When the main power is normal, it will supply power to the load through the exchange equipment and meanwhile the battery charger intelligently charges the spare battery. When the controller detects main power cut or the main power is too low, it will send out inverse signal and start the inverse power supply. At the same time, exchange equipment immediately outputs the inverse power supply continuously providing sine wave DC current. After the voltage of main power becoming normal, emergency power supply will return to main power supply.