When power is applied to the circuit, SCRl is off, so there is no bias-current path to ground; thus, L~317 acts as a current regulator. The LM317 is umnected to the battery through steering diode Dl, limiting resistor Rl, and bias resistor R2. The steering diode prevents the battery from discharging through the LED and the SCR when power is removed from the circuit.
As the battery charges, the voltage across trip-point potentiometer R5 rises, and at some point, turns on the SCR. Then, current from the regulator can flow to ground, so the regulator now functions in the voltage mode. When the SCR turns on, it also provides LEDl with a path to ground through R3. So, when LEDl is on, the circuit is in the voltage-regulating mode; when LEDl is off, the circuit is in the current- regulating mode.