The battery works off of full charge cycles. The example that Mophie gives is that you can charge it 10% for 10 times and it'll only increment 1 full charge. That's how it works. So if you're charging it 40% today, 20% tomorrow, 20% the day after, 10% the next and 10% the last, you've only charged it one full cycle. But it's recommended that you don't make it a habit. Or if you do, at least completely discharge your device biweekly to keep the battery healthy.