The precision voltage-monitor chip contains a temperature-compensated voltage reference. R1 divides down the battery voltage to match the built-in reference voltage of ICl (15 volts). When the voltage at pin 3 falls below 15 volts, pin 4 supplies a constant current of 7 mA to drive a small LED
About 0 volt of hysteresis is added with R2. Without hysteresis, the LED could flicker on and off when the monitored voltage varies around the set point, as might be the case on voice peaks during receive.