A manganese-based lithium ion battery was developed for hybrid electric vehicle (HEV) applications. The cell consists of an improved manganese spinel as the positive electrode and hard carbon for the negative electrode. The Mn-based Li ion cell for HEVs was developed by adding a high power density modification to a pure-EV (EV) cell of high energy density specification. It has a power density as high as 2000W/kg at 50% depth of discharge (DOD) and 25°C. Storage tests at various temperatures suggest a practical calendar life of more than 5 years. The 48-cell battery module was developed for use in an HEV. It also proved to have an excellent output power density of 1350W/kg at 50% DOD. Based on the excellent characteristics of the lithium ion battery, it seems very promising to apply this battery not only to EV, HEV and other motor-assisting drive systems but also to other high power applications.