12V Low Battery Indicator

[Suraj143]

I made this circuit.

The problem is when it reaches the trigger point (0.65-0.7) the LED is flickering.

How to overcome this?

 

[MikeMl]

You built a better thermometer than a battery voltage monitor. To cure the flicker, you need some positive feedback to provide hysteresis.

 

[Suraj143]

Hi.
You mean to add a 100K resister from 2nd BC547 Emitter to trigger point?

 

[MikeMl]

Hi.
You mean to add a 100K resister from 2nd BC547 Emitter to trigger point?

No, to get positive feedback, the second stage would have to be an inverting stage; not an emitter follower.

 

[Suraj143]

I have made this on a pcb now I cannot change the design.
Instead of the two voltage divider is it ok to insert a zenner diode in place of 56K resister?Will it also flicker the output?

 

[MikeMl]

Here is the behavior of your circuit. I varied temperature from -25degC to +50degC. Green is -25C, red is 0C, lt. blue is 25C and dk. blue is 50C. Note the slope after the LED begins to turn on. At this point, the circuit is very sensitive to slight variation of the battery voltage. I'm guessing that there is something varying the battery voltage (intermittent load?)

 

[MikeMl]

I have made this on a pcb now I cannot change the design....

I never commit a circuit to a pcb until I breadboard it and test it

 

[Suraj143]

Hi thanks for the input.

I added a 22uF cap across GND & the base of the 1st transistor.The jitter dropped much but has some more.I'm checking from my adjustable bench power supply by giving varying voltage.

 

[MikeMl]

Here is a much better circuit with about the same parts count. It has much better temperature (although not perfect) compensation, and has a much better rate of turn on. Compare the plots of LED current vs battery voltage as a function of temperature to your circuit.

 

[MikeMl]

Here is my favorite circuit for this task. Better gain with hysteresis, better temperature compensation, low parts count.

 

[MikeMl]

Here is a way of adding hysteresis to your circuit; the temperature compensation is still lousy.

 

[audioguru]

I would use an accurate LM10 which is an adjustable voltage reference and opamp in an 8-pins package.

 

[Suraj143]

Thanks mike that will work nicely.I'll give the update.

 

[ronsimpson]

Change Q1 into a TL431.
Change R3/R4 to divide down to 2.5 volts.
Change the collector resistor from 10k to 4.7k.

The TL431 is temperature stable and turns on at 2.5 volts.

 

[MikeMl]

Change Q1 into a TL431.
Change R3/R4 to divide down to 2.5 volts.
Change the collector resistor from 10k to 4.7k.

The TL431 is temperature stable and turns on at 2.5 volts.

Sort of like the circuit in post #10?

 

[avrilcory]

This is a simple circuit that will indicate a low voltage on a 12V lead acid battery. Many that have golf carts have a number of 12V lead acid batteries to maintain and this circuit will help protect your investment while not using these batteries. This circuit will light an LED when the battery voltage decreases to 11.6V and more make LED1 red stick bright.


Thanking You
Riello-ups

 

[Suraj143]

This is a simple circuit that will indicate a low voltage on a 12V lead acid battery. Many that have golf carts have a number of 12V lead acid batteries to maintain and this circuit will help protect your investment while not using these batteries. This circuit will light an LED when the battery voltage decreases to 11.6V and more make LED1 red stick bright.


Thanking You
Riello-ups

Where is the circuit?

 

[bortre]

Here is my favorite circuit for this task. Better gain with hysteresis, better temperature compensation, low parts count.

View attachment 83823

Hi there. I'm searching for a low-voltage indicator just like this, but for a 18650 LiMn battery instead.

As I've got no idea about electronics, besides reading the partlist, the circuit and soldering it alltogether I would like too ask, if it was too much trouble for you to tell me what components I would need for a 3.7V battery and the LED lightening up at a 3.3V ?

 

[MikeMl]

Lot of info here.

 

[bortre]

Lot of info here.

That's a nice information about the batteries, yes, but there's no low-voltage-indicator circuits shown there. It only says, that some batteries have PCBs to protect the batteries.

I'm using unprotected IMR-cells as I need high power to fire an atomizer, so my batteries have no PCB included and that's why I'd like to assemble a small low-voltage-indicator myself.