When charging an empty lithium ion battery pack,the charger output a constant current (C.C.), the battery voltage will increase slowly, when this voltage reaches N*4.20V  or N*3.65V ("N" means the serial number of battery cell pack, 4.2 is for Lithium ion battery, 3.65 is for LiFePo4 battery type), it's also called float voltage or CV point, the charger will use constant voltage methord to charge the battery pack continously. At this stage, charge current will decrease slowly. When the charging current drop to 0.1CC level, the battery is full charged. One variation is to start a timer when entering the CV stage (float voltage charging), even charge current is >= 0.1CC but the timer is out, the charging process will be cut-off automatically. 

Following curve is a real record data from P1004-18L battery charger, battery pack is 3600mA 3.7V Li-ion type

Lithium ion charging curve ( blue-Voltage, white-Current, green-Temperature)