Astronauts on the shuttle Endeavour conducted a fourth space walk at the International Space Station Friday. The crew used the mission to replace batteries that store power from the station's solar panels.

Astronauts Chris Cassidy and Tom Marshburn donned their protective suits and ventured outside the shuttle Endeavour to install four new batteries for the space station.

The space walkers fell slightly behind schedule when they struggled to access one of the batteries located in a tight spot on the station. Commander Mark Polansky assured the two that they could finish the work during the 7 1/2 hour space walk.

"Hey guys, I wouldn't worry about the timeline because the next two batteries are going to be real efficient with the slots, so you guys keep pressing at the exact same pace. You are doing great."

"Don't worry. We are not worried about the timeline."

During an earlier space walk, Cassidy was forced to return to the shuttle before finishing his work, because NASA technicians noticed a buildup of carbon dioxide in his suit. Officials said the former Navy commando was working so fast that the air-cleaning device could not keep pace with him.

The shuttle crew used the earlier space walk to install two new batteries which store the energy collected by the space station's solar arrays. The Endeavour will carry the old batteries, which weigh 168 kilograms each, back to Earth.

Astronauts also finished installation of the final segment of Japan's Kibo science module, which will be used to conduct experiments that expose materials to the vacuum of space.

One more space walk is planned to complete additional repairs and upgrades on the station. The Endeavour crew is set to complete its 16-day mission and return to Earth next Friday.