Static (solid state) frequency converters are used for converting mains power (50 Hertz or 60 Hertz) to adjustable frequency & voltage AC power source to simulate any country's grid power system to compatible with any appliances, especially the machine with electric motor inside it. By applying a solid state frequency converter, you can easy change 110V 60Hz, 120V 60Hz to 220V, 230V, 240V 50Hz and even 400Hz power supply.
When utility is available, and where batteries are disconnected from frequency converter input (by means of diodes or semiconductor switches which connect the battery to Inverter only when input power fails) there is no reason why not to use frequency converter output for feeding the battery charger. In certain UPS systems, a small output transformer followed by a rectifier and regulator for the charger circuit, is more economical and more efficient than alternate approaches.