An apparatus for regulating voltage and aerospace electrical power systems are implemented. A multi-functional apparatus for regulating voltage, according to one embodiment, comprises: a DC to DC converter (315), wherein the DC to DC converter (315) is controlled to operate in a first direction to receive a DC voltage from a first bus (329) and to output a first regulated DC voltage adjusted to charge a battery (307), wherein the first bus (329) receives power from a second bus (201), and is controlled to operate in a second direction to receive a battery DC voltage from the battery (307) and to output a second regulated DC voltage to the first bus (329) to power a load (231) independently or in combination with the second bus (201).