CAPE CANAVERAL, Fla.—A pair of NASA astronauts ventured out on their second spacewalk in under a week Monday to complete a four-year effort to modernize the International Space Station’s power grid. Over the weekend, flight controllers in Houston used the station’s big robot arm to replace the last pair of old-style batteries with a single better-quality one. Astronauts Mike Hopkins and Victor Glover needed to put the finishing touches on this newest lithium-ion battery to complete a series of spacewalks that began in 2017. “Beautiful day. Let’s go for a walk outside,” Mission Control radioed as the spacewalk got off to a late start. The space station is now equipped with 24 lithium-ion batteries to store power collected by the solar panels. The big, boxy batteries, surpassing 400 pounds (180 each, provide electricity for the orbiting lab when it’s on the night side of Earth. They’re so powerful that only …