The charging of lithium-ion batteries is generally carried out by the CCCV method as follows
When the voltage per cell exceeds 2.9V, start charging with the specified current, and after confirming that the battery voltage has risen and is charged to the specified voltage (e.g. 4.2V), transition to constant voltage charge by the CVCC function of the charger.
When the terminal voltage is charged with the specified voltage (e.g. 4.2V), the charging current gradually decreases.
This process is called constant voltage charge.