Below is a circuit designed to light up an LED if the battery voltage drops to a certain level. It's an LM358P Opamp in comparator mode. My problem is that it's not working as expected. The LED is lighting up even before the Input at Pin 3 (non-inverting input) drops below the Input at Pin 2 (inverting input). That happens when using a particular IC. I switch out the IC with another LM358P IC and I get no response at all. I switch out with an LM358G and the LED is always on. Funny behavior. Can anyone help fix the problem and make the circuit do what it's supposed to?

 

The lm358 is a dual operational amplifier how are you terminating the other amplifier?

 

At half a milliamp standing current, the monitor is going to shorten the battery life
10Ω is way too small for the zener bias resistor, and a 3V zener is a poor choice for a reference, because the zener voltage varies with current (and hence with battery voltage)
Use a 1.2V bandgap reference such as an LM385 or LM4040, both of which will work with 60uA bias current.
Then choose a low power comparator.

 

The lm358 is a dual operational amplifier how are you terminating the other amplifier?

Nothing is connected to the other amplifier.

 

Hello there, welcome to AAC!
The most straightforward way that I can think of to solve your predicament is to use an actual
Comparator. Do you have any flexibility in your design?

I like the opamp because it's cheap. haha

 

Below is a circuit designed to light up an LED if the battery voltage drops to a certain level. It's an LM358P Opamp in comparator mode. My problem is that it's not working as expected. The LED is lighting up even before the Input at Pin 3 (non-inverting input) drops below the Input at Pin 2 (inverting input). That happens when using a particular IC. I switch out the IC with another LM358P IC and I get no response at all. I switch out with an LM358G and the LED is always on. Funny behavior. Can anyone help fix the problem and make the circuit do what it's supposed to?

The schematic shows that the circuit is working perfectly.
As long as Pin 3 voltage is above Pin 2 voltage, the output will be high and the LED will be lit.

Interchange Pin 2 and Pin 3. Then it will do what you want it to do.

 

At half a milliamp standing current, the monitor is going to shorten the battery life
10Ω is way too small for the zener bias resistor, and a 3V zener is a poor choice for a reference, because the zener voltage varies with current (and hence with battery voltage)
Use a 1.2V bandgap reference such as an LM385 or LM4040, both of which will work with 60uA bias current.
Then choose a low power comparator.

The LM385 and low power comparator are too expensive.

 

Nothing is connected to the other amplifier

If you're terminals in the unused op amp are all left unconnected, there is a real risk that stray electrostatic fields will cause an input to go outside the supply rails. This can cause latch-up and destroy the whole chip.
https://www.analog.com/en/analog-dialogue/raqs/raq-issue-46.html

 

Welcome to AAC!

That happens when using a particular IC. I switch out the IC with another LM358P IC and I get no response at all. I switch out with an LM358G and the LED is always on. Funny behavior. Can anyone help fix the problem and make the circuit do what it's supposed to?

If you're using a single 18650 battery, you're violating the common mode input voltage range of the opamp. The output is also only guaranteed to be 4V less than the supply.

 

Below is a circuit designed to light up an LED if the battery voltage drops to a certain level. It's an LM358P Opamp in comparator mode. My problem is that it's not working as expected. The LED is lighting up even before the Input at Pin 3 (non-inverting input) drops below the Input at Pin 2 (inverting input). That happens when using a particular IC. I switch out the IC with another LM358P IC and I get no response at all. I switch out with an LM358G and the LED is always on. Funny behavior. Can anyone help fix the problem and make the circuit do what it's supposed to?

Low voltage for 18650 battery is about 2.5 VDC. The LM358 minimum supply spec is 3 VDC so it won't work at the required low battery levels. Therefore, the opamp/comparator needs to be able operate at a supply voltage below 2.5 VDC so the LED will remain on at low voltage.

 

Below is the simulation of a circuit using the low-cost TLV431 programmable reference as a comparator.
It turns on the transistor and LED (yellow trace) when the voltage at the Ref input (red trace) is below 1.24V, as determined by the Vb voltage (green trace) and the U2 pot setting.

This circuit is significantly more stable with temperature than one using a 3V Zener for reference.

 

the magic word for your 358 app. might be a "j-Fet"
___________________
there are a lot low power circuitry op amps comparators . . . also there are cmos voltage comparators in a bicycle tail lights - that (reversely to your case switch on an extra led if the batteries are fresh) - but the same thing can be wired for opposite function . . . if i'm correct
. . . just couple of loose compiled... (= never built , not tested/verified against misc. conditions . . . not optimized for component count )
...low power circuits
. . . it looks like the min supply for the 358 varies from +2 (±1) to +5 (±2.5) V https://www.google.com/search?client=firefox-b-d&q=lm358+min+supply and depending on particular variant may cause the problems . . . i tuned my apx. 3.6 ... 4 V Vs.min model to work ...

 

A 3V zener diode has terrible voltage regulation, especially when it has the extremely high current you are feeding it.
It might be 3.8V when the battery is 4.2V fully charged and both voltages drop as the battery voltage runs down.
I agree that the very high current in the zener diode will soon kill the battery.

Use an LM10 that is an accurate adjustable low voltage reference and an opamp in an 8 pins package.
When the battery voltage drops below the reference voltage then you can have the LED light on or turn off.
If the LED turns on when the battery is low then its current will soon kill the battery.

 

. . . https://www.apogeeweb.net/article/891.html , https://www.electronics-notes.com/a...diode-datasheet-specifications-parameters.php , compensated Zener , https://www.onsemi.cn/pub/collateral/hbd854-d.pdf , http://www.reallyreallyrandom.com/www/Zener_Theory_and_Design.pdf , https://sound-au.com/appnotes/an008.htm , a Vref https://www.ti.com/lit/an/snva514/snva514.pdf

 

Below is a circuit designed to light up an LED if the battery voltage drops to a certain level. It's an LM358P Opamp in comparator mode. My problem is that it's not working as expected. The LED is lighting up even before the Input at Pin 3 (non-inverting input) drops below the Input at Pin 2 (inverting input). That happens when using a particular IC. I switch out the IC with another LM358P IC and I get no response at all. I switch out with an LM358G and the LED is always on. Funny behavior. Can anyone help fix the problem and make the circuit do what it's supposed to?

View attachment 263896

as long as the battery is ok ,the led is on, so the circuit is ok
because ,when the + input is higher than the -input ,then the output is high.
change + and - to obtain what you want.
but I agree to all the other comments too.