Over the last several years, Apple has dramatically improved how it handles lithium-ion battery charging in iPhones, iPads, Macs, and Apple Watches. Across multiple system releases, the company moved from always charging to 100% to a more careful analysis of how much charge is needed based on your daily usage patterns.
Lithium-ion batteries have a remarkable number of charge cycles. You can charge them fully, drain them, and charge them full again hundreds of times without seeing much degradation in full charging capacity. This makes them ideal for smartphones, tablets, laptops—and electric vehicles.
But this category of battery has a problem that works a bit like pressurization. Fill a balloon to a certain amount, and it’s pliable and safe and deflates slowly over time. But try to estimate what its 100% maximum is, and you might err, and it pops suddenly. To avoid this, hardware manufacturers charge Li-ion batteries to a safe level below their theoretical maximum capacity.
Read more at Macworld.com