This study is designed to determine the deterioration state of the Li-ion battery by measuring the AC impedance spectrum. To this aim, the possibility of degradation discrimination was examined by changing measured environmental temperature within the practical operating range of the battery, and the following knowledge was obtained. (1) By lowering the ambient temperature below the room temperature, the impedance of the battery increases. The degradation state can be quantified even in the battery with medium and low levels of degradation. (2) Since the cell structure suppresses an increase in the imaginary component of the impedance, it was found that the deterioration progress made it difficult to discriminate the deterioration of the medium and low stage cells. (3) It is possible to distinguish a new battery from a used battery simply by lowering the ambient temperature below 0 degC. (4) It has been found that even in mass-produced batteries, since moisture that is inevitably mixed in during production causes internal condensation, the determination of deterioration at around zero environmental temperature may lead to misdiagnosis. (5) When the ambient temperature is lowered below the room temperature, both the electrolyte resistance and the charge transfer resistance increase, but an increase in the electrolyte resistance is more remarkable. It is appropriate to judge degradation at the ambient temperature around 10 degC where the charge transfer resistance has risen to about twice the case with the normal temperature even in the battery with medium to low levels of degradation.
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