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Simple 48V Automatic Battery Charger Circuit

Simple 48V Automatic Battery Charger Circuit

The following article explains a very simple yet effective 48v automatic battery charger circuit, utilizing very ordinary components. The circuit is highly accurate with  its over charge cut off features.

Circuit Description:

As shown in the circuit diagram, the main element in the circuit is the opamp IC 741, which has been arranged as a comparator.

Pin#3 which is the inverting input of the IC is referenced with a fixed voltage of 4.7V through the respective  zener/resistor network.

The other input is applied with the sensing voltage which is actually the voltage merged from the supply and the from the  battery, in other words the charging voltage which is applied to the battery for charging.

The resistor network at pin#2 along with the preset forms a voltage divider network which is initially adjusted such that the voltage at this pin stays below the voltage level at pin3, which is the reference voltage set at 4.7v by the zener diode.

The preset is set in such a way that the voltage at pin#2 rises above the 4.7 mark as soon as the battery voltage rises above 50V or the fill charge threshold level of the battery.

The moment this happens, the output of the opamp goes low switching OFF the mosfet, and cutting off the voltage to the battery.

Initially as ling as the battery voltage and the over all voltage from the 48V supply remains below the full charge threshold level of the battery, the output of the opamp stays high and the mosfet us kept switched ON.

This allows the voltage to the battery for charging, until the above explained threshold is reached which automatically inhibits the battery from further charging.

The mosfet can be selected as per the AH rating of the battery.

UPDATE: For converting this into a Solar version you can read this article

1) Using Mosfet Cut Of

2) Current Controlled Version of the above Design

current controlled 48V battery charger circuit

NOTE: The above diagrams mistakenly shows 48V as the input, the correct value is 56V.

NOTE: You will have to connect the battery first and then switch ON the input supply, otherwise the mosfet will fail to initiate for the charging process. Make sure the green LED remains illuminated after power switch ON, this will confirm the charging status of the battery.

3) Making a Fully Automatic Version

The above circuit can be upgraded into an over charge cut off, as well as low charge restoring  battery charger system, for charging 48V batteries.

The modifications enables the circuit to switch OFF the battery charging process at the set over charge threshold and restore back the process when the battery voltage falls below the low threshold value.

The 10k preset must be adjusted to set the full charge level while the 22k preset for detecting the lower threshold of the battery.

How to Set up the above Circuit:

For setting up procedure, the sample power supply should be connected across the points where the battery is connected, the mosfet does not require any attention initially. DO NOT connect the battery while carrying out this procedure.

Also keep the 22k preset link disconnected initially.

Apply the higher threshold level across the above mentioned points and adjust the 10K preset such that the RED LED just switches ON. Seal the adjusted preset with some glue.

Now reconnect the 22k preset link back into position.

Next, reduce the sample voltage to the lower threshold value and adjust the 22k preset such that now the green LED just lights up, while switching OFF the RED LED.

If you find no response from the circuit try using a 100K preset instead of the 22k preset.

Seal the adjusted preset as above.

The setting up of the circuit is over and done.

Please note that during actual operations, the above circuit will remain functional only as long as a battery stays connected at the shown points, without a battery the circuit will not detect or respond.

Feedback from Mr. Rohit

I have a 50-52v solar panel setup which is charging a 48v 78ah battery. What I want is when my battery is fully charged that is it reaches to 54v the battery charging stops and the supply which is coming from the solar panels is directed to another port from which we can charge any other device connected to the port. This charging should only continue till  the battery is above 48v. Once it reaches 48v the battery again starts charging on solar panels and the supply to the other port is stopped.

Hoping you will reply soon.

My Response to the above Circuit Request

You can try the last circuit from the following artcilehttps://www.homemade-circuits.com/48v-solar-battery-charger-circuit-with/replace the "load" with any other device any other preferred device.


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

211 thoughts on “Simple 48V Automatic Battery Charger Circuit”

  1. Hi Sir, we have an e-bike with 48v(12v x 4pcs 20ah) gel type battery. my problem is we have to monitor the charging process because the charger that comes with the unit is just an ordinary charger no auto cut-off no over charged protection etc. can you suggest a circuit that i can incorporate to my existing charger to make my battery safe from over-charging or to have an auto cut-off when fully charged?

    • Hi Amor, you can use the second circuit from the above article it will never allow the battery to get over charged if properly set.

          • Hi Sir, my battery is 4x12v 20ah (48v) what is the value of RX in my case?
            what particular P-channel mosfet can i use IRF9540 or IRF5210?

          • Hi Sir, what is the minimum and maximum input voltage i could use in the second circuit above? i have a regular charger with 60vdc output @ 2.8amp for ebike 48v 20ah, (4x12v)

            how big must be the heatsink for the mosfet?
            what will be the ideal battery full charge cut off for 48v 20ah gel type battery? 50v or 54v?

            • Hi Amor, the maximum input voltage should be 57 V, minimum should not be lower than 56V otherwise the battery will not charge fully.

              The heatsink will need to be determined with practical testing, I am not sure about the size.

              Full charge cut off should be at 57 V, or 56 V will also be OK.

          • Thank you Sir, is there any way that i could use it with my existing charger with 54v – 59v output indicated on its label. what is your recommendation Sir?

          • Is it possible to use a series of diode to make my 59vdc regular charger limit its output to 57vdc so that i could use it with your 2nd circuit above? if yes, what particular diode and how to do it Sir?

            • 59V won’t be a problem, because the second circuit preset will be adjusted to 57V, so it will anyhow cut off at 57V automatically, but make sure the input current is around 1/10th of the battery Ah or at the most be 1/5 Ah

  2. Hey Swagatam, I tried simulating your second circuit using LTSpice and couldn’t get the circuit to behave properly. A varied Rx between many different values and didn’t observe any consistent changes in the current being fed to the battery pack. I also tried getting rid of the diode connected to the gate of the mosfet and didn’t observe any changes with that either. What kind of diode did you use there? Also, what software did you use to simulate the second circuit? Thanks for your time.

    • Hi Nathanael, it is a simple design and should start working immediately when you test it practically. Rx is in series with the battery so changing its value will instantly effect the charging current, but only while the mosfet is ON. The gate zener diode is employed to block the offset leakage voltage from the opamp output so it’s important. I design and build circuits using my knowledge, experience, and calculations. I don’t use simulators, because often they produce misleading results.

      • Thanks for your feedback, I was able to get the charging current to change according to your equation. What is the purpose of the green diode in series with the 10k resistor? I’m currently working to build a charging circuit for a 37 volt lithium ion battery pack. I can supply just about any voltage to the circuit with buckboost converters, but I need the circuit to be able to detect when the charge voltage is above or below the 37 volts and either stop charging or start charging appropriately. Do you have any recommendations? Thanks again.

        • The shown preset can be used for adjusting the cut off threshold at 37V precisely.

          The green diode is a green LED which illuminates while the battery is charged, and shuts off when the battery is full

          • Hey Swagaram, I constructed the circuit and it works great! I started by having a 35 volt supply to charge my 37 volt battery and was able to get it to operate as desired for extended periods of time. I had to change out the rectifier for my source and now I am getting a 50 volt source for the circuit. I’m still charging the battery up to 37 volts, but I’ve run into a problem where my 4.7v zener running from pin 3 to ground burns out after a short period of time. I was wondering if it mattered which order I connected the battery and the source voltage (I’ve been connecting to the source first and then connecting the battery).

            • Glad it’s working Nathanael! The pin#3 zener is protected by the 22K resistor so It’s strange if it burns. It shouldn’t burn because the 22K would drop the current to a great extent safe enough for the zener. Still, you can try using a 1 watt zener and see how it responds.

              The battery should be connected first and then the input switched ON, although this may be not related to the zener burning, this is necessary to enable the the charging of the battery. If the supply is switched ON first, the opamp would sense a high voltage at pin#2 and switch OFF the mosfet, which would then disable the battery charging permanently until the input supply is switched OFf and ON again

          • I did some more trouble shooting and it turns out its the BC547s that are burning out. I connected the battery (slightly undercharged at 35 volts) and then connected the terminals for the supply and heard a pop from the circuit. I pulled the BC547 and it has gone bad. Any ideas on how to protect the BC547? I know now the zener at pin 3 is working properly.

            • Referring to the second circuit from top, the BC547 has no chance of burning because its collector emitter is clamped and protected at 4.7V by the zener, and its base is almost connected to the ground via RX.
              The only way it can burn is if Rx is faulty or open, in which case the entire voltage from the fet can enter the BC547 base and burn it.

              So I can’t figure out how it may be burning in your case?

              Please check the voltage across the C/E and B/E of the BC547, this will verify the actual conditions.

            • Replacing the 540 mosfet seemed to fix whatever was wrong with the circuit. I’m assuming the Rx=0.6/(max current) is somehow derived from equations regarding the BC547?

            • Glad you could solve it. Yes the 0.6 value is the approximate full switch ON base voltage for the BC547

  3. hai sir
    i want to use this circuit for 12v 17ah battery. please discribe the required modifications.do u have pcb for this circuit

    • Hi Rajesh, if you want to keep your battery always charged at the correct level and cut off when it reaches an overcharge, you can make the first design. Remove all the 22k resistors and replace them with direct links, and also remove the 15V zener at pin#7 of the IC. That’s all, now you can set it up as described.

  4. sir can this circuit be used as a BMS for lithium ion batteries and can it be wired in series or parallel the way BMS is usually wire sir

    • Marvin, yes it can used, but the cells cannot be monitored individually for series connected modules, same for the parallel connected modules, however the entire module can be monitored as a whole

  5. 24vdc बैटरी को चार्ज करने के लिए क्या चेंज करना होगा डिज़ाइन में 150 w 12 vdc के 2 पैनल हैं

  6. Sir, we want to design 48V battery level indicator. As after full charge battery voltage reaches to 54V. Our team is manufacturing a solar car with a battery of 48v. Our problem is when ever driver press throttle the battery voltage in battery level indicator fluctuate very much and then the car is stationary it doesn’t. So we want such battery level indicator which do not fluctuate when throttle is applied and show battery percentage while driving the vehicle.
    Hope you reply soon.
    Thank you Sir

    • Yashvardhan, if your battery level indicator is fluctuating due to an actual drop in battery voltage while throttling then that cannot be corrected, because if the battery voltage drops suddenly and rises then the indicator will also show corresponding sudden drop and rise in its reading….so first yu will have to investigate why the indicator is fluctuating, is it due to actual battery voltage drop or due to some other reason such as RF disturbance or noise spikes?

  7. Gud day sir, I av 2000watt stabilizer transformer, I want to use the transformer for 1kva inverter, when I calculated it d primary currently is 4.5amp while d secondary current us 83amp but I dnt no size of copper Guage to use

    • Hi Abdul, you can use the trafo for inverter purpose, but for inverter the mentioned specs for primary will become for the secondary and vice vers….wire size is not important, the measured are enough for the verification

      • I did get u sir, what I mean is Dat I want design a 1kva inverter using 2000watt stabilizer transformer. And I want to wind it myself. So hw can I get d number of turns and size of wire to be used. Sir I nid ur help

    • Dayo you can try the second circuit from top, and use it for your 24V/200AH battery.

      use IRF540 on heatsink for the mosfet.

      dimension the current sensing resistor for limiting 20 amp current

      feed 28V/20amp to the circuit

      • Thanks for the response, but the second circuit does not have overcharge cutoff and low level feature. How will that be possible sir.

  8. Hi Mr Swag,
    I constructed the last circuit today(for 12v battery)though without the current control bcos I could not found appropriate RX.

    With high threshold the red led came up but became brighter when I adjusted he 10k preset ok.
    But with low threshold, I adjusted the/22k/100k preset yet the red LED did not go off. Both LEDs came on. What did you think I did wrong?

    Do I need the 12v zener diode across gate and source of p channel bcos am using 12v supply.

  9. Dear Swagatam,
    How is possible to limiting to charging current with 3 amper.
    I want to charge 4 x 50 Amper battery set with 7 Amper transformer but I want to limit charging current with 3 Amper with this circuit.

    • Dear Yildirim,

      in the second circuit use Rx = 0.6/3 = 0.2 ohms, wattage = 0.6 x 3 = 1.8 watts

      therefore replacing Rx with a 0.2 ohm/2 watt resistor will do the job

    • Hi swag
      Using the last circuit from top, pls where is the Rx resistor? Is it the resistor connected to emitter of BC 546 to gate of MOSFET.

    • I still don’t get it. Is the 22k connected to 10k preset or 22k connected to zener diode 4v7 at the inverting input of op amp from 4th diagram from top.

      • for a 12V battery you can remove all the 22K resistors except the 22K preset. and also remove the 10k and zener at pin#7…all these resistors were used to safeguard the IC from the high 48V source. pin#7 can be directly connected to the positive line.

        and please do it only after proper understanding of each and every steps and stages…otherwise you may fail to get things working for you.

    • I still don’t get it. Is Rx resistor the 22k connected to 10k preset at non-inverting side or 22k resistor connected to zener diode 4v7 at the inverting input of op amp for 4th diagram from top.

      • I think you should try the second circuit first which is much simpler, the low voltage restoration can be added by adding a feedback link from pin#6 to pin#2

  10. Thanks Swag
    So, P-channel MOSFET what part Not can I use if I am to convert the circuit to 12V up to 200AH battery charger with current limit = 30A

    • Hi Segun, I have already explained which parts should be removed, please check my previous comments.

      the mosfet s not P-channel, it is N-channel IRF540

    • I really commend you for your great work and interest.
      Straight to the point.
      I am using the last circuit from the top (i.e 4th circuit) for 12V upto 200ah batt with xformer 15v/50A. I want to limit the current to battery to 30amp.
      Now(1) how do I calculate the current 30A limit
      (2) you told me to remove all 22k resistors with links and 15v zener and use N channel irf540 MOSFET right?

  11. Hi pls how can we adapt this circuit to charge any battery of 12v upto 200AH

    (1) what will the zener diode volts would be
    (2) wattage of resistors and preset resistor and ohm size
    (3) can it limit the current drawn from battery to 20A

    • for a 12V battery you can remove all the 22K resistors and replace them with a link.

      also remove the 15V zener, and the 4k7 resistor.

      rest all be as given , all resistor are 1/4 watt.
      pin#3 zener can be any zener between 4V and 9V rated at 1/2 watt

  12. I need to charge a 48V lithium Ion battery pack. It needs to have a current limiter installed. Can you tell me how to modify your circuit to control current? Then constant voltage after bulk charging?

    Thank you!


  13. Gud day sir, I av a project am working on title inverter type arc welding machine. I av finish constructing it. And is 48v system. When I tested it is working properly after a week later later I tested is jst starting but no melting the electrode as before. My question is wat can be d problem, bcos am thinking maybe the battery is low. Can u giv me ur opinion. Thanks.

    • Hi Abdul, yes most probably it could be due to low batt, because a welding inverter is supposed to draw heavy current and in the process the battery may drain relatively quickly…you can verify it with a voltmeter and possibly keep a voltmeter permanently connected with the system.

  14. Hello Sir,
    Sir can you refer a circuit for the following situation
    I have two battery packs, Let us assume one of them is charged. Now there are two parts one is the generator which charges the empty battery pack and the other part is the motor which uses the charged battery pack. Now what i want is when the motor has used the battery to the fullest i want a automatic battery switching circuit which switches the connection with this battery with the other one(being charged by generator till now). So that now newly charged battery from generator is used by motor and the other one starts charging.
    Please help.

  15. Thanks sir, I we try it n see.
    But sir i av 48v 100ah battery and I want connect dc fan rated 12v, 0.13amp to my inverter to b fan d mosfet .i nid a circuit diagram to limit d current. Bcos it as damage 2 dc fan. Pls I nid ur help sir

  16. Gud day sir, I av issue with the charging circuit setting , after complete the circuit via third circuit, I disconnected the 22k,then I vary the 10k the red led turn on, so I connected the 22k back, the green light turn off and the red light good off. But the question is the green light always on even I vary the 22k.pls I nid ur help about the setup

    • Abdulnoni, disconnect the positive side of the green LED from the existing position and connect it with pin#7 of the IC..this will solve the issue but the illumination might decrease….to correct this decrease the pin#7 resistor to 4k7, and also decrease the LED resistor to 4k7….do this and check the response…

      and also remember that the 22k feedback preset is arbitrarily selected…you can try other values such 100k preset or 220K preset for getting the correct lower threshold restoration adjustment of the circuit.

  17. Sorry sir, is a mistake the battery is 12v 100ah x 4 to in series to give 48volt 100ah. What type of mosfet will I use via third circuit

  18. Hello sir, I want to construct a 48v automatic battery charger and my battery rating is 12v 100ah via ur third circuit what type of mosfet will I use

  19. I have a circuit ,where I am getting the 52v 700mA output at my power supply but the problem is about the current. I require 5-7A to charge my battery pack because its is 19A pack.

    Please Kindly help me sir.

    Please suggest me a design to get 5-7A 52V supply to charge my battery pack using 1st or 3rd circuit diagram

  20. I tried the first circuit in that the LED is goes on during charging and when the battery is fully charged the LED goes off!

    Is I mentioned in the above statement is right? or not?

    • that’s perfect, If the green LED i getting OFF it means the opamp is correctly responding to the fully charge level, and the mosfet must also switch OFF in this state.

  21. I had set the #pin 2 as 4.7v without connecting the batteries and then i connected the battery pack.

    After the battery pack is fully charged up-to 48v but the circuit doesn’t cutoff

    • sorry, that’s not the correct way to set.

      you must feed 48V to the circuit without battery connected, then adjust the preset until the green LED just shuts down. That’s all, your circuit is now set.

      By the way for a 48V battery the full charge level should be 56V.

  22. This is very help full article. Thanks for your great support for beginners like me.
    Can i use the first circuit for 48v 7amps supply, which is going to be charge the 18650 Li Ion batteries of 48v 19amps.

    • Thanks Swagatam………
      Would it be hard to set this design up for 12 volts?
      Could the self regulating design be tweaked to shut off high and turn on low?
      BTW where are you located?

    • Hi Carl, there are plenty of similar circuits published in this website which you you can refer and customize as per your preferences…all of these employ opamps are self regulating type and the distance between the full charge and low charge can be adjusted as desired.

      you can check out the top sidebar section for getting all the links related to automatic battery chargers…

  23. Hi hello mr swagatam. I'm back again ask you again..let me ask you sir.. is this capable to charge 48V with 365AH? IF not what components to be replaced?thanks in advance sir

    • Hi Mark, yes you can use the designs for charging a 48V/365AH battery, but make sure that the input source has a current not more than 40 amps.

      and also make sure the mosfet is mounted over a large heatsink

  24. Hi Mark, you can use the circuits which are explained in the above article for charging any battery from 12V to 48V, simply by adjusting the indicated 10k preset accordingly

  25. Hi hello mr swagatam..im back again..let me ask you sir.. do you have a circuit that will be able to charge 12v, 24v, and 48v,.. I really need it mr sir swagatam.. thanks again

  26. Sir pls i have not seen the answer to that pin 14 and pin 10, that capacitor is it the one of polarity which pin is negative and positive please sir help me/. the first question that i asked before about the value of the CT and RT u have answered me that i should try any value but the pin 14 and 10 i did not know its polarity please help me sir or i should try any polarity? u know u are our lecturer thanks.

  27. …And pls Engineer Swag, this circuit of IRS2453 the capacitor in pin no 14 and pin no 10 is it ceramic or micro-farad with polarity?

  28. please Swag, my own request is still 280vdc in and 48vdc out for charging 4 batteries pls help me. and its like there are many John in this site may be i will change my name to Dayo thanks

  29. all mosfets work in an identical manner, so any mosfet matching the N/P and V/I specs will work here.
    you can try the ones which are rated to work with 60V or above

    It's better to build and check it practically instead of checking on a simulator.

  30. thank you, the first circuit is the easiest and the best one, although it is designed only for full charge cut off.

    the 3V zener can be a 1 watt type….all resistors are 1/4 watt

    the mosfet can be a IRF9540 for P channel, and IRF540 for N channel

    BC546 base resistor is a 10K

  31. Hello every body ,Please if you have any PCB of battery charger on 24V and Imin=10A,don't hesitate to help me and thank you in advance

    • 5amp is OK, but as you increase the charging current, the battery gets more prone to an early damage, and vice versa

  32. Sir I need battery charging circuit of 48v (12 *4)v and each battery capacity is 26 ah. I need automatic charging circuit.so will u please suggest which one I can use.

    • Nikhil, you can use the first circuit from the above article…make sure the DC input is 56V DC and with 3 ampere current

  33. Mr Swagatam Majumdar
    thank you for helping and directing me to the right path !

    in other post you told me to use the first circuit of this article

    i just wanna know is first circuit could detect low batter voltage ?

    i other word can i use first circuit for a battery backup which is standby ?
    when battery falls below 13.6 or 13.5 or less will it charge it automatically to the full level ?

    thank you again

    • yes it can, in the first circuit from the above article, the circuit will rapidly switch ON/OF when battery voltage reaches the set full charge threshold say 14V and drops to 13.9V….this gap can be increased to any desired value simply by adding a calculated resistor across the pin6 and pin2 of the IC

  34. Hello sir, pls I need an auto-cut off car battery charger circuit diagram to charge my 96volts/600amps battery bank for my inverter,pls my mail:eureka.deg911@gmail.com, tnx alot!

  35. Hello.
    I want to build the last circuit. I don't have 741 but i have some other op-amp's left from other projects, they are : TLC271,LM358,LM318N can i use any of them instead of the 741 ?

  36. Hello Swagatam i am a great fun of you and have been following your work. please can i use the above circuit to charge 36 v battery and can i replace the mosfet with relay, if yes please can you make the change for me ?

  37. Hi Swagatam Majumdar ,

    I have gone through your blog and I appreciate your efforts ! I wanna try the third circuit to cutoff charging at 56.8 v and reconnect charging at 44 v . Means if battery voltage reaches 56.8 v it should cutoff charging and wait till battery drained to 44 v . Will this ckt work for this purpose ??

    Thanks in advance 🙂

    • Thanks Saiq,

      yes, I have simulated the design in my mind thoroughly and I feel it would work perfectly if assembled and set correctly.

  38. I need a charger about 15 amps for 6 8volt(48v) trojan golf cart batteries is this circuit feasible for my application? thanks

    • You could probably build the following design and modify its turns in the following manner for achieving the desired output:


      use 20 turns for the secondary winding of the trafo and use 50V zener in place of the 12V zener, also make sure the primary is wound with two or three 0.4mm wires in parallel for allowing more current in to the trafo….similarly use two or more 0.6mm wires in parallel for the secondary winding.

    • ….the circuit presented in the above article is for enabling auto cut to the battery it's not a power supply circuit….you could probably use it in conjunction with the linked circuit in the above comment.

  39. Hi Swagatam,
    thx for the fast response, i really love this site,
    yes, second curcuit, i'm trying, i guess,there is a therotical misunderstanding or i did not understand properly, when i apply 52Vdc and PİN number3 must be just lower than PİN number 2(Zener) of the OPAMP. this is setting up the upper cut of level. till here fine.
    then connect the link and reduce the voltage to 49VDC and play with the 22K Pot to make PIN number 3 higher than PIN number 2 of the OPAMP. this setting up the lower treshold value.(On sunday i could not manage but ill try again),

    But, here is the therotical problem, when you connect the supply voltage(on second curcuit, where, it is printed as 52VDC) and battery(Where it is printed as per shematic), since supply voltage side is the sensing side, means, if my supply is, let's say 53VDC curcuit is just cut-off because supply is 53VDC and treshold was setted up 52VDC anything above 52VDC will trigger curcuit to OFF state, and if i reduce supply voltage to 51,5V then upper cut-off will never be utulize since supply voltage 51,5 always less than that of upper cut-off level 52VDC.

    could you kindly enlight me.

    • Hi John, thanks!
      I am sorry there was indeed a mistake in the last two diagrams, I have corrected them now with PNPs as the relay driver, however with the present diagram please follow the new setup procedure, that is now pin3 should be higher than pin2 at 52V and vice versa…I hope now everything will fall into place and you won't have to go through all those previous weird results:)

  40. Hi Swagatam,

    yesterday, i had time to test charger curcuit, i could not figure out, how does overcharge cut of feature works, Could you enlight me pls. here below what i have done,

    i have applied the 52Vdc sample voltage, where battery shown, and play with the 10K pot where mosfet just switched OFF. i have double checked at 51,9V curcuit does conduct and 52V curcuit cut-off(mosfet off) so over charge cut off setted up?
    i connected 22K link and applied 49Vdc where battery printed and play with 22K pot, do not respond, 100K do not respond, 470K do not respond. do you think more higher value pot will respond? or am i doing wrong set-up procedure.
    Then i connect the battery for charging purpose, but when i aplly 53Vdc where input shown, voltage level of battery claimed to 52,5V suddenly, but actually level of battery is 48V, start flickiring. mosfet start ON and OFF continuously. i could not fix hysteresis.

    Could you advise me pls, where is my mistake?

    • Hi John,

      Assuming you have used the second circuit,
      You must do the setting up procedures by checking the voltages across input pins of the opamp.

      After you have set the upper limit, connect the multimeter in DC V range across the preset pin of the opamp (pin3) and ground.

      Now in this position reduce the input to 49V and starting varying the feedback preset until the pin3 voltage just falls below the pin2 reference. You can add a 1N4148 diode in series with the feedback link to avoid any sort of leakages from the opamp output.

      A 470k preset would be more than enough.

      a diode may also be added in series with the base of the lower BC546 transistor for a better response.

      If you are connecting the battery as the load you must connect it first before applying the input voltage, also the input amp must be 1/10th of the battery AH for proper functioning of the circuit. For example if the battery AH is 20AH, the input must not be more than 3amps,

      Please confirm the above and see how it fairs.

  41. Hi Swagatam;

    regarding to your question, İ have managed to cut off at 43V and re-connect at 47V, if i be more spesific.
    1-)i constructed to curcuit except mosfet part, and sample voltage suplied, where battery is printed. i aplly sample voltage 48V(where battery printed), i check PİN number2, it was 3.6V due to zener reference voltage. PIN number3, i adjusted to 3.7V(By the help of 10K pot). then i connect to hysteresis link 100K and reduced to sample voltage 43V . i played with 100K pot till, where output of the OPAMP just switch to low logic.
    2-) then i experimented the curcuit at 43V, it was cutting output of the OPAMP, to low logic and at 47V, it was again reconnecting(making high logic output). then i replaced the PİN2 to PİN3 to see output of the opamp at 43V high logic and at 47V low logic. comparator part working superbly.there is no problem at all.
    3-) my trouble strated, when i try to switch OFF and ON the mossfet, first, i thought due to P-channel N-channel mosfet diffrence, but later, i realize that, its always ON regardless of the OPAMP condition(Low logic or High logic no difference,always ON due to same supply line of the source, meaning at 43V supply voltage, gate of the mossfet low logic, but Source of the mosfet 43V and drain also 43V and mosfett ON, at 47V gate is high logic 12V at the gate, source 47V and drain 47V,what ever the supply voltage mosfett is ON), when i try to increase output of the OPAMP to full suplly voltage,by this way gate would also receive full voltage (cancelling PİN7 limitation) then mosfett started to act but not perfomed well as well.in order not danger curcuit, i asked you, you said, dont do it(but i did not check thoroughly increasing the output of the OPAMP, you said bad idea).

    my question is, can i do, with this circuit 48V battery disconnect? ill constructed battery, where battery is printed and where you placed to 52V DC input, ill construct to load, and switch ON and OFF the load, by the help of this curcuit? if i manage to OFF and ON the mosfett then load also would be ON and OFF at desired level of battery since comparator part is working superbly.

    Could you advise pls?

    • Thanks John, It's great to hear that you have succeeded in making the circuit work as per the explanation.

      You are right, the second circuit can be used for connecting or disconnecting a particular load in response to the set battery thresholds, it could be a battery or any other load whatsoever.

      You may go ahead and make the second design for the intended operations, in fact you could even try the first circuit for the same, using an N channel would make the design much easier to configure as could be witnessed by comparing the above schematics.

  42. Hi Swagatam;

    i was bussy, just today had some time to experiment on circuit, evetually failed to switch off mosfet.
    Actually; i was worried both the case, about 2.2V during low logic of the curcuit and not switching off the mosfet(you said that drain and source voltage doesnt matter and PİN7 have nothing to do), as per your suggestion 3.3 zener solved the 2.2V problem, i mean low logic,i get 0v instead 2,2v. but mosfet, from the moment, curcuit is energized, become on, and never becoming off. it is very strange.

    i gave up on mosfet, i dont know, why, it is not becoming off, damn thing, always on, regardless of the low logic or high logic condition.
    Could you advise me about BJT, i shall connect base to zener 3.3 (PİN6), collector to positive supply and emiter to load, right? and how many pcs would be enough? no need to make some darlington pair etc right? simple 1 transistor instead of mosfet right?

    • Hi John,
      I think we are missing something here….the circuit uses a p-channel mosfet which will switch OFF with a positive that must be at least 5V more than its source voltage.

      It means we need to supply at least 5 + 48 = 53V in order to switch OFF this mosfet.

      oh that's a big blunder!!

      The BJT will also face the same issues, it will require at least 48 + 1 = 49V in order to switch OFF completely.

      And our opamo is helpless, it's not able to supply anything above 12V.

      So that's the problem we are facing presently.

      I would rather suggest that we switch to a N-channel device instead of the P-channel.

      All connections remain as is.

      we just need to put the N-channel in series with the negative supply instead of the positive, and swap the input pins of the op amp with one another.

    • …by the way the p mosfet would require just 48V in order to switch OFF in your case and not 48 + 5 as wrongly mentioned in my previous comment:)

    • John, I was just going through your first comment and it made me realize that perhaps the last circuit will need to be modified differently for your application.

      The above circuits are all configured for charging the battery and not for connecting or disconnecting a load with the mosfet.

      So I am a little confused here with your following statement:

      "What im trying to set up is; at 43VDC battery should dissconnect the load and when battery level reach 46VDC or 47VDC battery should re-connect to load. is it possible with third circuit above?:"

      I would like to have a little more clarification on this.

  43. Hi Swagatam;

    İf, i remove the opamp suply voltage component(PIN number7) what i mean is, removing 15V zener and 10K resistor, by this way output of the opamp voltage will be the same suplly voltage available in the battery.
    right now, output of the opamp is limited to zener voltage, which is max 14,6V(in high logic case). do you think, ill put the circuit in danger? by removing zener and 10K resistor at PIN number 7. the circuit can also operate without zener and 10K resistor at PIN number 7 ?


    • Hi John, I would have never put the zener at the IC supply if it could tolerate anything above 40 volts.

      The IC and the mosfet would get instantly damaged if the protection zener was removed.

      Moreover any mosfet would nicely work at around 12V, irrespective of their S/D specifications, so 14V is quite optimal, I thought you were worried about the leaking 2.2V which was not allowing the fet to switch OFF at full charge levels.

  44. Hello Swagatam;

    your site is really informative, thanks for the response.

    i have established the curcuit, without the mosfet part, operating just fine, i manage the trip at 45V and re-trip the 48V. but mosfet do not trip the load.
    Since by placing 15V zener at opam supply,you fix the output of the opam at 14,6V(High logic case), gate of the mosfet is receiving 14,6v at 48,49,50,51,52,53,54V supply voltage. At 45V sample voltage mosfett gate is receving 2.2V(Low logic) but mosfett do not make any cut out for the load. pls note that at 45V sample voltage, drain and source of the mosfett is 45V. gate is only 2.2V. how will mosfett will trip, when gate is receiving only 2,2V in low logic case and 14,6V at high logic case,in the other hand drain and source receiving 45V. i have try N-channel and P-channel both.

    Could you pls advise for the mosfett part.

    • Thank you John,

      In order to block the 2.2V and make it zero you can try adding a 3V zener in series with the gate of the mosfet. (anode to gate)

      Alternatively you could quit using fets and try BJT such as TIP142 for making things much easier.

      In any case the 3.3V zener would be required at the output of the IC.

      This should hopefully solve the problem.

  45. Hi Swagatam;
    İm a new bee in electronics,thx for the response, here below what i have seen, when, i try to build third circuit.
    1-)Since im observing battery discharge cut off side, i have feed 52Vdc sample voltage to battery end,(not mossfet side) .
    2-) i saw green LED lighted but not the red LED(red led never lighted actually,green led always lighted).
    3-)İ checked with the help of multimeter voltage at PIN number2 of IC and saw 3,6V
    4-)İ checked PIN number 3 of IC and voltage was 3,1. with the 10K preset i increase the level 3,8V.
    5-)İ checked voltage at the output of PIN number 6 , 2V low logic.
    6-) İ inserted 22K preset and feed the sample voltage 43V, nothing change, then i change to 22k to 100K preset, output of the İC became 14V, high logic.
    7-)i remove the sample voltage source and connected the actual 48V battery and connect the load at mossfet side, battery level reduced to 43V but mossfet did not cut off the load.
    😎 gate voltage for mosfet is maximum 14V when İC is high logic level and 2V when IC is low logic. but mossfet drain and source voltage is 42V. mossfet did not cut out out the load.

    do you think, did i do correct? if i did correct, then when battery level falls to at 43V i should have seen the load cut out from the battery suplly.
    Could you advise pls.

  46. Hello Swagatam;

    i build third circuit, when i aplly the 52VDC, as per the cicuit set up procedure, while playing 10K preset, red led supposed to lit or off but strangely from the moment apllyinng 52VDC greed led lit, and do not switched off what ever i try and red led do not light at all. so actually i could not setup circuit.
    Could you kindly tell what is the procedure to setup. pls note that, i follow your circuit set up procedure, with some trial and error also i could not fix. here below your comment which i have tried as well.
    don't bother about the LEDs remove them for the time being or use 10K resistors.

    Just check the output voltage while adjusting the preset. Do it in the following manner:

    Initially keep the 22K feedback preset link disconnected.

    feed approximately 52V DC to the circuit, with some trial and error adjust the 10K preset to make the output (where the battery would be connected) switch OFF. Flip the preset to and fro to confirm the adjustment….now glue the preset tight.

    Now reconnect the 22k preset link back into position.

    next apply a 46V DC input and adjust the 22k preset to restore back power at the output, if it gives problems increase the preset value to 100K.

    the circuit is all set now

    What im trying to set up is; at 43VDC battery should dissconnect the load and when battery level reach 46VDC or 47VDC battery should re-connect to load. is it possible with third circuit above?

    you kind response will be highly appreciated.

    • Hello John,

      It will definitely work if you do as per the abve stated instructions.

      It has to work.

      The idea is (with a 46/47V input) to make the pin3 voltage of the IC just higher than the pin2 voltage of the IC by tweaking the pin3 preset so that the output of the IC is forced to go high (RED LED ON), until this happens meaning as long as pin2 V is higher than pin3 V, the output stays low (green LED ON)

      You'll have to implement this by connecting the sample power supply input at the battery end….not at the mosfet end.

      You can check the proceedings as explained above with a multimeter.

      And as mentioned in the earlier explanation the feedback link should disconnected while setting the 46V threshold and must be reconnected and brought to use while
      setting up the lower threshold cut off

  47. Hello Mr. Swagatam;

    how about battery over discharge cut-off curcuit for 48V battery. assume that, when battery level reduced to 43V, curcuit should cut-off the load, and when battery level reach to 46V curcuit should supply the load. do you think it is possible to make using op-amp 741 such a circuit? i think it is easy when battery level at 43V cut-off is easy, but re-conducting at 46V will be difficult?
    Could you pls advise?

    • Hello John, yes it's possible, the above circuits are all designed to cut off and switch ON at the any desired high and low thresholds within the reange, so it's already there in the above shown design

  48. Hi Swagatam, I simulated circuits 2 and circuits 3 with TINA and I found out that the currents flowed through the battery were both more than 30A. The cause of these were found to be the very high gate source voltage across the p-mostfet (i simulated with IRFP9140). can you comment on this?


    • Hi Cham,

      You are right, we need to add 12V zener diodes across the gates of the shown p-channel mosfets for correct operations.


    • the 12V zener is to be connected between the gate and the drain of pMOS? what is the role of the OpAmp in this case? it doesnt seem to be helping in ensuring constant current does it?

    • the opamp cuts off the mosfet gate voltage when the battery is fully charged by applying a high logic at pin#6. this happens as per the setting of the preset a its pin3

      the opamp is for controlling over charge of the battery, not the current.

    • Dear Sir
      I have al-ready request the plz send to me 12vx4(48v ) Circuit when the battery discharge the battery bank to get 46v around . relay cut off with circuit diagram and install the parts in detail plz sir……

    • Dear Kailash,

      Try the second circuit in the above article.
      replace the mosfet with BC546 transistor. Connect its emitter to negative line of the circuit and connect a relay across its collector and positive, relay should be 48V rated.

  49. Swagatam, tomake onek dhanyodab eto sojoj kore bujhie debar jonyo..Amar kache MJE 13007 transistor ache, ami ki TIP122 er bodole seta lagate pari ?

  50. Hi Drubajyoti, thanks very much!

    yes you can use the first diagram in the above article, and use a supply of 56V approximately as the input.
    Without connecting the battery adjust the preset such that the LED just shuts off at 56V input.

    After this switch OFF power, connect the discharged battery and then switch ON power again for the required charging until the supply cuts off automatically.

    You can use a transistor TIP122 in place of the mosfet, use a large heatsink for it.

    The input current should be 1/10th of the battery AH, that is around 20/10 = 2amps

  51. Hi Swagatam, Ami battery byke charger toiri korchi, 12volt 20amp battery ache 4te. total voltage 48. ami ei page er 1st diagram ta toiri korbo thik korechi, etay to Hi voltage cut off ache, kintu 12 volt er battery te full charge hole 14.2 volt hoy, se karon e mot voltage hobe 56.8 volt ba pray 57 volt, ei circuit daigram e sei voltage e cut off kor jabe to ? ar ei prothom diagram tate ami ki mosfet use korbo seta ektu janao. tomar ei blog amader moto electronics premi manusher onek upokar e lagche. onek dhanyobad tomay, ei blog din e dine aro onek somriddhyo hok ei asha rakhi. – Dhrubajyoti

  52. Just a quick thought before I make myself sound stupid by saying I can solder but thats about it lol could I get a permanent magnet wind turbine generator the 48v version and boltbthat to the engine and run it through a wind turbine regulator I could be realy wrong but just thought the only diference is its my engine driving the motor rather than the wind. But no idea about wats amps snd volts calculations lol with me having 4x 12v 75 ah batteries what sort of wattage or amp or even voltage for it to work. Like I say just a thought lol i can repair most electronic or electrical things but dont realy understand the ohms law thing i like working on circuit boards but wouldnt know where to get a circuit board from like you have in your pics with nothing soldered onto it to start with. What I do have here dont know if it might be the same as what is on your pics, I hav 2x kemo m102n accumulator chargers these are ment to charge 2x 12v batteries up so I bought 2 of them a while ago thinking it would do the job but like I say when I looked at the wire diagram I would have ended up with a strait positive to negative short with the batteries still connected up.

    • shipment could be a big hassle…so may be not a feasible idea to get it done from India

      lol you wont blow up anything believe me, i'll show you how to do it step wise.
      could you please procure the IC LM196 or LM396 one for each battery in the meantime??

      As soon as you get them we can proceed with the construction, of course i am assuming that you know all the basics such as soldering, resistor color codes, mounting parts on pcb etc.

    • according to me, you will have to make the circuit that's given at the bottom of this article:


      you will need 4 of these with a common input….the individual 4 outputs will connect with the positives of the 4 batts.

      all the negatives should be made into a common rail and connected with the alternator negative.

      however the above set up would not give you an automatic cut, for that you will have to modify each individual circuit with additional parts as done in the following design:


    • Wow lol way to complicated for me the thing is if you look at how 4x12v batteries are wired up to get 48v if you joined something up to all the positive wires then used the earth surely at some point on the batteries you would be running positive strait to negative somewhere along the battery cables. Ive done a drawing of it and dont think it would work, the only other thing I could do is add another alternator to the engine wich is possible and have it rewound to give out 48v and isolate the earth and just go strait to the batteries, someone said a car alternator is actualy 3 phase and generates 110v its the regulator and rectifier that trims it down so isnt there a way to change the regulator myself to give out 48v and isolate the earth then some how work out what amps that would give me

    • My suggestions are lengthy but not complicated,moreover the idea is technically correct and extremely safe…it will take care of your batteries for years to come.

      Your idea could be quick but hazardous and non-friendly to the batteries.

      Modding an alternator?? do you think it would be an easy job? Even a slightest mistake could mean fire and explosions.

      Charging batteries by connecting them in parallel without appropriate isolation is never recommended.

  53. Hi mate thought id ask to see if you could help me atvall as no one else seems to be able to, I need to charge 4x12v 75ah lead acid batteries from a 12v input. This is going to go into my van so will be charging from van alternator, looked at the alternator and it says 120a is there anything you could suggest and is that possible. It would need to cut out when it had fully charged the batteries and could do with being quite a good amp setup to charge that bit quicker than say a 3a system. Thank you in advance

  54. It's a lengthy and confusing procedure, better to check it practically by attaching a 1k resistor across it's gate and positive, and confirming a lamp activation across its drain and ground by touching its gate to ground.

  55. dear sir,
    please modify the 3rd circuit in led resister value and ic pin 6 to mosfet gate resister value and 22k preset value

    • Dear Subramanyam,

      led resistor value is OK.

      pin6 voltage will be equal to the zener voltage at pin7 of the IC…use a 15v zener here.

  56. 48v,20ah(4 nos,12v*20ah battery series total volt 48v full charge 52v) escooter charger 48v 3a charger) and gate volt how much?

  57. 48v,20ah(4 nos,12v*20ah battery series total volt 48v full charge 52v) escooter charger 48v 3a charger) and gate volt how much?

  58. you can try the last circuit, replace the 22k resistors at pin3/2 of the IC with 1k resistors, and remove the 22k at pin7 with a direct link, also remove the zener at pin7.

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