Selectable 4 Step Low Voltage Battery Cut off Circuit

The post explains a circuit configuration which facilitates a multi-step low voltage selection and cut-off for the battery being used and monitored. The circuit was proposed by Mr. Pete.

The Circuit Request

Dear Swagatam,

Your assistance during the past few days has been incredible, thanks to which I think I have managed to get close to this 4 step low battery cut off circuit I need.

Having combined your info, a selector circuit I found in a different source and by adding my own ideas I came up with the following circuit:

I know there are issues which with my knowledge I cannot solve, but I think I am close (I will list these issues at the end of this post).

My knowledge is limited (I am neither an expert nor a novice in electronics), so I would like to hear your opinion and recommendations.

To my understanding, what I expect it to do is as follows:

LM324 will monitor the voltage and turn off the relay at the bottom of four scales: 18.5-20V, 20-22V, 22-24V, 24-28V adjusted by the various 10K presets.

The circuit starts (battery full) with the relay armed (load connected) and the 24-28V LED on (left hand side set of LEDs).

The selector circuit (top part) assures that only the selected OPAMP arms the relay although all of the OPAMP outputs will be high at the same time (this I need for a later function, see end of next paragraph).

During discharge, when 24V is reached the equivalent output (1) will go low and the relay will disarm, disconnecting the load. At the same time BC337 (NPN) will conduct and light up the three right hand side LEDs, to indicate scale options that are available.

Once a button on the selector is pressed for a lower scale (i.e 22-24V), the relay at the lower circuit will arm by the second OPAMP (output 7) and the load will be connected. Same goes for the equivalent left hand side LED. The same applies for the 20-22V scale, but only 2 LEDs will light from the right hand side set).

Now, if the last scale (18-20V) is selected, when 18V is reached the 4th OPAMP of the LM324 will disconnect the load once again but at the same time the SPDT relay at the top left of my picture which is connected as a latching relay will disarm and take power completely off of the circuit to minimize consumption as the battery will be very deeply discharged now.

In order to start the system again, manual reset will be required via the push button near the latching relay, once the battery is charged again.

The purpose of this circuit is for a solar system used to power an emergency communications station. This is the reason it cannot be set in a completely automatic way.

Although deep discharging of the batteries is not recommended, in such stations the operator will have to judge whether he must deep discharge the battery or not, in case the type of emergency makes it inevitable.

Under normal situations, the power will cut off at 24V automatically protecting the batteries, but it is important that the operator will have the option to continue drowning the batteries if necessary.

So here are my circuit issues:

1. I have a fear that in the way the circuit works, the outputs go high, not low when thresholds are met. If it is so the circuit will not operate in the intended way, especially the latch relay function.

2. I have a feeling that there will be a lot of small and big flaws in my additions to the design. I am confident in building it when it is all designed, but designing of circuits is a skill I only have to certain extend.

3. I wonder if I can substitute the relays of the top part of the circuit with some kind of low power MOSFET. If so, please give me a clue as to how to connect them.

4. I am also thinking that the disconnection of the load will cause the system to do some chasing effect, so I wonder if some small hysteresis can be added.

Please get back to me with you thoughts; I believe this can be a useful 4 step low battery cut off circuit if ever completed for many people (maybe by adjusting the thresholds to higher values, not everybody needs to drain their batteries so much).

My gratitude once again for the tones of information you have helped me to collect during the past week.


Simplified Diagram for the above Circuit Issue [Solved]


Parts List

All resistors can be 10K 1/4 watt
All presets = 10K
T1, T2 = BC547
Relay = 24V/SPDT
Scr = BT169
IC = LM324
D1 = 1N4007
Z1 = 6V/400mW

38 thoughts on “Selectable 4 Step Low Voltage Battery Cut off Circuit

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. My friend I think this is SPOT ON!!!!! You are a star! Plus you made me realize that maybe I am thinking too complicated sometimes (call it lack of knowledge)…

    Just to ask some questions before I start building it:

    1. This configuration energizes or de-energizes the relay to cut off the load? Preferably I would like to de-energize it.

    2. How about hysteresis? Do you think it will make the relay tremble once the load is disconnected and the battery voltage rises slightly?


    • Thanks Mate 🙂

      Your design may be lengthy but it has more possibility of functioning, my design may look simplified but can't confirm until we test it.

      Actually the previous diagram was not fully correct, I have it now rectified.

      Here's how it goes:

      We press T1 push button first to let the circuit get latched and powered. On pressing this push button the IC is momentarily energized and a positive feed back from the output of A4 latches T1 into position.

      Next we press the push button at the output of A1 and allow the first stage to begin. On pressing this PB, the relevant SCR gets latched which also triggers the relay driver stage…the output now gets connected with the battery.

      Once the battery voltage reaches below the set mark makes the output of A1 low and the relay breaks.

      The procedure is repeated for all the other outputs too.

      I hope you got it:)

  3. I assume "A" chip to be LM324, T1 BC547 and T2 BC337. Can you please correct me if I am wrong and also give appropriate values for R5, R6 and R11?

  4. Got it and looks fantastic!

    Will once the first stage expires and shuts the circuit down, will I have to press T1 button before I select the next stage? I was hoping that the T1 button would be necessary only for the very bottom stage…

    • If we go as per your circuit explanations in the above article, T1 must remain ON until the last opamp A4 is used….
      To break T1 latch without making use of A4 opamp, the power will need to be switched OFF manually.

      T1 switch is used here only for powering the circuit.

    • Yes, I got that after studying the circuit a little more. This is exactly as I wanted it to be!

      Do you expect power consumption of the whole circuit to be absolute zero when the T1 is off?

      Also, do you think REL 1 can be substituted by an appropriate mosfet (if yes how)?

      Last (not so important but it would be nice) can I connect one additional LED just before each SCR, returning their grounds through a transistor (let's call it T3) with its base linked to the emitter of T2 and arranged in a way that T3 will only conduct when T2 is off? This way, when a stage is over and the load disconnects, there will be an indication of which stages are still available.

      I will start building it soon after the weekend and get back with feedback so other people can benefit from the incredible amount of effort and time you've put into this! I still don't know how to thank you…


    • with T3 off no power can enter the circuit, so it will be zero.

      you can replace T2 with any suitable N-channel mosfet, gate replaces the base and the source with the emitter. The load may then be connected across +26V and the drain. Relay thus vanishes.

      yes extra LEds can be connected via T3 however now T2 is replaced by the mosfet so you will have to use it for the switching action.

      let's see what transpires.

  5. Could you please check this out:

    The additional relay has been placed there to either control the load, or (as in my case) to turn the inverter off (my inverter has remote ON/OFF switch terminals which are jumpered for normal use, or a external 230V and a few hundreds mA rated switch can be connected on them).

    I think I did this right but you can judge this.

    Is it possible to recommend the code for an appropriate mosfet for T2?

    Something else that crossed my mind just now, I think there is an important flow in my concept, that I totally neglected so far:

    Consider the voltage has gone bellow some stage and the relay has tripped. Then the sun comes out, the batteries charge but the circuit does not reconnect the load automatically (because of the SCRs, am I right about this). Can I use an additional LM324 to send the SCRs a pulse at voltages slightly higher than the cut-off thresholds of each stage? This way the system will change stages automatically when charging and manually when discharging. Or am I thinking too complicated solutions once again and it can be done in a simpler way?


    • Yes your diagram looks correct, but if you are keeping the relay then what is use of the mosfet?

      I couldn't get your second point. The above discharging process needs to be manual, and the charging needs to be automatic, is it?

      The charging condition does not need to have any relation to the above circuit, it should be done through a different opamp and a different relay according to me. This opamp will be only responsible for monitoring the charging voltage of the battery and will cut it off once full.


    • I got confused there, please forget the mosfet. I wanted to control an isolated output so I can turn off the inverter instead of disconnecting the 28VDC line under high current. So I will go back to your recommendation with the transistor and relay setup.

      About my second point. I am not talking about charging, this is done through a different charger. I am talking about switching stages while monitoring and connecting/disconnecting load. It should be done automatically during charging and manually during discharging. The bottom stage though, I would not mind if it still needs a manual reset even if the voltage rises again, but for the intermediate stages the stage switch should be automatic while voltage is going up.


    • I am getting confused here, if you connect the load automatically then what would the switches serve for??

      Irrespective of the battery charge level, the discharging process or connecting of the battery to the load needs to be done manually right?

      Not sure who is getting confused, me or you:)

  6. Hey mate,

    Let me show you what I mean schematically. I tried to fix the additions I have described, once again I hope you can check for mistakes.

    At the very left, I am using a single OPAMP in order to detect voltages above 24V. Once this happens, the SCR of the top stage gets activated, so the system resets and starts up automatically (except in the case that the bottom stage has tripped and T1 is off). I wonder if there is a native way to make 324 to work as monostable (one shot). Not that it matters but I would prefer it (only if I won’t have to use additional ICs).

    Now, apart of that, I have tried to do one more thing. This is why I have added the T5 and some additional passive components on the right hand side of the circuit. I hope it doesn’t look stupid but this is what I was thinking:

    I am trying to avoid the possibility of activating two stages at the same time. So, I am trying to bias on T5 by R17 and D6 through C1. But C1 should bias off T1 once any button is pressed and this should turn off any src that is on except the one that its button is pressed.

    Hope this makes sense, and I’d be thrilled if I managed to design it only with some minor flows…

    • OK, if you want to add a self-resetting trigger to the circuit, you will need to trigger the base of T1, not the top SCR, if you trigger the top SCR the battery will start discharging until it reaches 18V flat :O

      T5 and the additional components are not required, I couldn't figure why you felt the need of introducing them?? the SCRs are well isolated and never affect each other.

  7. Well, I am triggering the top SCR coz I want the system to reset once the voltage has gone up again to 24V. Plus when the bottom SCR has cut off, I don't want any monitoring so there is zero consumption. Manual reset will be required once the voltage goes below the bottom stage.

    I know the SCRs are isolated but consider this: if the voltage is 25V (i.e the top SCR is active) and someone presses the second stage button, then both the top and the second LEDs will lit (am I wrong about this?). I want to avoid this not only for aesthetic reasons, but also for being able to select a stage lower than the voltage of the battery at any given moment. When one button is pressed, I want that it cancels any other button that was pressed before, so only one stage can be enabled at any given time. This is why the components around T5 were introduced and it is very possible that my schematic is wrong, but this is what I wanted to achieve.

    One additional question if you don't mind: if + and – of each OPAMP are reversed, is it true that the OPAMP will work in the opposite direction (i.e cut off upwards instead of downwards)?

    • Actually the scrs would sense in the opposite way. A1 should be set for cutting off at say 24V, A2 at 22V, A3 at 20V and A4 at 18V. When A4 activates it's output will go low and T1 will get deactivated switching OFF the whole system.

      When you press the push buttons, the load gets connected with the battery and the battery starts discharging and when the battery level falls below a predetermined value, the relevant opamp output becomes low and the SCR latch breaks, which in turn deactivates the relay, and disconnects the load.

      After this if the user prefers to discharge the battery further down, he has the option to press the next push button until A4 opamp is reached which finally discharges the battery down to 18V and after this T1 is switched off making the whole system dead.

      You want the above process to be completely manual in the manner explained above.

      Now if you trigger any of the opamp through an external source it would make the relay activation and the battery discharging automatic……..I still cannot understand what is your exact need.

      Yes, if two buttons are pressed, say A1 and A2, then both the SCRs will get activated….but I couldn't understand how T5 in your diagram would handle this issue.

  8. Nice so far. Up to your 4th paragraph everything is perfect!

    Now, for your 5th paragraph:
    I do want the system to work completely manually just as you describe: When each stage disconnects the load there must be human interaction before a lower stage is activated. We have this correct in the diagram. What I am trying to achieve with the fifth OPAMP is the following: No matter what stage is selected by the user, once the battery has reached 24V in an upward manner (i.e when it is charging again after a deep discharge) the system must cancel the user selection and return to the A1 OPAMP option. I hope I explained it in a better way this time, sorry English is not my native language neither.

    Now, about the T5:
    What I was trying to do (obviously unsuccessfully, I have to trust your knowledge about this) was to make each SCR turn off all other SCRs once activated. I was trying to implement in the system the function of the selector circuit I have used in my initial schematic at the very top of this thread (the hand drawn one). The author of that circuit claims the following:

    “The PNP is biased on by the 1K and diode, until a + pulse through the .1 uF cap biases it off, briefly. That causes any SCR that is on to turn off. The + pulse occurs when any button is pressed. Whichever button is pressed gates the corresponding SCR on, which energizes the corresponding relay. The SCR stays on until the next time a button is pressed.”

    Please shed some light on it, I tried but, hey this is as far as I could go on my own…

    Thanks again,

    • OK, that means you want reset the circuit in case the battery charges up to 24V under any condition.

      For that you will have to connect the fifth opamp output to the common line of R7—R10 via a 10uF/50v capacitor.

      This will momentarily make the scr cathodes high, cutting of the biasing and thus the activated scr will break.

      Your T5 stage may not work according to me, and I am myself unable to find any suitable solution for this situation…..I am trying, though it looks difficult to me. let's hope we figure it out soon…

  9. Actually, I don't want to reset it. Once batteries charge up to 24V I want the top stage (A1 OPAMP) to activate just as if someone had pressed the A1 button (except if this is what you mean by reset).

    Thanks for being so patient with this. The group which I will make this circuit for is a volunteer group which restores communications in case of emergencies and natural disasters. Your name is definitely going on our benefactor's table.

    With your assistance I hope we will be solving these two final issues soon so I can start building the project… You are a star!

    • Thanks Pete! I appreciate your feelings!

      If SCR1 is the device which needs to be triggered then it may be done as shown in your diagram, but it will need to done via a 1uF cap in series with the 5th opamp output.

      The other issue, relating to the preventing of simultaneous scr triggering can probably be solved by cutting the push-button common point joint from the shown positive supply, and connecting it to the relay N/C contact. I hope you will understand the reason behind this.


  10. Yes I do understand it, but it cannot be done unfortunately. The relay switches off the inverter, it does not disconnect the battery, so it belongs to another circuit and needs to remain isolated…

    Did you have a chance to have a look at the selector switch diagram that a sent to you a few days ago? I got what the guy is doing, but I couldn't really implement it in our circuit. Maybe you could. Check the following link, fifth post from top (dated 2007-04-07 01:41:00):

    • The diagrams are drawn with dashed lines which is making it very difficult to follow and time consuming.

      My previous suggestion can be implemented by simply adding another relay with T2 and wiring its contacts with push button common joint.

      What do you think?

  11. That can be a solution.

    If you like you can check my initial circuit (at the top of this page, hand drawn). The top part of the diagram is exactly the one on that thread only properly drawn (no dashed lines).

    • The 1K resistor should be removed otherwise the relays might not operate, also the relay coils would work better at the anodes of the scrs rather than at the cathodes.

  12. OK! I will try to implement it to your circuit but I am not sure I can… I hope you won't mind if I post it to you once again for checking…
    Thanks again…

  13. Excellent!

    Sorry for spending so much of your time in the process of designing this circuit, but your effort has been extremely appreciated!

    I will start building it soon and report hopefully positive feedback so other people can also benefit from all your hard work.

    I hope I haven't spent all your patience and I can get back to you if some minor issue arises upon assembly.


    • Thank you Pete!

      I am sure your involvement and contribution would be appreciated too by the viewers.

      Feel free to express your thoughts and queries as you move ahead with this project and possibly many more in the future.

  14. Hi Swagatam. I know it was asked before, but still, if you can help me out, I'd really appreciate it.
    I'd like a circuit/ic for purely low voltage cut off. I have a 8-pack of AA (NiMH LSD) batteries, which should never discharge below 7.2V. I'm using these with a (car) 12V to 5V USB charger, which I'd like to use on the go. The setup of simply using a transistor and a variable resistor in conjunction with a TIP122 transistor for handling larger currents, resulted in draining the battery further than the absolute minimum of 7.2V. I'd like this to use this with no relays (as they use too much power).

    I'm looking at 0.5-1.5A on the primary side (batteries) and 1-2.5A on the secondary 5V side. For charging both the phone and the tablet.

    I just hate it, when a circuit like this is used in a 1$ devices. I don't want to buy of the shelf products.

    Thank you really much!

  15. can this schematic/ equipment be modified for use on a 144 vdc battery used in a Honda high bread.
    I made a charger from another site but need a safe way to discharge so as to have a good balance when recharging.


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