In a lithium-ion battery, the crossing of its charging–discharging curves represents an unambiguous current capacity. The reproduction of a complete charging–discharging cycle defines the maximum possible charge for a current battery state. When obtaining of the curves experimentally, one should consider the duration of polarization or depolarization. It is reported that the battery capacity depends considerably on the value of the battery’s internal resistance as this value defines the location of the actual charging–discharging curves relative to the nominal curves, i.e., the point of their crossing shifts. The present study aimed to develop a diagnostic algorithm for the batteries of electric vehicles for a complete charge–discharge cycle, considering the processes of polarization and depolarization. The mathematical expressions for determining the internal resistance and the complete charge of the battery were obtained. The novelty of the present study is in the fact that by applying extrapolation to a simple electrical model of a lithium-ion battery, the degree of degradation and the current charge could be predicted without the requirement of lengthy experimental procedures.