scr battery charger
The SCR will be the rectifier.
The top two transistors are actually powered at DC by the battery itself.
If the battery voltage is below the threshold set by the pot (it's discharged), then the top right transistor will be off.
The left transistor will then be on and it can turn on the SCR through the LED and diode. That however can only happen when the AC input polarity is such that the top is positive. At that point the SCR fires and power is supplied to the battery for one half of the aC cycle. When the polarity reverses the SCR turns off. It will be on the next AC cycle.
If the battery current is too high, the voltage across the two 1R8 resistors will turn on the BC547 transistor and charge up the lower 47uF cap. The transistor cannot turn off the SCR, but it will prevent it from firing the next time the AC has the right polarity. The cap can keep the transistor off for several AC cycles.
When the battery has charged up the top right transistor will be on, keeping the top left transistor off. That means the SCR can no longer fire and the battery will no longer charge. But if its voltage drops, then the charging cycle will begin again.
Not that since the battery powers the top of the circuit, it will not work with a completely dead battery. That is why you have the switch which will temporarily turn on the top left transistor with help from the AC when the polarity is top positive.
I am not 100% sure about the role of the top 47uF cap, but I suspect it's meant to prevent the top right transistor from turning on when the SCR fires, because the voltage increases when that happens.