Discharging your LIB(Lithium Ion Battery) fully (or less than 2.4 volt per cell) is bad for the battery. Whenever you do that, it can be said that a small part of your battery dies(they lose their charge forever). Don’t store your batteries depleted as there is a chance that they will die completely or become very ‘weak’.
You cannot resore bad LIBs by overloading or heating (or praying). You have to buy a new battery. They DO degrade over time as some cells naturally lose their ability to give/gain electricity.
Although it is said LIBs don’t have memory, it is not entirely true. LIBs have gauges that monitor the performance of the cells. If you do a lot of small charges, it won’t let those gauges to monitor a full battery potential, causing an invalid indication of charge levels. A complete charge/discharge should be done when the battery capacity seems reduced, that will calibrate the gauges and they will provide your phone with correct charge level status. A full charge-discharge cycle could be done for every 30 or so partial charges.
LIBs have a self life. Do not buy them to store them. They will die whether you use them or not. Don’t buy LIBs to use them in the future. Buy them when you actually need them.
LIBs have short lives (in comparison with NiCa batteries). You should expect to buy a new battery in two to three years after it is manufactured. It is caused by internal oxidation and there is nothing you can do to stop it.
The worst LIBs treatment is keeping it at 100% charge level at high temperature( like under the direct sunlight).
The most heathy LIBs charge level is 40%. This is the level of charge you buy them with.
LIBs do not like heat. For example, while always at 100%, typical LIB in laptop, at a temperature of 25℃ will lose 20% of its full capacity per year. This capacity loss is reduced to 6% at 0℃ and increased to 35% at 40℃. So keep the battery( and so your phone) from over exposure to the direct sun and don’t let it overheat at charge. (but while charging, the battery will be hotter than the outside environment for sure)
LIBs like frequent partial charges/discharges than full charges/discharges.
Cars “fast-chargers” over time degrade your battery life a little as they give too much energy to your battery too fast (high voltage). Trickle charge(USB) is the best. The general idea is that slower is the charge, longer will be life of your battery.
Here is a chart showing how the various feature of your device consume the power.