When you leave the battery plugged overnight, the battery gets completely charged, it is still left plugged. This can be bad for battery life in the long run. When the smartphone charges up to 100%, “trickle mechanism” is activated in order to keep it at 100%. This causes high stress and high tension state. This condition wears down the chemistry within the battery.
Therefore, it is extremely important to unplug the battery from the charging device as soon as the battery is fully charged.