The portable batteries charger has been designed to provide an opportunity to load NiCd batteries even in the outdoors from a 12 V battery of a vehicle.
When the circuit is connected to the vehicle battery, D2 lights only if NiCd battery was connected with the correct polarity. Since even on the discarded batteries we have a certain tension, T1 opens and D2 flashes.
As the NiCd battery starts to charge, thru R and D6, lights D6. In the meantime C4 is slowly loading thru P1 and R4. The value of these components determines IC2 monostable period and thus loading time of the NiCd battery. For the components values shown in the schematic, this period can be adjusted with P1 between 26 and 33 minutes. The charching can be interrupted with the reset button S2.
The loading current thru the NiCd battery is determined by the value of R, which can be calculated with the formula: R = (12 – (0.7 +1.3 x nr of elements) / Ic) Ω where Ic represents current loading – in this case because the duration chosen for loading it is twice the face value of battery capacity.
Resistance R must dissipate a power – Ic2 xR watt.
Finally do not forget to make sure that your NiCd battery allows rapid loading and be careful when making this operation will not exceed a half hour!
This portable battery charger has not been tested.