My MBA has had "service battery" on it for years and I've ignored it because it still worked fine. But a few weeks ago, I kept having problems with the charge dropping down to zero so I downloaded coconutBattery and it said that my battery capacity was at 30%. I ordered a replacement battery but then about a week later, the capacity was suddenly at 50%. Now today, it's at 60%. How does the battery's life keep going up?