This should help.
There are also free applications to calibrate your battery and improve its performance, i recomend you to get Battery Health from the App Strore, its very simple to use and gives tips to improve battery life.
You may need to let the battery completely discharge such as leaving it on overnight without going to sleep. I'd do that at least once a month. Then completely recharge it; leave it charge at least another hour longer than when the bettery says it's fully charged. Doing that regularly should help. Post back if it didn't seem to help.