A lithium ion battery and a method of power conservation for the same are provided. A bare cell including an electrode assembly with a plurality of electrodes and a protective circuit are coupled together via the plurality of electrodes. The protective circuit includes a remaining charge control circuit that internally detects a long-term nonuse state of the lithium ion battery and blocks current between the bare cell and the protective circuit when the long-term nonuse state is detected. The method includes: detecting whether the battery is used as a power source for an external instrument; determining and recording a value of an accumulated nonuse time when the lithium ion battery was not used as a power source; and blocking an internal current and converting the battery to a long-term nonuse mode when the value of the accumulated nonuse time is above a value indicative of a predetermined time.