Capacity Deterioration Characteristics of Li-ion Batteries for Mobile Terminals

The power consumption of mobile terminals is growing rapidly due to installation of video phone function, increased use of i-motion and i-appli brought about by introduction of pake-hodai, installation of applications supporting future digital TV functions and so forth. Currently, the power required for these applications is supplied by Li-ion batteries. This type of batteries has higher energy density than Ni-Cd and Ni-MH batteries, and the increased usage of these batteries contributes significantly to the downsizing of current mobile terminals. Ensuring the quality of Li-ion batteries is thus an important issue. As Li-ion batteries are used for extended periods of time, the voltage and capacity of the batteries decreased. This phenomenon is known as battery capacity deterioration. When battery deterioration occurs, the operation time of mobile terminals may be shortened, or they will suddenly become unusable. The capacity deterioration depends on the usage conditions of individual users such as usage frequency and time, and given the present circumstances, it is difficult to predict the deterioration period. Obviously, this is a factor that lowers the quality of battery-powered products, for instance leading to increased failure rates of mobile terminals. Moreover, the mechanism of battery capacity deterioration itself has not been investigated sufficiently, and it is difficult to predict a battery’s lifetime accurately under the present circumstances. In this article, we propose a evaluation method of capacity deterioration of Li-ion batteries assuming actual operation conditions of mobile terminals (conditions under which users use mobile terminals) and validate the method by comparing with measurement data. Moreover, we conduct a material analysis and internal impedance analysis of various factors causing capacity deterioration, and show the ideas for lifetime prediction and detection method.