Astronauts on board the International Space Station (ISS) have successfully completed their second spacewalk in 2017, which was aimed at upgrading batteries that connect to the main power system.

NASA astronaut Shane Kimbrough and Thomas Pesquet of the European Space Agency (ESA) stepped outside the orbiting lab at 12:20 p.m. EST (17:20 GMT) almost half hour earlier than planned. It was the fourth spacewalk for Kimbrough in his career and the first for Pesquet..

In the six-hour long spacewalk, they installed six new lithium-ion batteries with adapter plates, replacing the station's previous nickel-hydrogen batteries. These were sent to the ISS in December through a Japanese HTV cargo craft.

NASA has said nickel-hydrogen batteries will continue to be a part of its power upgrade for the ISS over the next couple of years and more batteries will be delivered to the station in future.

Each power channel on the station has three strings of batteries, with each string containing two pairs of nickel-hydrogen batteries. And each battery pair has to be switched out with one lithium-ion battery and an adapter plate, according to The Verge.

Earlier this week, robotic ground controllers used the "Dextre", a Special Purpose Dexterous Manipulator that works as a two armed robot, to move three new lithium-ion batteries for the latest spacewalk.

Spacewalkers Thomas Pesquet (left) and Shane Kimbrough being helped to suit up by astronaut Peggy Whitson (center) NASA