CHARGE STATE ESTIMATION METHOD FOR LITHIUM ION BATTERY AND CHARGE STATE ESTIMATION DEVICE FOR LITHIUM ION BATTERY

A method of estimating a state of charge of a lithium ion battery, which uses a lithium phosphate compound having an olivine-type crystal structure for a positive electrode and uses an active material exhibiting no potential fluctuation due to an insertion/desorption reaction of lithium for a negative electrode, based on a voltage change rate. The method includes: storing a map representing a correspondence relationship between the voltage change rate and the state of charge at a plurality of charging/discharging rates; obtaining a candidate for a present state of charge from an actually measured voltage change rate, by referring to the map; and obtaining the present state of charge through a comparison between the candidate for the state of charge obtained during the charging and the candidate for the state of charge obtained during the discharging.