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You could do it with a fixed voltage reference and a comparator.
For example, if your circuit runs at 9V and you want it to warn you when the battery drops below 7V, use a zener circuit to feed (7/2)=3.5V into one input and a voltage divider to feed Vbat/2 into the other input. When (Vbat/2) < 3.5V (meaning battery voltage is below 7V), the comparator output would switch and you can use the output to indicate a good or bad battery status (e.g. light up a low battery LED, or feed the status into a microcontroller).
I have a simple circuit using a TL431 (or LM431). As shown, it was optimized for a 12V lead-acid battery, and trip point is adjustable from 10.7V to 11.6V. If you want a different trip point, the LED lights when the voltage on the REF input to the TL431 is less than 2.500V, so recalculate the voltage divider consisting of R8, R6 and the 1K trimpot (R3 & R4 in the simulation).
The Plot shows the total current drawn by the circuit as the input voltage sweeps from 10V to 12V for different positions of the Trimpot wiper (0<k<1000). Note that the current drawn by the circuit when the input voltage is above the trip point is less than 1mA. For the value of R5 shown, the LED current is about 6mA when the input voltage is less than the trip point. If you want more LED currrent or change the operating range, you might have to adjust R5