An improved battery charger for charging a battery from a dc power source includes a DC-to-DC converter comprising a transformer and a switching transistor connected in series with the primary winding of the transformer. The transformer has its primary winding connected to the dc power source and has its secondary winding connected to the battery to be charged. The switching transistor is controlled to be alternately turned on and off in order to develop across the secondary winding an induce voltage which is rectified to be applied to the battery for charging the same. The secondary winding of the transformer is electrically connected to the dc voltage source in such a manner that the induced voltage of the transformer is in an additive relation with the terminal voltage of the dc voltage source.
