Laptop batteries have a certain life-span. It is difficult to tell whether a battery is near dying just by looking at it, but the software gives it away. The System Profiler on a MacBook gives away the health information of the battery.
In the Apple menu at the top left, click on About This Mac. Then click on More Info? Then go to the Power section and check out the cycle count. The MacBook Pro battery longevity is about 800. The battery health is normal when it cycles in the 200s. If the battery is not acting right, it will say something like “Check Battery.”