I repurposed my son's old 2011 Air. He'd been using Bootcamp 100% of the time but had Yosemite installed. When I booted back to Yosemite it had the Service Needed warning on the battery. I then booted off a Mavericks installer to wipe the hard drive and return it to one partition. After the Mavericks installation completed it still said Service Needed. I then upgraded to El Capitan and after that it now says that the battery is Normal. Does that sound normal?