Six NiH₂ batteries support the Telescope’selectrical power needs during three periods: when demand exceeds the Solar Array (SA) capability, when the Telescope is in Earth’s shadow, and during safemode entry. The batteries reside in Support System Module (SSM) Equipment Section Bays 2 and 3. These units have extensive safety and handling provisions to protect the Shuttle and its astronauts. The design and operation of these batteries, along with special nondestructive inspection of each cell, have allowed these units to be “astronaut-rated? for replacement during a servicing mission.

To compensate for the effects of battery aging, astronauts installed a Voltage/Temperature Improvement Kit (VIK) on each of the batteries during the SM3A servicing mission. The VIK provides battery thermal stability by precluding battery overcharge when the HST enters safemode, effectively lowering the Charge Current Controller (CCC) recharge current.

The Solar Arrays (SAs) recharge the batteries every orbit following eclipse (the time in the Earth’s shadow). The recharge current is controlled by the CCCs. Each battery has its own CCC that uses voltage-temperature measurements to control battery recharge. Fully charged, each battery contains more than 75 amp-hours. This is sufficient energy to sustain the Telescope in normal science operations mode for 7.5 hours or five orbits. The batteries provide an adequate energy reserve for all possible safemode contingencies and all enhancements programmed into the Telescope since launch.