Astronauts performing a spacewalk on the International Space Station have upgraded the batteries on the power grid on the ISS. NASA says that all three lithium-ion batteries are now functional, a big step forward in the effort to upgrade the solar power system on the space station, which is starting to show its age.

Commander Shane Kimbrough and Peggy Whitson, along with the help of a robot named Dextre, worked to replace the nickel-hydrogen batteries on the space station, which are now about a decade old. The batteries are about the size of half a refrigerator, making it a painstakingly slow process.

The new batteries were delivered last month in a cargo capsule sent by the Japanese. They have installed three of the six batteries, and will install the remaining three next Friday. Overall, there are 48 nickel-hydrogen batteries that make up the solar power system on the ISS, and it will take two to three years to change them out.