This paper presents a lightweight, high-performance Lithium-ion Battery System developed jointly by Nissan Motor Co. and Sony Corp. for electric vehicle (EV) use. Electric vehicles are generally powered by a battery pack consisting of numerous cells connected in a series. Management techniques to elicit the maximum performance of the battery pack are needed, including a function for monitoring individual cells to prevent them from over-discharging.Because of high cell voltage, lithium-ion batteries enable the number of cells in a battery pack to be greatly reduced compared with other types of battery systems. They also allow accurate detection of the battery State of Charge (SOC) based on the battery voltage. These characteristics are conducive to the application of battery pack management technology. These concepts provided the basis for the development of a Lithium-ion Battery System for EV application. This system consists largely of battery modules, cell controllers, and battery controller, which calculates the battery power and remaining capacity and conveys the results to the vehicle control unit. In addition to its cell monitoring function, the cell controller also controls the charging current bypass circuit according to the voltage of each individual cell during charging so as to ensure that all cells are fully charged. By making use of the superior characteristics of lithium-ion batteries, this system provides highly accurate, real-time calculations of power available from the batteries while the vehicle is in operation. Moreover, it can also calculate with high accuracy the remaining capacity of the batteries, which has been an unresolved issue in EVs so far.This paper gives an overview of the Lithium-ion Battery System based on the characteristics of these batteries, and presents the results of tests conducted with the battery system installed in a test vehicle.