Depending on the year/model you're using, the capacity count will help you determine when to replace the battery
The older laptop, I believe from Jan 09 need to have the batteries replaced after a 250 cycle count.
Just an FYi for those who have Apple Care. Apple will replace your battery if it has not gotten to 180 cycle count.
This usually equates to about 6-8 months.
When you get close to 180 cycle count, call Apple claiming your computer keeps shutting off when it gets to 50%.
Apple will ask you to run a diagnosis to check the battery. They will see that there is nothing wrong with the computer but, will still send you a new battery to replace the old one. Repeat this process again when it gets to 180 cycle count again, or before your apple care expires. This time, after they run the diagnosis and see there's nothing wrong, they probably won't send you a new battery being that they sent you one already a few months ago. At this point, or before they run the diagnosis, hold the power button down until it goes off. When back at startup, they'll run the diagnosis and no problem will show up. But being that they were on the phone, believing it shut off, they'll send you a new battery. Get it???
I've done this for the last 3 years, and have gotten a fresh battery free of charge each time.
My lil contribution to NT.