A battery charger onboard a vehicle, including: - a main voltage converter including: - at least one transformer including at least one primary and at least one secondary, - at least one upstream bridge of controlled transistors supplying the primary of the transformer, - at least one downstream bridge of controlled transistors which are supplied by the secondary of the transformer, providing for, when current- and/or voltage-controlled, generating a first voltage to supply a battery to be charged, the downstream bridge including notably at least one first leg and one second leg, - a transistor control circuit, said battery charger characterized in that it further includes: - at least one isolating switch connected, notably directly, to a middle point of the second leg of the downstream bridge, providing for, when open, using the downstream bridge to deliver the first voltage, and when closed, using the first leg of the downstream bridge, in order to generate, by controlling all or some of the transistors of the downstream bridge supplied by said battery, a second voltage that is different from the first voltage, - a capacitance arranged between a low-potential output of the downstream bridge and a terminal of said at least one isolating switch not connected, notably directly, to the middle point of the second leg of the downstream bridge, and in that - the control circuit is configured to use only the transistors of the first leg of the downstream bridge, while keeping the other transistors of the downstream bridge open, so as to use at least one magnetizing inductance of the secondary of the transformer in order to generate said second voltage. - a main voltage converter including: - at least one transformer including at least one primary and at least one secondary, - at least one upstream bridge of controlled transistors supplying the primary of the transformer, - at least one downstream bridge of controlled transistors which are supplied by the secondary of the transformer, providing for, when current- and/or voltage-controlled, generating a first voltage to supply a battery to be charged, the downstream bridge including notably at least one first leg and one second leg, - a transistor control circuit, - at least one transformer including at least one primary and at least one secondary, - at least one upstream bridge of controlled transistors supplying the primary of the transformer, - at least one downstream bridge of controlled transistors which are supplied by the secondary of the transformer, providing for, when current- and/or voltage-controlled, generating a first voltage to supply a battery to be charged, the downstream bridge including notably at least one first leg and one second leg, - a transistor control circuit, - at least one isolating switch connected, notably directly, to a middle point of the second leg of the downstream bridge, providing for, when open, using the downstream bridge to deliver the first voltage, and when closed, using the first leg of the downstream bridge, in order to generate, by controlling all or some of the transistors of the downstream bridge supplied by said battery, a second voltage that is different from the first voltage, - a capacitance arranged between a low-potential output of the downstream bridge and a terminal of said at least one isolating switch not connected, notably directly, to the middle point of the second leg of the downstream bridge, and in that - the control circuit is configured to use only the transistors of the first leg of the downstream bridge, while keeping the other transistors of the downstream bridge open, so as to use at least one magnetizing inductance of the secondary of the transformer in order to generate said second voltage.