The circuit is actually a half-wave rectifier. It only charges the battery on every half cycle. The plug pack doesn't like this as it leaves residual flux in the core of the transformer and causes it to overheat. But that's the only drawback with the circuit.
The SCR turns on during each half cycle and current flows into the battery.
A voltage is developed across the two 1R8 resistors (in parallel) and this voltage is fed into the 47u electrolytic. It charges and turns on the BC547 transistor.
The transistor robs the SCR of gate voltage and the SCR turns off. The energy in the 47u feeds into the transistor but after a short time it cannot keep the transistor turned on.
The transistor turns off and the SCR switches on and delivers another pulse of current to the battery.
As the battery charges, its voltage increases and this is monitored by the "Voltage Monitor" block.
The circuit is very complex and one way to look at the operation is to consider the top rail as a fixed rail and as the battery voltage increases, the rail connected to the negative terminal of the battery is pushed down.
This lets you see how the "Turn On" transistor is activated and how the "Voltage Monitor" components create voltage drops across each of them.
The "Voltage Monitor" components consist of a transistor and zener diode as well as an 8k2 resistor, the 1k pot, a 1k5 resistor, a 150R resistor and a signal diode.
The signal diode is actually part of the flasher circuit and we discuss its operation later.
As the voltage across the battery increases to 13.75 volts, each resistor in the "voltage detecting network" will have a voltage drop across it that corresponds to the resistance of the resistor. The diode will have a constant 0.7v across it.
The voltage on the wiper of the pot will be about 3.25v and the voltage across the zener will be 10v. This leaves 0.6v between the base and emitter of the Voltage Monitor transistor.
This voltage is sufficient to turn the transistor ON.
When the Voltage Monitor transistor turns ON, it robs the "Turn On" transistor of base-emitter voltage and the circuit...