As general rules, NiCd batteries can be trickle charged at C/30. ``C'' is the capacity specified in AH (amp-hour). This means a battery of 3AH capacity can be trickle charged at 100mA. A completely drained battery should be charged at C/30 for 40 to 45 hours before they are fully charged. There is no harm in trickle charging beyond 45 hours.
Some batteries, both NiCd and NiMH, are designed for ``rapid charging''. Most newer NiMH batteries can be charged at 1C. This means a battery rated at 3AH can be charged at 3A. In rapid charging, charge duration should be used only as a safety measure. When charged at 1C, a timer circuit should turn off the charger in 1.2 hours. In addition to the charge period, one can also use the battery voltage as a hint as to when a battery is fully charged. Each cell peaks at about 1.55V and then drops to a lower voltage as it is being charged.
When a NiCd or NiMH battery is discharged, be sure not to overdischarge. When a cell in a battery is 1.1V or less (at average drain rate), the battery should be considered drained.