Two American astronauts ventured outside the International Space Station on Friday to upgrade the orbiting outpost's electrical system.

Americans Shane Kimbrough, the 49-year-old commander of the six-person crew aboard the ISS, and flight engineer Peggy Whitson, 56, officially ended their expedition at 1855 GMT.

During their approximately six-and-a-half hour spacewalk, the pair installed new adapter plates and hooked up electrical connections for three of six new lithium-ion batteries installed on the station's truss, according to the US space agency NASA.

Japan's uninhabited H-II Transfer Vehicle, or HTV, delivered the batteries last month. The Canadian-built Dextre robotic arm began the installation to pave the way for the astronauts' work.

The new batteries are expected to last as long as the ISS is expected to remain in use -- until at least 2024.

Kimbrough and Whitson were assisted by European Space Agency astronaut Thomas Pesquet, 38, and Oleg Novitskiy of Russia from inside the station. The entire orbital ballet was conducted by the Johnson Space Center in Houston, Texas.

Kimbrough and Pesquet are scheduled on January 13 to conduct another expedition to connect the other three lithium-ion batteries.

The vessel that brought the new batteries will be unplugged in February after being loaded with waste and old equipment, including nine of the old nickel-hydrogen batteries. It will then plunge back into the atmosphere and disintegrate over the Pacific Ocean.

Friday's spacewalk marks Whitson's seventh, matching the record previously set by fellow American Suni Williams for the most spacewalks by a woman.

It is also the 196th overall spacewalk for space station assembly and maintenance.