US Patent 9199543 Bi-directional battery voltage converter

A method of charging batteries in a vehicle electrical system having a system bus, a first switch selectively connecting a first battery with the system bus, a second switch selectively connecting a second battery with the system bus, and a controller monitoring a voltage of the vehicle electrical system and controllably opening and closing the first switch and the second switch. The method includes closing the first switch and opening the first switch if the voltage of the vehicle electrical system traverses a threshold value. The method also includes closing the second switch and opening the second switch if the voltage of the vehicle electrical system traverses the threshold value.