When the battery voltage is low the current flows into the gate terminal of SCR1 through resistor R5 and diode D7. Therefore the SCR1 will conduct heavily for every half wave cycles and charges the battery with large currents. The voltage of the battery is sensed (or detected) by the serially connected resistor R3 and the preset resistor PR2. The R3 and PR2 form voltage divider circuit. The PR2, in this circuit, can be considered as two resistors: PR2 (u) and PR2 (l).