An apparatus for estimating a state of charge, SOC, of a battery, comprising: a sensing unit configured to measure voltage, current and temperature of a battery; a memory unit; and a processor operably coupled to the sensing unit, the apparatus being characterised in that the processor is adapted to perform: setting a first reference voltage based on first SOCs and according to the voltage of the battery, wherein the first SOCs are stored in a table in the memory unit, as calculated in advance using current of the battery measured at every charge/discharge cycle of the battery and mapped to corresponding voltages of the battery, wherein said setting includes calculating a SOC deviation of the first SOCs for each charge/discharge cycle at every voltage of battery, wherein the SOC deviation is calculated based on a deviation of the first SOCs of the battery corresponding to the voltage of the battery at a first charge/discharge cycle and the first SOCs of the battery corresponding to the voltage of the battery at the other charge/discharge cycles; and setting, as the first reference voltage, a voltage of the battery, at which the SOC deviation is minimum; wherein the processor is adapted further to perform: calculating a second SOC according to the voltage of the battery using an equivalent circuit model corresponding to the battery; setting a voltage region of the battery equal to or lower than the first reference voltage as a first voltage region, and a voltage region of the battery higher than the first reference voltage as a second voltage region; and estimating the SOC of the battery as a first SOC calculated at an initial charge/discharge cycle among the first SOCs in the first voltage region and as the second SOC in the second voltage region. a sensing unit configured to measure voltage, current and temperature of a battery; a memory unit; and a processor operably coupled to the sensing unit, the apparatus being characterised in that the processor is adapted to perform: setting a first reference voltage based on first SOCs and according to the voltage of the battery, wherein the first SOCs are stored in a table in the memory unit, as calculated in advance using current of the battery measured at every charge/discharge cycle of the battery and mapped to corresponding voltages of the battery, wherein said setting includes calculating a SOC deviation of the first SOCs for each charge/discharge cycle at every voltage of battery, wherein the SOC deviation is calculated based on a deviation of the first SOCs of the battery corresponding to the voltage of the battery at a first charge/discharge cycle and the first SOCs of the battery corresponding to the voltage of the battery at the other charge/discharge cycles; and setting, as the first reference voltage, a voltage of the battery, at which the SOC deviation is minimum; wherein the processor is adapted further to perform: calculating a second SOC according to the voltage of the battery using an equivalent circuit model corresponding to the battery; setting a voltage region of the battery equal to or lower than the first reference voltage as a first voltage region, and a voltage region of the battery higher than the first reference voltage as a second voltage region; and estimating the SOC of the battery as a first SOC calculated at an initial charge/discharge cycle among the first SOCs in the first voltage region and as the second SOC in the second voltage region. calculating a SOC deviation of the first SOCs for each charge/discharge cycle at every voltage of battery, wherein the SOC deviation is calculated based on a deviation of the first SOCs of the battery corresponding to the voltage of the battery at a first charge/discharge cycle and the first SOCs of the battery corresponding to the voltage of the battery at the other charge/discharge cycles; and setting, as the first reference voltage, a voltage of the battery, at which the SOC deviation is minimum; wherein the processor is adapted further to perform: calculating a second SOC according to the voltage of the battery using an equivalent circuit model corresponding to the battery; setting a voltage region of the battery equal to or lower than the first reference voltage as a first voltage region, and a voltage region of the battery higher than the first reference voltage as a second voltage region; and estimating the SOC of the battery as a first SOC calculated at an initial charge/discharge cycle among the first SOCs in the first voltage region and as the second SOC in the second voltage region. calculating a second SOC according to the voltage of the battery using an equivalent circuit model corresponding to the battery; setting a voltage region of the battery equal to or lower than the first reference voltage as a first voltage region, and a voltage region of the battery higher than the first reference voltage as a second voltage region; and estimating the SOC of the battery as a first SOC calculated at an initial charge/discharge cycle among the first SOCs in the first voltage region and as the second SOC in the second voltage region.