the newer Nickel-Metal Hydride batteries have no memory effect unlike the older nickel cadmium. they are cheap and last a long time
http://data.energizer.com/batteryinf...al_hydride.htm
Memory/Voltage Depression
The issue of "memory" or voltage depression has been a concern for many designers of devices, using nickel-cadmium cells. In some applications where nickel-cadmium cells are routinely partially discharged, a depression in the discharge voltage profile of approximately 150 mV per cell has been reported when the discharge extends from the routinely discharged to rarely discharged zones. While the severity of this problem in nickel-cadmium cells is open to differing interpretations, the source of the effect is generally agreed to be in the structure of the cadmium electrode. With the elimination of cadmium in the nickel-metal hydride cell, memory is no longer a concern.