Washington, July 25 (DPA) US astronauts Chris Cassidy and Tom Marshburn Friday undertook an intense spacewalk to change batteries on a solar panel of the International Space Station (ISS).

During seven hours and 12 minutes of work, the astronauts replaced four batteries that collect and store energy from the station's solar panels. The old batteries are to return to Earth aboard the space shuttle Endeavour.

The spacewalkers had more work to do than originally planned after a spacewalk Wednesday was cut short due to problems with carbon dioxide buildup in Cassidy's spacesuit. A piece of equipment that filters the gas from the air inside the suit was not working properly, but Cassidy was never in imminent danger, NASA said.

The spacesuit has been repaired and the astronauts are trained in what to do in case of an emergency, NASA said.