Even though a charger turns off the juice when your phone reaches 100%, the charger will continue to top off the charge during the night, says Bradshaw. Such a “trickle charge” attempts to keep it at 100% to compensate for the small bit of charge that your phone just naturally loses on its own. So your phone is constantly being bounced between a full charge and a bit below a full charge. These trickle charges can lead to higher ambient temperatures for your phone, which can reduce capacity over time.