An apparatus includes an energy storage device, a bi-directional DC-to-DC voltage converter coupled to the energy storage device, and an input device. A voltage bus is coupled to the bi-directional DC-to-DC voltage converter and to the input device. The apparatus also includes a controller configured to control the bi-directional DC-to-DC voltage converter to convert a charging energy on the voltage bus into a charging energy suitable for charging the energy storage device during a charging operation and to monitor a voltage of the energy storage device during the charging operation. The controller is also configured to control the bi-directional DC-to-DC voltage converter to convert the charging energy into a charging energy configured to maintain the voltage of the energy storage device at a pre-determined value.