This paper describes a battery model that has been developed to predict whether the Extreme Ultraviolet Explorer spacecraft batteries will support proposed loads for user-specified time periods (usually about 2 weeks). For given orbit, attitude, solar array panel, spacecraft load, and time period, the model is able to calculate minute-by-minute values for the net power available for charging the batteries plus minute-by-minute values for the voltage, current, and state of charge. The model's calculations are explained for the beginning-of-charge, constant-voltage charging, and discharge phases. A comparison of predicted values with telemetry data shows good correlation. The model can be adapted to changing spacecraft conditions through the use of empirical data from ground tests and telemetry, and the model can be customized for other spacecraft. Nomenclature A, B = amplitude constants a, b = time constants, s c/d = (total charge supplied to the battery)/(total charge removed from the battery) per orbit D = solar-array-panel degradation factor eff = energy conversion efficiency factor / = battery current, A /o = battery current at the start of the constant voltage charging phase, A L = level of operation of the battery p= power available to charge the battery, W PI , PI -maximum power produced by each solar array panel at the beginning of the mission, W p sap = power from the solar array panels, W •P sc = power to the spacecraft load, W R = distance between the sun and the EUVE spacecraft, AU r = internal resistance of the battery, £2 T = temperature of the battery, °C t = time from start of constant voltage charging phase, s V = battery terminal voltage, V Viimit = maximum voltage of the battery that is allowed by the voltage limiter, V y = time from EUVE launch, years fti, fii = angles between the sun's direction and the normal lines of the first and second solar array panels
