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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iceng9 months 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.


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

Yes True

aryajaya iceng1 month ago

where is the Input and Output terminal?

help me please.

iceng aryajaya1 month ago

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

12V-Drop bat .GIF
aryajaya iceng1 month ago

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

iceng aryajaya1 month 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 iceng1 month ago

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

iceng aryajaya1 month 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 iceng1 month 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 aryajaya1 month 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...

aryajaya iceng1 month ago

across zener mean paralel with zener?

iceng aryajaya1 month ago

Yes parallel :-)

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 iceng1 month ago

will test it with SLA battery.

or can it try with adjustable DC power supply?

iceng aryajaya1 month ago


aryajaya iceng1 month ago

here the pic

aryajaya iceng1 month ago


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

iceng aryajaya1 month 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 iceng1 month ago

thanks ......

tutdude98 (author)  iceng9 months ago

I will try to do this since it requires few parts

aryajaya1 month ago

use this for my DIY boombox speaker.

Use reset button, thanks.

tutdude98 (author)  aryajaya1 month 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

yes. Use iceng circuit.

rickharris9 months 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)  rickharris9 months 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)  Quadrifoglio9 months ago

Im gonna try to do that

Nice find.

iceng9 months ago

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

Downunder35m9 months ago

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.