The automotive industry is placing high importance on technologies that can reduce CO2, even in a highly fuel-efficient compact car. One major technology is Stop & Start(S&S) System, with a combined energy regeneration system.A key component of the system is a power supply storage device that has high-charge acceptance, light weight, and compact size. We believe a Lithium-ion (Li-ion) battery completely meets these requirements.For the battery, there are three key points: 1Battery cell specification2State Of Charge (SOC) detection method3Temperature management for Li-ion battery.We have already proposed the battery cell and the SOC detection during SAE 2013, and now we are going to introduce “Temperature Management”.If the temperature of a Li-ion battery operates over 60 degrees Celsius, the battery could be severely damaged. Therefore, temperature management of the battery is very important.Conventionally, the temperature is managed by several thermistors and a cooling fan. However, this method is difficult to install in a compact car where cost and space are limited.We have developed our battery pack with the flexibility that allows it to be put in a compact car. The pack has two features: 1One thermistor measures the temperature of one of the cells, and then the temperature of all the cells is calculated using a secondary temperature off of the control board.2The temperature of the battery is controlled by natural cooling.We succeeded to install this system in a compact car through our new temperature management.