A battery charging generator which is capable of charging two series connected batteries in a motor vehicle electrical system where the series connected batteries are used to energize the cranking motor of a motor vehicle. The generator is of the alternating current type and has first and second three-phase full-wave bridge rectifier units built into the end frame of the generator each of which is capable of supplying charging current to one of the batteries. The generator has a built-in transistor voltage regulator which senses the voltage applied to the main battery of the system by the first bridge rectifier and which regulates the output voltage of the generator to a predetermined value. The second bridge rectifier supplies charging current to an auxiliary battery and is connected with the stator winding of the alternating current generator by means of a three-phase transformer. The transformer, like the bridge rectifiers, is built into the end frame of the generator and isolates the two bridge rectifiers of the system. The generator is arranged with a 12 volt and a 24 volt output terminal which only need be connected directly to the batteries when installing a generator on a motor vehicle.