Ni-Cd batteries, and to a lesser extent Ni-MH batteries, suffer from what's called the "Memory Effect". What this means is that if a battery is continually only partially discharged before re-charging, the battery "forgets" that it has the capacity to further discharge all the way down.
To illustrate: If you, on a regular basis, fully charge your battery and then use only 50% of its capacity before the next recharge, eventually the battery will become unaware of its extra 50% capacity which has remained unused. Your battery will remain functional, but only at 50% of its original capacity. The way to avoid the dreaded "Memory Effect" is to fully cycle (fully charge and then fully discharge) your battery at least once every two to three weeks. Batteries can be discharged by unplugging the device's AC adaptor and letting the device run on the battery until it ceases to function. This will insure your battery remains healthy.