NOVELTY - A lithium secondary battery, comprises: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and a polymer electrolyte composition comprising a polymer electrolyte, a non-aqueous organic solvent, and a lithium salt.    USE - The lithium secondary battery is useful for industrial and commercial applications.    ADVANTAGE - The lithium secondary battery is highly efficient, effective and reliable.    DETAILED DESCRIPTION - A lithium secondary battery, comprises: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and a polymer electrolyte composition comprising a polymer electrolyte, a non-aqueous organic solvent, and a lithium salt; where the content of the polymer electrolyte is 9-20 wt.%, based on the total weight of the polymer electrolyte composition; and where the polymer electrolyte comprises a first monomer represented by: A-polyesterpolyol-B, where, the polyesterpolyol has a molecular weight of 100-10,000,000 and is the product of a condensation reaction between an alcohol derivative having 2-6 terminal OH groups and a dicarboxylic acid derivative, A and B are groups that are linked by means of terminal OH groups of the polyesterpolyol and are independently selected from CH2=CR-C(=O)-, CH2=CR-C-CH2-, CH2=CR-, CH2=CR-O-C(=O)-, CH2=CH-CH2-O-CH2=CHS(-O)2- or CH2=CR-C(=O)-O-CH2CH2-NH-C(=O)-.    R=a 1-10C hydrocarbon or a 6-10C aromatic hydrocarbon.    An INDEPENDENT CLAIM is a method of controlling a short circuit resistance of a secondary battery, comprising: measuring an open circuit voltage (VOC) of the secondary battery; calculating a short circuit resistance (RSC) using the measured open circuit voltage (VOC); calculating a heating value (W) according to time using the calculated short circuit resistance (RSC); calculating a time having the maximum instantaneous heating value, using the change in heating value (W) according to time, determining the short circuit resistance (RSC) value at the time having the maximum instantaneous heating value as a critical short circuit resistance; and adjusting an amount of a polymer electrolyte used in the secondary battery, according to the determined critical short circuit resistance.