This is an 12V battery charger circuit that can automatic battery charging, they uses the power SCR and UJT transistors are main parts.
And they will not work except the battery 12V to be charged with right polarity, so prevent errors as well. The battery voltage controls the charger and when the battery is fully charged, the charger will not supply current to the battery anymore.
Note: You can see various battery chargers below:
The battery charging current through the SCR MCR3818, when it is triggered into the conducting state by the UJT 2N4851 relaxation oscillator. And the oscillator is only activated when the battery voltage is low. Vb2b1 of the UJT derive the battery voltage that to be charged, and since Vp = Vd = Vb2b1 ; the higher Vb2b1, the higher Vp. When Vp exceeds the breakdown voltage of the zener diode ZD1, the UJT will stop to work and the SCR1 will not conduct.
T1 =Primary – 30 turn #22 , Secondary – 45 turns #22
R1 = Series resistance to limit current through SCR so use MCR3818 at 20 amps.
The setting VR1, we can charge the battery as needed.