Low Power Battery Level Circuit

A question occurred to me: how do smoke detectors monitor for battery voltage ?  Obviously it has to be a low power consumption circuit.  The smoke detectors I have emit a short beep when the 9 volt battery is low.  I actually did this with the DC Beeper kit too (the short beep when the battery is low):

http://wb9kzy.com/dcbeeper.htm

Why would I care about how a smoke alarm battery low detector circuit works ?   Because I've had multiple 9V Nimh rechargeable batteries go bad on me - I *think* it's due to the fact that a cell will be damaged if the battery voltage goes below a certain point, losing the voltage from that cell and driving it in reverse.  So it would be nice to have a low power battery monitor for those 9V batteries.  I suppose a DC Beeper could be used but it's somewhat complicated, large and expensive.

I looked at the datasheet for this old Motorola smoke alarm IC:

https://futurlec.com/Datasheet/Motorola/MC145012.pdf

It's a CMOS part with a bunch of functions but the battery monitor seems to be basically a CMOS voltage comparator:

The kit PICs have built-in comparator hardware but the questions are how much power would be required AND are there enough pins available ?   Another approach might be a cheaper version of the DC Beeper, maybe just a PIC based comparator and a piezo  or  LED ?

I've got to look into this more.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm