformer T. A rectifying circuit (b) comprising Db and Cb is provided for a winding Tb. A charging device Ra connected to one output of the circuit (a) is connected to the + side of a secondary battery E1. The + side of E1 is connected to an output terminal Tol and the + side of E3 through a switching circuit S1. The - side of E1 is connected to the other output of the circuit (a) and connected to the - side of E3 through S2. Rb connected to one output of the circuit (b) is connected to the + side of E2. The + side of E2 is connected to the + side of E4 through S2. The - side of E2 is connected to the other output of the circuit (b) and connected to the - side of E4 through S3. The - side of E4 is connected to an output terminal To2. S4 is provided between the + side of E1 and the - side of E4.