An Apple Knowledge Base article says:
For computers with Mac OS X v10.4.x or later, use the steps in this article to determine the cycles on your Mac notebook’s battery.
Use of your Mac notebook’s computer battery accrues in the form of “charge cycles.” You may use your battery after it reaches its maximum cycle count, but you may notice a reduction in your battery life. Knowing how many charge cycles are on your battery and how many are left can help you determine when a battery replacement is required. For optimal performance, replace your battery when the maximum cycle count is reached.
Follow these steps to access information on your Apple computer’s battery, including the cycle count.
You can check it out at:
For more information on batteries consult: