A serial-to-parallel converting apparatus , comprising: an input plate connected to positive electrodes and negative electrodes of a plurality of battery cells , respectively, wherein the input plate has circular shape and is divided into equal areas; an output plate spaced apart from the input plate and connected to an external terminal ; a rotary plate rotatably interposed between the input plate and the output plate to electrically connect the input plate and the output plate , the rotary plate being configured to convert serial connection and parallel connection of the plurality of battery cells by rotating; and a case configured to accommodate the input plate , the output plate and the rotary plate ; wherein the input plate , the rotary plate and the output plate are stacked on each other, and the input plate is completely fixed to the case and not able to move linearly or rotate; wherein the output plate is configured to be movable toward the input plate or away from the input plate ; wherein the output plate is in contact with the rotary plate to move toward the input plate or away from the input plate together with the rotary plate ; wherein a fixing protrusion or a fixing groove is formed at the output plate and the case , respectively, and the fixing protrusion and the fixing groove are coupled to restrict rotation of the output plate ; wherein an insulation region is formed in at least a portion of the rotary plate ; wherein the rotary plate has a simultaneous connection region to which a positive electrode of one cell and a negative electrode of another cell are simultaneously connected by the rotation of the rotary plate . an input plate connected to positive electrodes and negative electrodes of a plurality of battery cells , respectively, wherein the input plate has circular shape and is divided into equal areas; an output plate spaced apart from the input plate and connected to an external terminal ; a rotary plate rotatably interposed between the input plate and the output plate to electrically connect the input plate and the output plate , the rotary plate being configured to convert serial connection and parallel connection of the plurality of battery cells by rotating; and a case configured to accommodate the input plate , the output plate and the rotary plate ; wherein the input plate , the rotary plate and the output plate are stacked on each other, and the input plate is completely fixed to the case and not able to move linearly or rotate; wherein the output plate is configured to be movable toward the input plate or away from the input plate ; wherein the output plate is in contact with the rotary plate to move toward the input plate or away from the input plate together with the rotary plate ; wherein a fixing protrusion or a fixing groove is formed at the output plate and the case , respectively, and the fixing protrusion and the fixing groove are coupled to restrict rotation of the output plate ; wherein an insulation region is formed in at least a portion of the rotary plate ; wherein the rotary plate has a simultaneous connection region to which a positive electrode of one cell and a negative electrode of another cell are simultaneously connected by the rotation of the rotary plate . <EOS>