My mom has a MacBook Air 13" (mid 2012). It was running Yosemite and had a battery issue. As soon as I disconnected the power cable the computer would go blank and would only be usable when connected to the power cable. I have a genius bar appointment later today. While at work I thought I would do an update to Sierra. Now with the MacBook Air updated to Sierra it runs normally. In Yosemite the battery had "service" displayed, in Sierra it shows condition "normal". Could it simply have been an OS version or should I still look at replacing the battery?