Constant-voltage (often called constant-potential) chargers maintain nearly the same voltage input to the battery throughout the charging process, regardless of the battery's state of charge.
What is constant current charging?
Constant current charging is when the charger supplies a set amount of current to the battery, regardless of the voltage. This stage is used to overcome any internal resistance in the battery so that it can be charged as quickly as possible. After the initial constant current stage, the charger then switches to a constant voltage mode.
Why does the charging current decrease as the battery charges?
Since the voltage is constant, the charging current decreases as the battery charges. A high current value is required to provide a constant terminal voltage at anearly stage of the charging process.
However (quoting you): charging at a constant voltage (say 4.2V) so long as the maximum current is limited to a reasonable value for the cell means you will have constant current charger till your cell is at ~95%. Up to this point the voltage across the battery will be less than 4.2V if you measure it.
What is the difference between pre-charging and constant current charging?
Pre-charging is when the battery is initially plugged in and is drawing a very small amount of current in order to get the chemical reaction started within the battery. Constant current charging is when the majority of the charge is applied to the battery.
How do you charge a battery?
There are three common methods of charging a battery: constant voltage, constant current and a combination of constant voltage/constant current with or without a smart charging circuit. Constant voltage allows the full current of the charger to flow into the battery until the power supply reaches its pre-set voltage.
The current will remain constant until the voltage rises to 28V. At this point the power supply will transition to constant voltage mode and the current will decay to zero when the battery is fully charged. The charge current is controlled to avoid overheating and the float voltage limited to avoid over-charging.