Batteries which are used in airborne electrical systems must provide a very high discharge current for APU starting and offer virtually maintenance free service. To achieve this kind of performance for hundreds of operational cycles, it is important to consider the behavior of a battery under actual operating conditions. Several functional parameters such as terminal voltage, state of charge, internal impedance and temperature are continuously changing depending on past and present operating conditions.In order to evaluate the available energy which a battery can deliver at any given time, it is necessary to maintain a log of the State Of Charge Index (SOCI). This numerical indicator is based on a functional relationship between terminal voltage, current flow, elapsed time and internal temperature. These parameters are continuously monitored and periodic measurements are converted into a standard index factor.The battery readiness to provide a desired output power for a specified period of time is calculated from several index factors and the net current flow during that same time frame.The significant features of this control regime are: Adaptable to NiCad and Lead Acid Batteries; also, the battery state of readiness can be established with reasonable accuracy for both types of batteries. Most importantly, this control regime is continually optimized for best performances.