Kevin Purdy, iFixit:
All batteries wear out over time, but they don’t wear out at the same speed. You and someone else, given the same brand-new laptop on the same day, could have quite different battery life after two years—maybe as much as a 40 percent difference. It depends on charging levels, heat, how you store it, and avoiding the deadly zero-charge.
Here’s the best advice for keeping your laptop battery running as long as it can on its little pack full of chemicals. Don’t worry if you can’t swear an oath to them: like 8 cups of water a day, or 10,000 steps, they’re guidelines, and your life may have present exceptions. But following as many as you can will yield good results over years of use.