VOLTAGE AND CAPACITY STABILITY OF THE HUBBLE TELESCOPE NICKEL-HYDROGEN BATTERY

The power system of the Hubble Space Telescope includes two orbital re placement units, each containing three nickel-hydrogen (Ni-H-2) batter ies of 88 Ah capacity. Since launch in April 1990, the batteries have completed 23 000 charge and discharge cycles and continue to meet the power demands of the satellite. The voltage, capacity, and pressure ch aracteristics of all six batteries were analyzed to determine the stat e of health of the battery and to identify any signs of performance de gradation. The battery pressures have changed to varying degrees. The end-of-charge pressure for battery 4 increased by 96 psi, while that f or battery 3 decreased by 37 psi. The voltages of the individual cells show a decay rate of 0.69 mV per 1000 cycles, and the capacity of the batteries has apparently decreased, possibly due to the system being operated at a lower stage of charge. Autonomous battery operation invo lving charge termination at a preselected voltage continues to restore the energy dissipated during each orbit. The accumulated data on volt ages and recharge ratios can be used to design new temperature-compens ated voltage levels for similar missions that employ Ni-H-2 batteries.