Some android apps notifies user that the charging is up to 80%, and tells to unplug the charger. Batteries are not allowed to remain at 100% charge since it is harmful to the battery and the battery charging percentage rapidly drops to around 90% as explained by researches. This is a precaution implemented by OEMs
Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. In terms of longevity, the optimal charge voltage is 3.92V/cell. Battery experts believe that this threshold eliminates all voltage related stresses.
Lithium-ion has not yet fully developed and is still improving. Much advancement has been made in longevity and safety while the capacity is increasing incrementally. Today, Li-ion meets the expectations of most consumer devices but applications for the EV need further development before this power source will become the accepted norm. Environmental conditions, not cycling alone, check the longevity of lithium-ion batteries. The worst situation is keeping a fully charged battery at elevated temperatures. IC in the battery communicates battery parameters to the Operating system. Drivers in the OS, that read the values from the battery and amongst other things, display the battery percentage.
The capacity of a rechargeable cell or battery changes over its life. The definition of the battery life or cycle life of a battery is number of cycles that a cell or battery can be charged and discharged under specific conditions, before the available capacity falls to a specific performance criteria - normally 80% of the rated capacity. Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling
A discharge/charge cycle is commonly understood as the full discharge of a charged battery with subsequent recharge, but this is not always the case. Batteries are seldom fully discharged, and manufacturers often use the 80 percent depth-of-discharge (DoD) formula to rate a battery. This means that only 80 percent of the available energy is delivered and 20 percent remains in reserve. Cycling a battery at less than full discharge increases service life, and manufacturers argue that this is closer to a field representation than a full cycle because batteries are commonly recharged with some spare capacity left.