Operating pistol-grip power tools, the astronauts are loosening bolts that hold the batteries on the P6 -- or port 6 -- segment of the truss. Spacewalkers on the STS-127 mission now scheduled for launch aboard Endeavour on June 13 will remove and replace the batteries, which have been in orbit since December 2000.
The station's electric power system employs advanced nickel-hydrogen batteries to store electricity generated from solar energy collected by its four massive American solar wings. The batteries are charged as the station circles the sun-lit side of Earth and then power outpost systems when the complex is on the dark side of Earth. The station passes from the sun-lit to the dark side of Earth 16 times a day.
Click here for an MP3 primer on the ISS Power System
The batteries are made up of two units that each contain 38 individual pressure vessel battery cells. The station has 24 batteries -- or 48 battery units -- that each have a 6.5-year design life. The batteries are designed to charge-and-discharge 38,000 times and have a predicted five-year mean time between failure. They are made to be easily replaced by spacewalking astronauts.The first set of six batteries were integrated into the P6 truss segment, which was launched aboard Endeavour on Nov. 30, 2000. The on-orbit cycling of the batteries began in December 2003. The remaining 18 batteries were launched on later assembly missions.
Data beamed back from the P6 batteries show they have performed within their design specifications. Click here for an internal NASA report on the initial Battery Start Up
All aspects of the ISS battery hardware -- including design, development, assembly, test, and operation -- are managed by the NASA Glenn Research Center in Cleveland.
The replacement batteries to be installed in June were procured under a $145 million contract awarded to station prime contractor Boeing in 2003.
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