Washington, July 14 : Two NASA astronauts will conduct a pair of spacewalks on July 16 and July 21, to finish a 3.5-year effort to upgrade the International Space Stations power system, said the US space agency. The spacewalks will begin around 7.35 am, and could last up to seven hours, NASA said on Monday. NASA astronauts Chris Cassidy and Robert Behnken will replace ageing nickel-hydrogen batteries with new lithium-ion batteries delivered to the station on a Japanese cargo ship in May. The replacements will be on the starboard 6 truss' 3B power channel. During the July 16 spacewalk, they plan to remove five of six older nickel-hydrogen batteries for the truss' power system and install three new lithium-ion batteries, as well as accompanying hardware. The last nickel-hydrogen battery will be removed from the truss and stowed on July 21, when Behnken and Cassidy venture out on the 300th spacewalk involving US astronauts since Ed White stepped out of his Gemini 4 capsule on June 3, 1965. In all, 12 spacewalks will have been performed since January 2017 to change out batteries for eight power channels used to route electricity on the station, NASA said. These will be the 230th and 231st spacewalks in the history of space station assembly, maintenance, and upgrades. They also will be the ninth and 10th for Cassidy and Behnken, who will join former NASA astronauts Michael Lopez-Alegria and Peggy Whitson with the most spacewalks by Americans. Cassidy arrived at the space station in April, taking command of Expedition 63. Behnken, who is serving as a flight engineer for the expedition, arrived at the station in May with fellow Commercial Crew astronaut Douglas Hurley on SpaceX's Crew Dragon Demo-2 test flight. /IANS