US astronauts Chris Cassidy and Tom Marshburn on 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.

Wednesday's spacewalk saw Cassidy and astronaut Dave Wolf replace only two batteries instead of the four 170-kg batteries that had been planned. That work was cut short due to the spacesuit problems.

Each solar panel on the station has six batteries, which were designed to be changed after six and a half years. Because they are so large the work must be done while NASA is still operating the space shuttles that are capable of carrying large loads, the space agency said.

A fifth and final spacewalk is planned for Monday, when astronauts will put a video camera on the Japanese Kibo laboratory's new porch, which was installed earlier in the mission allowing experiments to be exposed to the extremities of space.