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 83Whkg−1, a specific power output of 3380Wkg−1 at 50% state of charge (SOC) for 10s. The performance of the 3kWh pack was evaluated using the 4-cell module data: the specific energy was 75Whkg−1 and specific power output was 2250Wkg−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.