This paper presents a semi-empirical model for predicting capability and life span of small size Lithium ion battery packs. The model consists of a simple Battery Management System (BMS) model, an existing electro-thermal model as well as calendar and cycle aging models. In this work we aimed to realize both fast calculation and high flexibility by using a simplified thermal and cycle aging model with Rainflow method, a method usually used for evaluating material fatigue. This paper details the mathematical structure of the model. The methodology is applied to a LiCoO2/carbon BB-2590 type battery pack utilized for small Unmanned Ground Vehicles. Moreover, simulation results of a capability test of an on-board battery, a 10 year calendar life test and a cycle test with 500 charging and discharging cycles are shown.