A method of detecting connection failure of a parallel connection cell, the method comprising: a reference data acquisition step of acquiring reference data when a cell connection failure occurs by generating a current interruption device, CID, operation or a parallel connection line open condition in a battery being discharged; a monitoring step of monitoring actual driving discharge data generated on the battery being discharged by an operation of an external device; a cell connection failure first detection step (S310-S330) of comparing, from an actual driving discharge data monitored in the monitoring step, whether there is a section matching the amount of change in the discharge voltage value due to a CID operation or an open parallel connection line in the reference data and first detecting whether a cell connection failure occurs due to the CID operation or the open parallel connection line in the corresponding battery according to a comparison result; and a cell connection failure final detection step (S410-S450) of finally confirming whether a result of the first detection is appropriate as that the cell connection failure occurs in the cell connection failure first detection step; characterized in that the cell connection failure final detection step comprises: a charging current formation step of forming a charging current in the battery first detected as being in a cell connection failure state due to a CID operation or an open parallel connection line in the cell connection failure first detection step; a DCIR measurement step of measuring Direct Current Internal Resistance, DCIR, of the battery while charging current flows through the battery by the charging current formation step; a DCIR comparison step of comparing whether a DCIR value of the battery measured in the DCIR measurement step is out of a predetermined DCIR reference range; and a connection failure final determination step of finally determining that a cell connection failure occurs due to a current interruption device, CID, operation or an open parallel connection line in the corresponding battery when the DCIR value of the battery is out of the predetermined DCIR reference range according to a result of the comparison. a reference data acquisition step of acquiring reference data when a cell connection failure occurs by generating a current interruption device, CID, operation or a parallel connection line open condition in a battery being discharged; a monitoring step of monitoring actual driving discharge data generated on the battery being discharged by an operation of an external device; a cell connection failure first detection step (S310-S330) of comparing, from an actual driving discharge data monitored in the monitoring step, whether there is a section matching the amount of change in the discharge voltage value due to a CID operation or an open parallel connection line in the reference data and first detecting whether a cell connection failure occurs due to the CID operation or the open parallel connection line in the corresponding battery according to a comparison result; and a cell connection failure final detection step (S410-S450) of finally confirming whether a result of the first detection is appropriate as that the cell connection failure occurs in the cell connection failure first detection step; characterized in that the cell connection failure final detection step comprises: a charging current formation step of forming a charging current in the battery first detected as being in a cell connection failure state due to a CID operation or an open parallel connection line in the cell connection failure first detection step; a DCIR measurement step of measuring Direct Current Internal Resistance, DCIR, of the battery while charging current flows through the battery by the charging current formation step; a DCIR comparison step of comparing whether a DCIR value of the battery measured in the DCIR measurement step is out of a predetermined DCIR reference range; and a connection failure final determination step of finally determining that a cell connection failure occurs due to a current interruption device, CID, operation or an open parallel connection line in the corresponding battery when the DCIR value of the battery is out of the predetermined DCIR reference range according to a result of the comparison. a charging current formation step of forming a charging current in the battery first detected as being in a cell connection failure state due to a CID operation or an open parallel connection line in the cell connection failure first detection step; a DCIR measurement step of measuring Direct Current Internal Resistance, DCIR, of the battery while charging current flows through the battery by the charging current formation step; a DCIR comparison step of comparing whether a DCIR value of the battery measured in the DCIR measurement step is out of a predetermined DCIR reference range; and a connection failure final determination step of finally determining that a cell connection failure occurs due to a current interruption device, CID, operation or an open parallel connection line in the corresponding battery when the DCIR value of the battery is out of the predetermined DCIR reference range according to a result of the comparison.