About battery cycles
When you use your Mac notebook, its battery goes through charge cycles. A charge cycle happens when you use all of the battery’s power—but that doesn’t necessarily mean a single charge.
For example, you could use half of your notebook's charge in one day, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two. In this way, it might take several days to complete a cycle.
Batteries have a limited amount of charge cycles before their performance is expected to diminish. Once the cycle count is reached, a replacement battery is recommended to maintain performance. You can use your battery after it reaches its maximum cycle count, but you might notice a reduction in your battery life.
たとえば1日でバッテリーの半分を消費し、そのあと満杯まで充電し直すとしましょう。明くる日も同じことをした場合、charge cycleは2回ではなく1回と数えます。ですから1回のcharge cycleに数日かかることもあります。