Battery Low Monitor
Could a little transistor circuit do the job?
ZD1 provides base current to TR1 and as long as about 0.7v is dropped across TR1's B-E junction it will conduct. This effectively robs the base bias from TR2, turning it off so the LED will be off.
As the battery volts falls below a set value (4.7v ?) the zener will no longer have its (4.7v - 0.7v for TR1 B-E junction = 4.0v) bias, it will stop conducting and so cause TR1 to turn off. TR2 will now have its base current returned and start to conduct so causing LED1 to light.
If the transistors are both BC109 then the hfe is around 500 so R1 can be (4V/(20mA/500)) = 100k
R2 = 4v/20mA = 200R
ZD1 will need some guesses but around 3v9 or 4v1 should do.
Will it work ???