You can check the condition of your battery by clicking the battery icon on the top right, where you’ll see one of four conditions: Normal, Replace, Replace Now, or Service Battery. Common sense suggests replacing the cell if prompted, but you can dig deeper into the issue by checking the cycle count on the back end. Hold down the Alt key and click the Apple icon at the top left, and choose System Report/Profiler. From there, select Power and check the battery’s current cycle run.
If it's indeed time for a replacement, you’re afforded three options. One is having Apple take care of it with service ranging from $130 to $200. You can see if the MacBook is still under warranty or protected under AppleCare. Or, take the gamble on a third-party battery and install it yourself. Doing so won’t void your warranty -- but any damage caused during replacement will. If you're feeling risky, Next Level Tech provides a simple video tutorial on how to do it yourself.