Sorry for the confusion, let me try again. There are three main things that reduce lithium ion cell life:
- Number of cyces (500 is ideal)
- Age since manufacter (they start to deteriorate before coming out of the factory)
- Time spent away from ideal/storage voltage (4.0 volts)
Theres not much to be done about cycles, its simply how much you need the device. Theres nothing to be done about age, its simply how long youve had it + how old it was when you bought it. But that last one can be helped. Lithium ion cells operate between about 3.0 and 4.2 volts. Depending on the nature of the particular chemestry and design of the device, 0% may be anywhere between 2.5v and 3.5v. 100% is 4.2v across most systems and chemestries. But all lithium ion cells prefer being around 4.0v most of the time. Above this voltage reduces cell life and below this voltage reduces cell life. Charging a cell to 4.1v instead of 4.2v, for example, can double the cells life while only sacrificing about 10% of the cells per charge runtime.
The problem is that Apple doesnt offer a charge to 4.0v option, nor even the ability to know which % shown equals 4.0v. When you plug it in before a trip, it charges to 4.2. When you leave your iPad plugged in over the weekend, it charges to and stays at 4.2 for a weekend. When you leave your MacBook plugged for an entire month, it charges to and stays at 4.2 for an entire month. The more you do this, the sooner and farther your cell's capacity will fall. To reduce exposure to 4.2v, I unplug my devices as soon as they are charged. Natural use then reduces time above 4.0 and increases time near 4.0.
The best trick for keeping a battery healthy is unplugging power as soon as you hit 100%.