Nickel Cell Charging Voltage required

Q1) If I have 3 Nickel cells how much input voltage do I need?

A1) Just Prior to termination on a new cell near 28C the voltge is 1.45V per cell.  Over the age of the cell and over temperature this value could approach 1.6V per cell.  One wants to stay in the constant current mode so the voltage regulation should be set slightly higher than the worst case for a healthy cell.  The setting sould be between 1.7 and 2.0V per cell.  I like 1.7V per cell so a three cell series pack would require an output regulation of 5.1V.  Depending on your charger topology and if it can go to 100% duty cycle and the drop out of the charger one may need 1 to 3V more voltage to achieve the 5.1V regulation.  Thus an input votlage of 6 to 9V may be required.....make sure that the charger stays in CC mode during the full charge up to at least 1.6V per cell.