An Uninterruptible Power Supply comprising: an input configured to be coupled to a power source and to receive input power; an interface configured to be coupled to a battery and to receive DC battery power; a bi-directional converter coupled to the interface and configured to provide DC charging power, derived from the input power, to the battery in a first mode of operation and to convert the DC battery power into backup DC power in a second mode of operation; an output configured to be coupled to a load and to provide output power to the load derived from at least one of the input power and the backup DC power; and a controller coupled to the bi-directional converter and configured to operate the bi-directional converter at a first frequency in the first mode of operation to generate the DC charging power and to operate the bi-directional converter at a second frequency in the second mode of operation to generate the backup DC power; a converter coupled to the input and configured to convert the input power into DC power; a DC bus coupled to the converter and the bi-directional converter; and an inverter coupled to the DC bus and the output, wherein the bi-directional converter is configured, in the first mode of operation, to receive the DC power via the DC bus and convert the DC power into the DC charging power, and in the second mode of operation, to receive the DC battery power, convert the DC battery power into the backup DC power, and provide the backup DC power to the DC bus, wherein the inverter is configured to provide the output power to the load derived from at least one of the DC power and the backup DC power; wherein a first voltage gain applied by the bi-directional converter in the first mode of operation is a function of the first frequency set by the controller and a second voltage gain applied by the bi-directional converter in the second mode of operation is a function of the second frequency set by the controller; said first voltage gain being defined by the ratio of an output voltage associated with the DC charging power divided by an input voltage associated with the input power; and said second voltage gain being defined by the ratio of an input voltage associated with the DC battery power divided by an output voltage associated with the backup DC power; and wherein, the controller is operable to maintain said second frequency at a resonant frequency when the output is at a nominal level, is further configured to maintain the second frequency at the resonant frequency even when the output voltage falls below the nominal level, provided that the output voltage remains at or above a minimum acceptable value. an input configured to be coupled to a power source and to receive input power; an interface configured to be coupled to a battery and to receive DC battery power; a bi-directional converter coupled to the interface and configured to provide DC charging power, derived from the input power, to the battery in a first mode of operation and to convert the DC battery power into backup DC power in a second mode of operation; an output configured to be coupled to a load and to provide output power to the load derived from at least one of the input power and the backup DC power; and a controller coupled to the bi-directional converter and configured to operate the bi-directional converter at a first frequency in the first mode of operation to generate the DC charging power and to operate the bi-directional converter at a second frequency in the second mode of operation to generate the backup DC power; a converter coupled to the input and configured to convert the input power into DC power; a DC bus coupled to the converter and the bi-directional converter; and an inverter coupled to the DC bus and the output, wherein the bi-directional converter is configured, in the first mode of operation, to receive the DC power via the DC bus and convert the DC power into the DC charging power, and in the second mode of operation, to receive the DC battery power, convert the DC battery power into the backup DC power, and provide the backup DC power to the DC bus, wherein the inverter is configured to provide the output power to the load derived from at least one of the DC power and the backup DC power; wherein a first voltage gain applied by the bi-directional converter in the first mode of operation is a function of the first frequency set by the controller and a second voltage gain applied by the bi-directional converter in the second mode of operation is a function of the second frequency set by the controller; said first voltage gain being defined by the ratio of an output voltage associated with the DC charging power divided by an input voltage associated with the input power; and said second voltage gain being defined by the ratio of an input voltage associated with the DC battery power divided by an output voltage associated with the backup DC power; and wherein, the controller is operable to maintain said second frequency at a resonant frequency when the output is at a nominal level, is further configured to maintain the second frequency at the resonant frequency even when the output voltage falls below the nominal level, provided that the output voltage remains at or above a minimum acceptable value.