Personally I think i'd have gone down the comparator route, I've done something similar with a comparator (may have been a 339), a zener, a handful of resistors and a 2N2222 to drive an NC relay........ The primary reason for this was low quiescent drain (down to several uA, as it's for a vehicle),

That's certainly a good option.
But an LM339 takes more than several μA, more like 800μA (which is still a very low drain).
And there's also current for the zener and other bias currents.

Below is the circuit with an LM339/393 comparator.
Take your pick.

 

Okay, take three.
Here's the circuit modified so that it turns on the 24V relay (and the LED) when the voltage drops below the set point and latches until its reset by S1.

View attachment 147262

Hi again, will it be easy to modify to 12 v system, and set point at 11,8v ?
I know i will have another relay. but what else to change ?

 

Just thinking here; but if your voltage drops below a set point you want the LED to come on and to activate a relay. The relay remains active (as per your requirement) until you push a reset button. If so - what happens, assuming a fully charged battery, during start up? The voltage is likely going to drop below your trigger point, so you're going to have to reset the relay every time you start the engine.

I may not have a solution to your problem, but I can be good at finding the problems with any solution. It's my nature.

 

Just a thought but an ATTINY UP running Bascom Basic language
could cut down on parts count, and add flexibility for changes/upgrade.

https://www.mcselec.com/

ATTINY44 would probably do. Has onchip Vref and A/D.

For future reference.

Regards, Dana.

 

Hi again, will it be easy to modify to 12 v system, and set point at 11,8v ?
I know i will have another relay. but what else to change ?

Change R2 to 10k and R3 to half the value shown in the .step list.