This charger for NiCd batteries (can be used for NiMh) allows simultaneous charging of up to seven NiCd batteries connected in series. This number can be increased if the supply voltage increases by about 1.65 V for each battery. Given that T2 is mounted on appropriate heat radiator, the input voltage can be increased to a maximum of 25 V.
Typically these loads to NiCd batteries a current equal to one tenth of the battery capacity.
If current value is greater than indicated, the charging time must be reduced proportionally, to prevent damage to battery elements.
The charging current is controlled by P1 between 0 mA to 1A current can be measured with a voltmeter connected parallel of R3 (which, to avoid additional calculations, has value, convenient to 1ohm).
In case you find it hard to buy a BD 679, it can be replaced with any type power NPN Darlington, with output parameters (collector voltage / current collector) at 30 V and 2 A.
Lowering the value of R3, maximum output current can be increased to over 1 A. Circuit consume 15 mA current in standby at 12 V.