12v battery low voltage disconnect with relay?

Could i make 12v battery low voltage disconnect with relay and resistor?
I was thinking if put resistor in series with relay coil i could lower current so relay would barely work on 12v but when it drops to 10v voltage after resistor would be so small that relay would disconnect

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iceng1 year ago

A 10Vz or 9Vz Zener Diode supplies 5 ma to an NPN 2N2222 1A, Hfe=100 gain transistor which drives 150 ma relay at 12 VDC but Drops out when the supply goes below the Zener voltage.

aryajaya iceng10 months ago

where is the Input and Output terminal?

help me please.

iceng aryajaya10 months ago

Please make the effort to click the pics to see the whole image !!!!

12V-Drop bat .GIF
aryajaya iceng10 months ago

already try it. Use zener 11v and it will disconnected at 2.5v, not 11v????

iceng aryajaya10 months ago

Hard to verify the wiring from your picture and there is no relay which explains the PM "let me know for relay connection coil, where is the positive and ground connection terminal"

If it disconnects at 2.5v how are you detecting that ?

The transistor needs a load use another 4.7K resistor in place of the relay coil...Then the when the voltage across the resistor drops to zero or the voltage of the transistor with respect to GND = ground goes to the supply voltage..

You can assume the transistor is turned OFF and any 12v relay coil (has_no_positive_terminal) will release the contacts to the No_Power position.. That is the position the relay has when taken out of the shipping container as determined with an ohmeter..

BTW, how are you reducing the voltage from 12.4v to 11v ?

If the voltage has a momentary voltage drop the full relay ckt will lock out..

aryajaya iceng10 months ago

i use step down module for test the schema. Let me try to take picture of the wiring.

iceng aryajaya10 months ago

You do take nice clear pictures..

I think I see the relay is the dark blue blob on the right..

I assume you test by starting over 12 v and slowly lower the analog voltage (NO_PWM) !

Add an LED and a resistor to indicate when the relay is ON/off...

As usual click the pic to see the entire circuit image..

aryajaya iceng10 months ago

now it work. Cut off on 10.9v. But have another problem. When I cut off on 11.4v, this circuit not working. But when i charge battery on 12v, it working again.

Any solution for this situation?

iceng aryajaya10 months ago

The circuit has hysteresis, that means you need to get it back into a higher voltage. Hysteresis is important to prevent oscillation....

After all this simple circuit was designed as a battery cut out only....

Add a momentary push button across the zener to restart the relay if you want...

JoseMI99 iceng6 months ago

Hi iceng, I
am implementing this circuit with a 22Vz Zener to cut-off two serial 12v lead

The circuit
responds cutting-off when the voltage of the batteries goes below 22v and
rearms at around 23V (actually the batteries serial voltage is around 24V as I
have 3 power diodes in series with the input, which reduce the measured voltage
at the Zener in 2V approx.).

How could I
increase the hysteresis? I would like to make this circuit rearm only after the
batteries serial voltage is above 25V.

My problem
is the type of charge I am connecting, as it commands a motor and the voltage
read at the Zener goes down when the motor is turning fast, so the relay
cuts-off, but as the power is not any more feeding the motor, the voltage read
at the Zener goes again up enough to re-activate the relay, so the motor
continues turning while the relay is very fast switching between on and off.

I would be
very glad if you or somebody else could put some idea or tip on how to increase
the hysteresis of this circuit.

Thanks in


JoseMI99 JoseMI996 months ago

Sorry for the strange order of the comment, I copied it from a word and I cannot correct it after several trials..

aryajaya iceng10 months ago

across zener mean paralel with zener?

iceng aryajaya10 months ago

Yes parallel :-)

aryajaya aryajaya10 months ago

i know now. It need some short connection for reset.

This is the final board but not with reset button, change resistor value to find some cut off voltage, maybe will be good use VR to adjust the cut off voltage. this circuit Will be use for BOOMBOX portable MP5 and bluetooth Speaker.


aryajaya iceng10 months ago

will test it with SLA battery.

aryajaya aryajaya10 months ago

or can it try with adjustable DC power supply?

iceng aryajaya10 months ago


aryajaya iceng10 months ago

here the pic

aryajaya iceng10 months ago


One more time, if i want disconnected for 11v, what component i must replace?

iceng aryajaya10 months ago

Use an 11Vz zener diode at $o.30


When you go there (read the data sheet) and nextime try to ask your own question so you can bestow a Best Answer...

aryajaya iceng10 months ago

thanks ......

MuhammadQ29 iceng10 months ago

11.5 vdc low cutt off this schematic 10vz replaced 11.5vz

iceng MuhammadQ2910 months ago

Yes True

tutdude98 (author)  iceng1 year ago

I will try to do this since it requires few parts

aryajaya10 months ago

use this for my DIY boombox speaker.

Use reset button, thanks.

tutdude98 (author)  aryajaya10 months ago

woah, i forgot about this project, nice man

You used schematic that iceng posted? I guess its time for me to go back to making this

aryajaya tutdude9810 months ago

yes. Use iceng circuit.

rickharris1 year ago

Voltage sensitive relays exist


You really need one for this to be reliable.

maybe tell us more about the application as well.

tutdude98 (author)  rickharris1 year ago

I want to make portable power pack that would be protected so i cant damage it
I would use 2x12v batteries since i have 2 sla laying around

Or if you are intent on DIY http://www.reuk.co.uk/Simple-Low-Voltage-Disconnect-with-NE555.htm. It does go into resistor selection and adds off at 12V and on at 12,4V (or 11.8 and 12.8 or what ever you choose) to your capabilities.

tutdude98 (author)  Quadrifoglio1 year ago

Im gonna try to do that

Nice find.

iceng1 year ago

Most relays do have a higher pull-in voltage and a lower drop-out voltage.

Since you need the power cut when the battery is low and then charge it you have several options available:
1. Simple battery monitor - it disconnects the load once a critical low voltage is reached, the other guys linked some already.
2. A dedicated battery isolator as used in 4WD and campers - they work fully automatic, if the input voltage is high enough they allow to charge the battery otherwise they block this way, once battery is going however they won't protect the output and shut off. IMHO only really good with two batteries.
3. Better than 1 and more versatile is a solar charge controller. They do exactly what you want and offer a dedicated charging circuit so you connect a proper DC source of 15 to 24V and the battery will be charger, plus it all works as well while charging.