A battery system comprising: a plurality of battery packs (1, 2), each battery pack having: a cell assembly (1a, 2a); and a battery management apparatus comprising: a sensing unit (110, 210) detachably mounted to the battery system and configured to measure a voltage of the cell assembly included in the corresponding battery pack; a balancing circuit (120, 220) having a balancing resistor (121, 221) connected to a charging and discharging path of the corresponding cell assembly in parallel and a balancing switch (123, 223) for electrically connecting or disconnecting the cell assembly and the balancing resistor; and a processor (130, 130', 230) operably coupled to the sensing unit and the balancing circuit, wherein, when it is detected that the battery packs are electrically connected to each other in parallel, the processor is configured to determine whether balancing is required between the voltages of the cell assemblies, and control the operation state of the balancing switch, the battery system being characterized in that when it is determined that the balancing is required, the processor is configured to calculate a balancing current of an operation state of each of the balancing switches by using balancing circuit data, compare each of the calculated balancing currents with a reference balancing current, and wherein the control of the operation state of the balancing switch is based on the comparison result. a plurality of battery packs (1, 2), each battery pack having: a cell assembly (1a, 2a); and a battery management apparatus comprising: a sensing unit (110, 210) detachably mounted to the battery system and configured to measure a voltage of the cell assembly included in the corresponding battery pack; a balancing circuit (120, 220) having a balancing resistor (121, 221) connected to a charging and discharging path of the corresponding cell assembly in parallel and a balancing switch (123, 223) for electrically connecting or disconnecting the cell assembly and the balancing resistor; and a processor (130, 130', 230) operably coupled to the sensing unit and the balancing circuit, a cell assembly (1a, 2a); and a battery management apparatus comprising: a sensing unit (110, 210) detachably mounted to the battery system and configured to measure a voltage of the cell assembly included in the corresponding battery pack; a balancing circuit (120, 220) having a balancing resistor (121, 221) connected to a charging and discharging path of the corresponding cell assembly in parallel and a balancing switch (123, 223) for electrically connecting or disconnecting the cell assembly and the balancing resistor; and a processor (130, 130', 230) operably coupled to the sensing unit and the balancing circuit, a sensing unit (110, 210) detachably mounted to the battery system and configured to measure a voltage of the cell assembly included in the corresponding battery pack; a balancing circuit (120, 220) having a balancing resistor (121, 221) connected to a charging and discharging path of the corresponding cell assembly in parallel and a balancing switch (123, 223) for electrically connecting or disconnecting the cell assembly and the balancing resistor; and a processor (130, 130', 230) operably coupled to the sensing unit and the balancing circuit, wherein, when it is detected that the battery packs are electrically connected to each other in parallel, the processor is configured to determine whether balancing is required between the voltages of the cell assemblies, and control the operation state of the balancing switch, the battery system being characterized in that when it is determined that the balancing is required, the processor is configured to calculate a balancing current of an operation state of each of the balancing switches by using balancing circuit data, compare each of the calculated balancing currents with a reference balancing current, and wherein the control of the operation state of the balancing switch is based on the comparison result.