I have explained a little bit about that in "
How do I test a charger", it is something I hope to improve one day.
A perfect LiIon charge curve looks something like this:
Note: There are two charge curves on top of each other.
The yellow line is where the charger says full and hopefully stops charging.
Notice the cross over at 174 minutes, the optimal is if green line starts to drop at the same time the red goes to horizontal. Most chargers has a more soft change, this will make them slower, and will not damage the battery.
The part where the green line is horizontal is called constant current (CC) and the part where the red line is horizontal is called constant voltage (CV), hence the label CC/CV charger.