Why A Battery Never Gets Back To 100%

I always assumed a battery wore out from being used. It mostly does not.

Inside a cell, lithium just shuttles back and forth — nothing is consumed by design. What takes capacity away is a crust that grows on the anode, and that crust is built out of lithium which never shuttles again. The part I did not expect: it keeps growing while the phone sits in a drawer, and it grows faster when the phone is full and warm. A spare phone left at 100% loses more in two years than one cycled every day around half charge.

I built an interactive page for it — a cross-section of a cell with the ions moving, and the crust thickening as the years run. Move the charge ceiling, the temperature, or the hours it spends on the charger, and it all re-runs.

Have a look, and move the controls yourself.