A charging method to charge a lithium ion battery in a short amount of time at low cost without degradation of the lithium ion battery is provided. A lithium ion battery includes a lithium compound containing iron as a positive electrode active material. The lithium ion battery is charged through a quasi-constant voltage charging procedure. The lithium ion battery includes a lithium compound as a positive electrode active material used for a positive electrode material. With the lithium compound, a voltage in a flat voltage section of a charging characteristic curve is higher than a voltage of a battery including the lithium compound containing iron as the positive electrode active material. The lithium ion battery may be an assembled battery including a plurality of electric cells connected in series.