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NASA astronauts conduct their third spacewalk in 3 weeks

Expedition 36 Flight Engineer Chris Cassidy is pictured in July 9, 2013, during a six-hour, seven-minute spacewalk at the space station. By Ashley Strickland, CNN

(CNN) -- Early Thursday, NASA astronauts Chris Cassidy and Robert Behnken conducted a spacewalk outside of the International Space Station to replace nickel hydrogen batteries with lithium ion batteries for one of the station's power channels.

This is the culmination of the power system upgrades to the space station that have occurred over the last three and a half years to replace 48 aging nickel-hydrogen batteries with 24 new lithium ion batteries.

The upgrades will require 12 spacewalks in total, since January 2017, to replace the batteries. One lithium ion battery installed last year shorted out and will be the final battery replacement installed on an upcoming spacewalk.

Thursday's spacewalk began at 7:10 a.m. ET and ended six hours later at 1:10 p.m. ET.

Both astronauts are veteran spacewalkers. This was the ninth venture outside for both Cassidy and Behnken, according to NASA.

Behnken, along with NASA astronaut Doug Hurley, launched from the United States and joined Cassidy on the space station on May 31. They were aboard SpaceX's Crew Dragon during the Demo-2 mission.

This spacewalk, similar to ones the two astronauts have conducted recently, focused on replacing batteries for one of the power channels on the far starboard truss of the station.

During their last spacewalk on July 1, the astronauts worked on tasks scheduled for later spacewalks, routed power and ethernet cables and laid the groundwork for future power system upgrades. These cables will provide better views on future spacewalks, according to NASA.

These power system upgrades, however, are nothing like replacing batteries in your remote. The new batteries each have a mass of 428 pounds.

Cassidy and Behnken removed six older nickel-hydrogen batteries and replaced them with three new lithium-ion batteries on Thursday.

Hurley helped Cassidy and Behnken into their spacesuits and supported the astronauts from inside the space station.

The battery replacements, which will have a 20-year lifetime, will put the station in a much better configuration for the long term, said Kenneth Todd, deputy International Space Station program manager, during a recent NASA press conference.

Behnken recently discussed the spacewalk, and why it's important to replace the batteries, during a call to the space station from CNN Innovation and Space Reporter Rachel Crane.

"When the space station is in the sun, it's collecting energy and it needs to store for when it's in the dark," he said. "And so those batteries, as they're cycled time and time again, they wear down and need to be replaced. And so periodically that maintenance is required."

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