The mounting of lithium-ion batteries (LIB) in hybrid electric vehicles (HEV) calls for the configuration of highly robust control systems. When mounting LIBs in the vehicle, it is important to accurately ascertain and precisely control the state of the battery.In order to achieve high durability, it is important to configure highly reliable systems capable of dependably preventing overcharging as well as to have control technology based on software that can contribute to extended battery life. The system configuration applies an overcharge prevention system that uses voltage detection with an emphasis on reliability. Furthermore, a method for varying the range of state of charge (SOC) control in the vehicle according to the battery state is implemented to assure durability.In order to achieve this, battery-state detection technology was developed for the purpose of correctly detecting and judging the battery state. This battery control system is able to enhance the durability while obtaining the maximum battery performance.This paper reports on the configuration of such systems, methods of state detection, and applications based on detection results.