Comparator without active V(ref)

[Mp3]

I have looked all around on google for this and i am turning up nothing.

Let's say I want to have a comparator that will take my battery voltage and compare it to a desired value of 2.5v. The purpose is to activate a LED when the battery level has dipped below 2.5v. To let me know that the battery level is getting close to empty.

I can't seem to find out any way to achieve this without providing 2.5v somehow, and by the time it dips below that point I obviously won't have a way to provide 2.5v.

Are there comparators out there which can accept a specific resistance for V(ref) and do the job i need?

[Siwastaja]

You use a reference voltage significantly lower than the battery voltage will ever be, and divide down the battery measurement using a resistor divider network.

For example, you could use a 1.25V reference and compare that to your battery voltage divided by two using a divider consisting of two resistors of the same value. High values like 470 kOhm would be desirable in order to not to drain the battery, but then you need to look at the input bias current spec of the comparator not to cause a significant error. You also want to add a capacitor (say 100nF) in parallel with the lower resistor to filter out any noise and interference which easily couples when you use large value resistors.

0.8V to 1.25V are usual reference voltages easily available.

There are also ICs that combine a reference and a comparator.

[Mp3]

You use a reference voltage significantly lower than the battery voltage will ever be, and divide down the battery measurement using a resistor divider network.

For example, you could use a 1.25V reference and compare that to your battery voltage divided by two using a divider consisting of two resistors of the same value. High values like 470 kOhm would be desirable in order to not to drain the battery, but then you need to look at the input bias current spec of the comparator not to cause a significant error. You also want to add a capacitor (say 100nF) in parallel with the lower resistor to filter out any noise and interference which easily couples when you use large value resistors.

0.8V to 1.25V are usual reference voltages easily available.

There are also ICs that combine a reference and a comparator.

That's immensely helpful. thank you, I will also seek out those combined IC's you mentioned as i'm sure one is tailored to the purpose i seek.

[exe]

Those are called supervisors. There are ready devices that are as as few cents: https://lcsc.com/products/PMIC-Supervisors_934.html . Like this one: https://lcsc.com/product-detail/PMIC-Supervisors_AnaSem-VDD251MNTA_C697875.html . Beware that parametric search on lcsc in this section is unreliable, many cheap parts don't have proper entries, like the one I mentioned. Also, most them accept only voltages as high as 6 or 7V.

[Electro Fan]

might be something in here that is useful
https://youtu.be/BRtE_IavUEA