The present invention employs more than two secondary batteries; most output of one battery is supplied to load, and rest output is used to charge another battery for improving the battery efficiency and prolong the battery usable time (discharge time). The object of the present invention is to provide a power supplying device and battery mode switching method by using the secondary battery cells. According to the present invention of using more than two secondary batteries that; if either one of the secondary battery sets for discharging mode, the device including that; an inverter for delivering a DC output voltage of the secondary battery, which is in discharging mode, when one of the more than two secondary batteries is discharged; a relay performing the switching operation for connecting to other different secondary battery of more than two secondary batteries to the inverter, at other point, the secondary battery in discharge mode of more than two secondary battery is connected to the inverter; and a controlling unit for alternately supping the power of the more than two or secondary batteries to the load by controlling the switching operation of the relay via the inverter, a switching unit alters charging or discharging mode, the anode (+) and cathode (−) of each DC terminal, which is mutually and fixedly connected, and each moveable terminal of the secondary battery is switched to control the battery mode.