In this circuit, the battery is charged with a constant current that is generally one-tenth of the battery capacity in ampere-hours.
This constant-current battery charger circuit is divided into three sections: constant current source, overcharge protection and deep-discharge protection sections.

This NiCd battery Charger can charge up to 7 NiCd batteries connected in series. This number can be increased if the power supply is increased with 1.65V for each supplementary battery. If T2 is mounted on a proper heatsink, the input voltage can be increased at a maximum of 25V.

This automatic NiCd charger for 9V NiCd batteries is using 555 timer properties and is very easy to build. Why is an automatic 9 volts NiCd battery charger? Because you can leave the battery for charging as much as you like: it will be always completely charged and ready for use when is needed. It wont be overcharged and it will not discharge.