Battery charging apparatus

A battery charger non-concurrently performing a first operation to charge a main battery and a sub-battery by using an external power supply, and a second operation to charge the sub-battery by using the main battery, comprising:

• a power supply circuit connected to the external power supply and configured to generate an AC voltage;
• a transformer including a primary winding, a first secondary winding, and a second secondary winding, the primary winding being connected to the power supply circuit;
• a secondary-side circuit along a power supply route connecting the first secondary winding to the main battery, the secondary-side circuit being configured to perform, in a first time period for which the first operation is performed, an operation to rectify an AC voltage induced in the first secondary winding and supply the rectified voltage to the main battery, and, in a second time period for which the second operation is performed, an operation to convert a DC voltage from the main battery into an AC voltage and supply the AC voltage resulting from the conversion to the first secondary winding;
• a conduction angle adjustment circuit along a power supply route connecting the second secondary winding to the sub-battery, the conduction angle adjustment circuit being configured to adjust a conduction angle of an AC voltage induced in the second secondary winding;
• and
• a control circuit configured to control the power supply circuit, the secondary-side circuit, and the conduction angle adjustment circuit, wherein
• the secondary-side circuit is a full-bridge circuit including a plurality of arms connected in parallel, the arms each including a plurality of parallel-connected bodies connected in series, the parallel-connected bodies each including a switching unit and a rectifying unit connected in parallel, and
• in the first time period, the control circuit controls the conduction angle adjustment circuit so that the conduction angle adjustment circuit is in a conductive state at a timing at which a voltage having a positive or negative finite value not equal to 0 V is induced in the primary winding.