Dual battery supply monitor?

Hi all - I came across this site today whilst researching a white whale of mine....a simple LED monitor that indicates whether one or both 9v PP3 batteries are failing. So far I haven't found an existing answer so I figured I'd open it up to greater minds. I came in off this thread:

https://www.electro-tech-online.com/threads/6v-battery-low-voltage-monitor.127101/

....which is excellent, however I'm uncertain how to apply this to my own application. I have an audio preamp which runs off a -9v/0v/+9v supply, developed by two series PP3 batteries. The aim would be some form of LED indication of a low battery condition in one or both batteries. Preferably one that doesn't add its own problem load to the battery supply or push noise into the audio circuit.

I'm genuinely at a loss and well beyond my knowledge comfort zone! Any kind geniuses seeing a solution here?

If you refer to post #4, then there will be the voltage drop over the 1M/100k resistors - so shuld be calculated to just below 0.8V at the point where battery voltage is defined as low.

My **broken link removed** (ps, the poster doesn't know the difference between an LED and a LCD)

Wire it through a push-to-test button so it doesn't kill the battery being monitored.

Erh, I didn't get it if the poster was thinking about an actual monitor, not just a simple led indication.

btw - an easy way to get an battery voltage drop indication where it's supposed both batteries in series should have equal voltage over the poles:

The resistor values is just a starting point - OP may need to increase R2 if it's too sensible to voltage drop of either batteries. Also don't care about values R4 R5, they need to increase to match supply voltage of ~18V.
Red led: V2 < V1, green led: V2>V1.

At least that is intension - error may occur as it get late.

Thanks guys.

Grossel - the circuits in that thread are not too useful, at least that I am aware of. The objective is to have a single 5mm LED - whether it be single colour or multicolour indicator - that lets us know when one or either battery is low. The sum voltage across both batteries could be used, however the gold standard would be knowing the status of each battery with reference to the common point rather than across both ends of the supply. This is where I am falling flat on my face.

MikeMl - nice little modules, and yes....LCD/LED the product I am modifying has a dumb full-time blinking LED that offers no idea of remaining battery capacity. Unless it doesn't blink, by which point it's sort of obvious and a little too late! Thanks for the ref. because I might buy a couple of those for other experimentation....

Grossel - sorry, I replied before your second post. Wow, that looks simpler than I expected and very easy to integrate in the existing audio circuit. The rest of the preamp is a differential amplifier formed from a dual op-amp that forces a clever ground lift to buck noise, hence the need for a balanced supply. Or at least, works in a simpler manner with one. A quad op-amp should be able to keep component count low and integrate a battery monitor into the main circuit itself. Very very elegant, thank you.

It does seem to have a fault in the logic in that if both batteries were at the same low voltage each, we'd get no indication of a battery life problem? This is the most likely case as each battery should be drained at the same rate as each other. Two batteries at 6v would show the same lack of impending supply issues as two at 9v.

<deleted: brain blurted out something stupid and patently redundant that doesn't solve any issue, thanks to it being late >

Any go/no-go LED on/off tester just gives you a single level, while a voltmeter gives you about 1000 levels in 10mv steps.

Um, yes - in the rare case where two batteries in series have a mad painstaking equality where voltage decreases equally, it will lie.
Another possibility of course is to use a constant current source and series of resistors - should be possible to solve in a similar way using two opamps.

[edit]
something like that - except current source must be adjusted to below 9μA, or resistor adjustet slightly down - you get the picture.

Here's the LTspice simulation of a simple circuit that will turn on the LED D3 if either voltage drops below about 6V.
The threshold is determined by the value of zeners D1 and D2.
The LED should be a high brightness type to minimize its required current for good visibility.


Edit:
Here's a somewhat more complex circuit using a quad package micropower opamp that provides a very stable trip point as determined by reference U1.
The trip point can be adjusted with pot U4 (shown adjusted for a trip point of ±6V).