For both Nickel Cadmium and NiMH, we use a common algorithm. The charge process consists of a four phase charging cycle, producing superior results in both chemistries.

Soft Start Mode - Used when the battery has been discharged. This gradually increases the charge current over the first two minutes of the charge process.

Fast Charge Mode - A constant current applied until termination.

Topping Mode - This mode completes the charge process, supplying C/10 for two hours after termination of the Fast Charge Mode.

Maintenance Mode - This mode supplies C/40 continuously to counter the self discharge of the battery.

Figure 2 - shows the complete charge process. During the first two minutes of the charge cycle, the battery is supplied an increasing current starting at .2 times the selected fast charge current. At the end of two minutes, full fast charge current is reached. Once termination has occurred, charge current switches to a C/10 charge rate to "top off" the battery,. This process lasts for about two hours. The final mode of operation is a steady current at C/40 to compensate for the natural self-discharge of the battery.

The charge controller samples battery voltage once per second, constructs a voltage curve from the data points, and calculates the first derivative of the voltage curve. When the slope of the voltage curve goes to zero, the fast charge mode terminates. In addition to using the voltage slope method for terminating the fast charge phase, the temperature slope method can be used.

Figure 2