Hubble Space Telescope on-orbit NiH2 battery capacity performance (1990 to 2002)

This work summarizes the Hubble Space Telescope nickel-hydrogen (NiH₂) battery capacity performance from launch to the present time. Over the life of HST vehicle configuration, equipment degradation and equipment failures have had a significant effect on the capacity of the HST batteries. Changes made to the charge system configuration in order to protect against equipment failures and to maintain battery thermal stability resulted in undercharging of the batteries. This undercharging resulted in decreased usable battery capacity as well as battery cell voltage/capacity divergence. This cell divergence was made evident during battery onorbit health checks by a more shallow slope of the discharge curve following the discharge knee. Early efforts to improve the battery performance have been successful. Onorbit health check data indicates increases in the usable battery capacity of all six batteries as well as improvements in the battery cell voltage/capacity divergence. Additional measures have been implemented to improve battery performance, however, failures within the HST Power Control Unit (PCU) has prevented verification of battery health. As this PCU fault prevents the execution of on-orbit capacity testing, the HST Project has based the battery capacity on trends, which utilizes previous onorbit battery capacity test data, for science mission and servicing mission planning. The servicing mission 3B, scheduled for February 2002, will replace the faulty PCU. Following the servicing mission, on-orbit capacity testing will resume.