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July 21 (UPI) -- Two NASA astronauts on a spacewalk Tuesday completed a years-long project to upgrade the power system at the International Space Station. Bob Behnken and Chris Cassidy installed a final, new lithium-ion battery, adding to 23 of the new power cells that replaced 48 aging nickel-hydrogen power cells.
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The space station is powered largely by solar power stored in batteries that are accessible only from space.
The spacewalk began at 7:12 p.m. EDT and lasted 6 hours, 29 minutes, the 10th such outing for both astronauts, according to NASA.
"Well it's a little more comfortable on the 10th one than on the first one," Cassidy said as he re-entered the space station. "The view is always amazing, though."
It was the fourth outing since June for Behnken and Cassidy, and the final spacewalk for Behnken before he is scheduled to return home Aug. 2 on SpaceX's historic Crew Dragon demonstration mission.
Behnken and Cassidy are now among only four astronauts in history to walk in space 10 times; Michael López-AlegrÃa and Peggy Whitson are the other two.
Behnken and Doug Hurley arrived at the space station May 31 as part of the historic return to human spaceflight from U.S. soil. The mission also marked the first time Elon Musk's SpaceX carried people into space.
NASA extended the SpaceX mission -- originally planned for five days -- to more than eight weeks so that the battery swaps could be finished.
Behnken and Cassidy also installed a protective unit that stores tools, providing them with thermal and physical protection outside the Space Station.
And, the spacewalkers successfully prepared the exterior of the space station's Tranquility module for a new commercial airlock built by Houston-based NanoRacks.
SpaceX plans to deliver the specialized airlock later this year that will enable public and private research on the outside of the space station.
Behnken and Hurley are due to splash down off the Florida coast less than 19 hours after they leave the space station Aug. 1.
