A battery charging system comprising: a first connector to be connected to both ends of a battery of a charging providing car; a second connector to be connected to both ends of a battery of a charging target car; a converter connected between the first connector and the second connector and configured to convert a voltage transmitted from the battery of the charging providing car through the first connector and transmit the converted voltage to the battery of the charging target car through the second connector; and a controller configured to control an electrical connection of the first connector and the second connector and to be connected to each of a battery management system of the charging providing car and a BMS of the charging target car to control a voltage conversion ratio of the converter based on a state information on each of the batteries transmitted from each of the BMSs (210, 310), wherein the first connector comprises: a first positive electrode relay configured to connect a positive electrode of the battery of the charging providing car and a positive electrode of an input terminal of the converter; a first negative electrode relay configured to connect a negative electrode of the battery of the charging providing car and a negative electrode of the input terminal of the converter; a first capacitive element connected to the input terminal of the converter in parallel; and a first precharge unit connected to the first positive electrode relay in parallel, wherein the first precharge unit comprises a first precharge relay and a first precharge resistor connected to the first precharge relay in series, wherein the second connector comprises: a second positive electrode relay configured to connect a positive electrode of the battery of the charging target car and a positive electrode of an output terminal of the converter; a second negative electrode relay configured to connect a negative electrode of the battery of the charging target car and a negative electrode of the output terminal of the converter; a second capacitive element connected to the output terminal of the converter in parallel; and a second precharge unit connected to one of the second positive electrode relay and the second negative electrode relay in parallel, wherein the second precharge unit comprises a second precharge relay and a second precharge resistor connected to the second precharge relay in series. a first connector to be connected to both ends of a battery of a charging providing car; a second connector to be connected to both ends of a battery of a charging target car; a converter connected between the first connector and the second connector and configured to convert a voltage transmitted from the battery of the charging providing car through the first connector and transmit the converted voltage to the battery of the charging target car through the second connector; and a controller configured to control an electrical connection of the first connector and the second connector and to be connected to each of a battery management system of the charging providing car and a BMS of the charging target car to control a voltage conversion ratio of the converter based on a state information on each of the batteries transmitted from each of the BMSs (210, 310), a first positive electrode relay configured to connect a positive electrode of the battery of the charging providing car and a positive electrode of an input terminal of the converter; a first negative electrode relay configured to connect a negative electrode of the battery of the charging providing car and a negative electrode of the input terminal of the converter; a first capacitive element connected to the input terminal of the converter in parallel; and a first precharge unit connected to the first positive electrode relay in parallel, wherein the first precharge unit comprises a first precharge relay and a first precharge resistor connected to the first precharge relay in series, wherein the second connector comprises: a second positive electrode relay configured to connect a positive electrode of the battery of the charging target car and a positive electrode of an output terminal of the converter; a second negative electrode relay configured to connect a negative electrode of the battery of the charging target car and a negative electrode of the output terminal of the converter; a second capacitive element connected to the output terminal of the converter in parallel; and a second precharge unit connected to one of the second positive electrode relay and the second negative electrode relay in parallel, a second positive electrode relay configured to connect a positive electrode of the battery of the charging target car and a positive electrode of an output terminal of the converter; a second negative electrode relay configured to connect a negative electrode of the battery of the charging target car and a negative electrode of the output terminal of the converter; a second capacitive element connected to the output terminal of the converter in parallel; and a second precharge unit connected to one of the second positive electrode relay and the second negative electrode relay in parallel, wherein the second precharge unit comprises a second precharge relay and a second precharge resistor connected to the second precharge relay in series.