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12v battery low voltage disconnect with relay? Answered

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



Best Answer 2 years 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.


where is the Input and Output terminal?

help me please.

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

12V-Drop bat .GIF

awesome ckt, thx from Bangladeshi......

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


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..

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

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..


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?

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...

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


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

across zener mean paralel with zener?

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.



will test it with SLA battery.

or can it try with adjustable DC power supply?



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

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

I will try to do this since it requires few parts


3 months ago

Thanks to everyone who has contributed to this thread. I have built one of these to protect a lead acid battery from being trashed by a buck converter that would otherwise run forever.

My remaining problem is that I occasionally get conditions in the load that pull down the voltage below the protection level for approx 1 second. I assume that the relay can be kept energised temporarily by addition of a capacitor... but I am a Software engineer with only a few scraps of Electronics, so 1) I would appreciate a hint as to where to put it (I am guessing across the poles of the relay coil - and since it'll be an electrolytic cap, - respecting the polarity of the existing circuit).

2) Any suggestions as to the size of the cap would be useful. The voltage drop is of the order of 4v and the current through the relay coil is probably in the order of 20mA..... oh, if only I had a storage 'scope, this would be a breeze.

Thanks in advance.


4 months ago

Lvd circuit diagram. @iceng, kudos to you. Could u pls send me the circuit diagram for the 12v battery low voltage shut off.

use this for my DIY boombox speaker.

Use reset button, thanks.


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

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


2 years 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.