Measure the Capacity of a Mac Battery Using Coconut Battery

Does it seem like your Apple notebook just won't hold a charge anymore? Perhaps the battery no longer has the capacity that it once did.

Steps

Download and run Coconut Battery. This application (which does have also have a widget version) will tell you how old your computer is, what the current capacity of your battery is and how many load-cycles your battery has run through.
Understand when there is a need to replace a battery. Carefully understand the result given by the app. If you realize that there is a major difference in the original and current capacity of the battery then you must think about replacing.
If you find that your battery performance is less than acceptable, replacement batteries are generally between $100 and $200. It's always better to use the Genuine Apple products and it is always a better decision to avoid the usage of batteries manufactured by some third-party vendors.