Cody,
It has been a while since I designed battery chargers, but I believe that most chargers do not charge with a simple voltage source. The reason for this is that the charging current could be excessive and damage the battery. What is commonly used is a current limited "float" voltage. Therefore, if the cell (or battery) is depleted, then the current limiter determines the maximum current into the cell. As the cell is charging, then the cell voltage will rise towards the "float" voltage. As the cell reaches the "float" voltage then the charging current goes to zero (in this simple charging technique).
Another common charging technique is to use a "trickle" current that continues to flow even after the float voltage is reached.
* When the cell is not completely charged, then the electrical energy into the cell is converted into chemical energy. Since the charging process is not 100% efficient, then there is a portion of the electrical energy that is converted into thermal energy and increases the temperature of the cell to allow for the thermal energy to be transmitted away from the cell. If the charging current is too great, then the temperature rise in the cell can causes damage to the cell. For this reason we have to be careful not to use excessive charging current.
* When the cell is completely charged, then the electrical energy into the cell is all converted into thermal energy and can give rise to cell damage if that continued current is too great. The "trickle" charge is selected to "top off" the charging process without causing damage to the cell. One will note from the discharge curve that charging the cell to the last millivolt does not provide much extra capacity in the cell.
HH,
Glenn