We participated in a national project, to develop lithium batteries for use in fuel cell hybrid electric vehicles (FCHEVs), etc., in Japan as a battery developer. The lithium battery in an FCHEV system has to supply the system with not only power but also energy. We have developed an elliptic single cell, a 4-cell module, and an application-specific integrated circuit to monitor and control the cells. Our developed single cell has achieved a specific energy of 83 Wh kg(-1). a specific power output of 3380 W kg(-1) at 50% state of charge (SOC) for 10 s. The performance of the 3 kWh pack was evaluated using the 4-cell module data: the specific energy was 75 Wh kg(-1) and specific power output was 2250 W kg(-1). Introduction of a lithium-ion battery into the FCHEV definitely suggested a reduced fuel consumption to 2/3 of that without the lithium-ion battery based on our measurements and calculations for the 10-15 driving mode. (C) 2008 Elsevier B.V. All rights reserved.