There have been questions as to whether leaving your smartphone plugged in for a long time will damage the battery. The truth is when you plug your phone for a long time, it will stop charging when it attains full bar which will subsequently switch to full trickle effect to maintain it at full bar.
Majority of us while growing up especially in Nigeria have perhaps one time in our life heard that battery gets spoil when plugged in for a very long time. The truth is there are no negative effects when plugged for long.
Phones produced nowadays now use Lithium or lithium polymer. Wikipedia defines, “lithium-ion battery or Li-ion battery as a type of rechargeable battery in which lithium ions move from the negative electrode to the positive electrode during discharge and back when charging” while it explains its predecessor, nickel–cadmium battery, also known as NiCd battery or NiCad battery as a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. The former do suffer from memory effect. With lithium ion battery, your phones are perfectly safe and you can charge them for as long as you want without battery life span being affected.