A low battery indicator can be designed using this electronic scheme.
Current through circuit not greater than 1 mA and therefore it is not an additional big task for the battery.
This low battery indicator is constructed with an operational amplifier in comparator connection, which compares the battery voltage, applied to the inverting input voltage divider R1-R3-P1, with a reference voltage of approximately 4.7 V applied to noninverting input. Due to small current through the diode Zener reference voltage will always be exactly 4.7 V. When the battery voltage falls, the voltage at the inverting input decreases more rapidly than the noninverting input, as always the comparator basculate at the same voltage (that can be fixed very precisely by P1). When the battery voltage is at a normal level, the voltage at the inverting input of A1 exceeds the zener voltage (in which case, the comparator output is practically negligible). If voltage exceeds the Zener voltage drop on R3 + PI, basculate comparator, which makes the output level of increase to that of the battery voltage