All batteries die over time, and OS X offers a quick way to determine if yours is on its way out.
Topher KesslerMacFixIt Editor
Topher, an avid Mac user for the past 15 years, has been a contributing author to MacFixIt since the spring of 2008. One of his passions is troubleshooting Mac problems and making the best use of Macs and Apple hardware at home and in the workplace.
If you have a laptop system, then it helps to ensure the battery is working properly, since as with any system, over time and use the battery will begin to lose its charging capabilities and give you less and less running time. To compensate for the gradual loss in charging capability, it may help to regularly calibrate your battery to ensure its charge is accurately reported to the system, but this will not give you any indication of the battery's overall health.
To check the condition of your battery, first go to the Energy Saver system preferences and check the option to show the battery status in the menu bar (this should be enabled by default on most laptop systems). In the battery menu that appears you will be able to see the power source and estimated charge time remaining in addition to links for opening the system preferences; however, if you hold the Option key when opening this menu, then you will see a Condition option appear that will tell you the health status of your battery.
The available health conditions are the following:
Normal -- the battery is working as expected
Replace Soon -- the battery should hold ample charge to allow you to work, even though it holds less than its original capacity
Replace Now -- the battery will not hold enough charge to give you ample work time
Service Battery -- the battery is experiencing an error (regardless of how new it may be) that could result in an unstable capacity. While it should continue to function, it is recommended you have it serviced.
With these options available, if your battery is only giving you a few hours of running time or is not charging fully even after calibrating, then be sure to check its health using these options to see if the system is detecting a problem with the battery. If not, then you might need to look into what programs and services are running that could be draining the battery, but if the system does report poor battery health then you should have the battery replaced.
This information about the battery can also be found in the System Information utility (available by generating a full-system report in the About this Mac dialog box in the Apple menu). By selecting the Power section in the generated report, you will see charge and health information about your battery, including the number of complete charge cycles it has been through and the estimated condition determined from these charge cycles (the same that is reported in the battery menu).
A charge cycle is a full discharge and recharge of your battery, and the system determines the number of these by summing up every partial use and recharge of your battery. For instance, if you fully calibrate your battery and discharge it followed by a recharge, then that counts as one charge cycle. If you use half of your battery one day and charge it, then a quarter of it the next day, followed by a quarter the final day, and then charge it back up, you have in these uses put another charge cycle on the battery.
The number of charge cycles your battery is capable of will vary between 300 and 1,000 depending on the system you own, which under most circumstances should give you at least several years of battery life, but this will vary for each system and how it is used. You can look up the estimated maximum charge cycle count for your system in this Apple Knowledgebase document.