Increased interest in global warming requires rapid improvements in CO2 reduction efforts. The automotive industry is placing high importance on CO2 reduction technologies.Using Lithium-ion (Li-ion) battery pack Stop & Start (S&S) system with combined energy regeneration is an effective technology to reduce CO2 emissions.Power supply storage is very important for the S&S system. High charging acceptance, low weight, and compact size are required. A Li-ion battery is the optimal power supply that meets these requirements. It has high charge acceptance per weight. Furthermore, we developed simple system structure which eliminates the need for the DC-DC converter. By utilizing a Li-ion battery that has voltage characteristics similar to the Pb battery there is no need for a converter to make adjustments between the two power supplies.The Li-ion battery's range of capacity must be managed appropriately as overcharge and overdischarge causes extensive damage to the battery. Therefore, battery State of Charge (SOC) is very important.We considered a method which detects open circuit voltage (OCV) while driving. The S&S system has a stable electric discharge area for a long time. The battery condition is stable in the area. By measuring in this area, and considering the frequency characteristic of the impedance of a battery, it becomes easier to detect SOC correctly even if the current sensor has an error.