P-channel power Mosfet Q1 is connected as a switch in the positive battery lead, controlled by LM311 comparator IC1. IC1 monitors the battery terminal voltage and compares a portion of the battery voltage from trimpot VR1 with a fixed +7.2V at the inverting (-) input. This reference voltage is derived from the battery voltage via a shunt regulator circuit using zener diodes ZD2 and ZD3.