Comparison between ELAN test results and an improved Zimmerman model

ELAN (Essais de Longevite d'Accumulateurs Nickel-cadmium) is a joint CNES-ESA program carried out in the European Space Battery Test Centre (ESBTC) at ESTEC, whose main objective is to provide an extensive data base on the lifetime performance of nickel-cadmium (NiCd) batteries in support of low earth orbit (LEO) spacecraft projects. This test program includes twenty NiCd batteries of twelve cells each which are subject to LEO cycling with voltage-limited taper control under various Depth of Discharge (10%, 20%, 30% and 40%) and temperature (-5°C, 5°C, 15°C and 25°C) conditions. These tests have been running for more than ten years now and provide a mine of information for people interested in battery modeling. However the huge amount of data available (about 20 GB !) made it necessary, first of all, to record only one cycle per thousand, then to develop a data base and EXCEL® based software able to provide direct access to the measured variables (battery current and cell voltages, amongst others) of any cycle. For a given cycle, this software is capable of extracting the variations of several variables and displaying them automatically. Among the widely known battery models, the Zimmerman model based on a non-linear capacitance seems to represent the behavior of a battery quite well. However, the initial model turns out to he imperfect for wide ranging variations. That is why this model needs improving. The solution which was found consists of using two different capacitances for the charge and the discharge, which are updated at each step of calculation, as functions of the temperature and current. A tool has been developed specifically to allow easy comparison of the simulation and the test results. This comparison has been carried out for every ELAN test and shows a reasonable correlation between the model and the test results.