WASHINGTON, July 22 (Xinhua) -- Two astronauts conducted a spacewalk on Wednesday and replaced two old batteries for the International Space Station (ISS), NASA said.

    According to NASA TV, astronauts David Wolf and Chris Cassidy began their spacewalk about 30 minutes ahead of schedule, exiting the space shuttle Endeavour's decompression chamber at 10:32 a.m. EDT (1432 GMT).

    It was the third spacewalk in five days for the Endeavour crew.

    They were supposed to replace four batteries for the ISS. However, NASA decided to end the spacewalk earlier than planned because of higher than normal carbon dioxide levels in Cassidy's spacesuit. The remaining batteries will be replaced on a future spacewalk.

    These batteries, storing the power collected by the space station's solar wings, are quite critical. The old ones were launched in 2000. NASA is uncertain how long those original batteries might last and wants new ones installed before the old ones die.

    Endeavour lifts off on July 15 from Kennedy Space Center in Florida after five delays, on a track to the ISS. The shuttle will undock from the space station on Tuesday. Landing is set for July 31.