If you have Xiaomi PB, you will notice that when it charge a phone to full status, it will STOP charging even if you leave it connected to powerbank the whole night. Your phone will start draining its own battery instead. This is a very good safety feature.

If you charge via normal charger, it will change to trickle charge that supply minimal amp to keep your phone at 100% till morning. At trickle charge, both phones and charger are cool too touch coz only minimal current goes thru.