Battery Charge controller Circuit using LM324 Comparator IC

A charge controller circuit for battery makes you hassle-free by not bothering about switching the charger with different battery levels, the main advantages being improved life and low current consumption as it switches off the charger when battery full charged. So far we have published a simple battery charge controller circuit using 555 IC where many of our readers commented that it was difficult for them to set the threshold levels and also it seemed like it was harder to analyze working of the same.

For the above reasons, I have decided to introduce another one where you could find it easier to set the threshold levels. Furthermore, this charge controller design makes it easier for debugging. The main component of this circuit is a LM324 comparator IC which has four inbuilt comparators inside; here we are making use of only one. You can use any other comparator ICs instead of this one, like LM358, LM317, LM339 etc. I have used lm324 here owing to its high availability. For basic electronics learners see what is a comparator.

Read: Battery charging circuit with battery level indicator

As the threshold levels to turn ON and turn OFF the charger are different (hysteresis), it helps to avoid oscillation problems of the relay due to leakage current of battery. The threshold levels can be set by varying the potentiometer. As we are switching the charger with the help of relays, you can connect any capacity battery. High current transformers are recommended when using higher capacity batteries to decrease the charging time. For small capacity batteries you should use a series resistance to limit the charging current and that’s not necessary for higher capacity batteries.

This charging controller circuit can be applied with any system that makes use of rechargeable batteries like emergency, UPS, inverters, telephone receivers etc. Here we have used LEDs to indicate the charging status. Also at the bottom I have uploaded a video demonstration to guarantee you that this charge controller schematic is 100% working.

Battery charge controller circuit diagramBattery-charge-controller-circuit-using-lm3241

Click on the image for enlarged view

Components Required:

  1. LM324 IC
  2. Zener diode 5.1V
  3. Bridge IC/1N4007X4
  4. Diode 1N4007
  5. Transistor BC548X2
  6. Led Green
  7. Capacitor 1000uF
  8. Resistor 1KX4, 100K
  9. Pot 50K/47K
  10. Relay 12V/10A
  11. Transformer 0-15V/2A


  • Step down transformer steps down the line voltage to 15 volts and bridge rectifier does the AC to DC conversion process (rectification), it’s followed by the capacitor filter which removes AC ripples.
  • The power supply for the working of charger circuit is given from the battery (as it should always be monitoring the battery voltage level even when the mains supply is disconnected).
  • The non-inverting terminal of the comparator is connected to a zener diode and 1k resistor which are used to make a reference voltage of 5.1 volt, and its inverting terminal is used to monitor the battery level.
  • Usage of zener diode instead of resistors makes sure that the reference value is independent of battery level and temperature variances.
  • In fact the comparison is not between reference voltage and the actual battery voltage, but between a proportional value of battery level and reference value. This proportional value is achieved by using a potentiometer.
  • If the battery level is below the Lower Threshold Point (say LTP), then the reference voltage becomes higher than the proportional battery value. Thus output of comparator becomes positive.
  • Then the transistor is switched ON (as output of comparator is connected to the base of transistor BC548 through a 1k resistor) followed by the relay. I have used a freewheeling diode along with the relay to de-energize the inductor.

See:Working of relay with animation

  • And when the comparison goes reverse, output of comparator is low and transistors are OFF resulting in the disconnection of supply.
  • Output of comparator is also connected to another BC548 through a 1k resistor which make hysteresis. That means when the charger is ON, transistor is ON and it makes a 100k resistor parallel with the respective voltage divider, then the drop across the divider will again decrease increasing the charging level.
  • Upper Threshold Point (voltage level when charger is switched OFF) can be set by varying the potentiometer.
  • The difference between UTP and LTP (hysteresis) can be varied by changing the 100k resistor, decreasing the resistance increases the hysteresis and vice versa.
  • A 8.2ohm 10 watt resistor connected in series with the circuit helps to limit the charging current. It’s not required for higher capacity batteries as the charging time will increase.

That’s all! Let me know through the comments box below if something confuses you.

80 thoughts on “Battery Charge controller Circuit using LM324 Comparator IC

  1. Hello again sir. I alrdy made this ckt but it only charges the battery when I lowered the pot and doesnt stop at 13.8v what should I do? Thanks

      • Dear Yaseen,

        I have built the circuit.

        Kindly explain me how to set the upper thresh hold voltage.

        What i feel or my ambiguity is, that once that charging circuit is connected to the battery, the voltage across battery will apparently be equal to charging voltage, which in my case is 16 volts.

        As the voltage across variable resistor has increased apparently, the proportional voltage drop will decrease and the input at the inverting pin of lm324n will increase than the reference voltage of 5.1 volts, resulting in No output of comparator and the relay will switch off the charging circuit before the battery actually gets charged.

        I know you have explained the working of 100k resistor for this purpose, but i cant get it.

        If you explain it in more detail that how the circuit is judging the UPPER THRESH HOLD VOLTAGE, then it will be of great help.

        anticipating your kind response

        Best Regards
        M. SAlman Ayub

  2. Hi sir, may I know the approximate value of resistance of the pot in ur video? My crcuit only charges when I lowered the pot and doesnt stop at 13.8v. Am I doing it right or I have mistaken something? Thank u for reply sir yaseen

  3. may I know what load you are using to lower your battery level fast so that I can check my ltp and utp? I am using 12v/ 7.2 Ah

    • Hi Kabhi,
      I apologize for your time by trying our circuit. But I wonder I why you did’t get output? The circuit is 100% working in our i-St@r lab and tested and found to be working. For your satisfaction we had given the Video demonstration also. Please double-check the components and test the circuit each section with multimeter. Please put your progress as the following comments here. We will assist you for getting the accurate output. Once again I beg your pardon for spending time for the circuit.

  4. Dear Brother Yaseen,
    Assala mu alay kum warahmatullah.Thanks for your “Battery charge controller circuit diagram” may be its work with 12V Battery charger,but some time i need 24V,36V,48V auto cut battery charger circuit,so please help me how i can use your circuit for 24V,36V,48V.

    May Allah bless u.
    Kamal Hossan,
    Tel : +8801711987611.

  5. Hay,,Good job…Actually I build this circuit on proteus simulation…

    I don’t know why it’s not working…I just put the same components as yours..any help please!


  6. kindly let me know that who to set the threshold…..i am not getting ant output when i exceed the pot +ly…means increasing pot make it not to generate any output.

  7. sir I have connected ckt as given above…it is charging but my LED is not blinking….i checked o/p using multimeter… n is it possible to set threshold for 3v battery….n also using this ckt which all batteries can be charged???
    reply asap

  8. its work with 12V Battery charger,but some time i need 24V,36V,48V auto cut battery charger circuit,so please help me how i can use your circuit for 24V,36V,48V.

  9. Thanks i have test the cct its working, please i want to use it to charge 24v&48v what value do i change 100k to for 24v , 48v?

  10. say me how to what are the steps or where and where and how should check the output …after completion of connection ?

  11. I built this circuit. I used IC LM741 to replace the LM324. I also replaced the 5.1 Volt Zener with a 4.7 Volt Zener. I also replaced the 2 Transistors with 2N3904. I used all NEW Parts. This is not the first circuit I have built. I have a good knowledge of what I am doing. However, this circuit doesn’t work for me. Yes, I have adjusted the POT. The problem is, once the battery is connected, the relay closes and no matter where the Pot is set, the relay remains energized in the ON position. The Voltage on the Base of the Transistor controlling the relay is receiving .8V. When the POT is adjusted, the LED does go On and Off, however, the voltage on the Base of the Transistor doesn’t change and remains at .8V I have replaced the LM741 but it doesn’t chance the problem. When I used the LM741, I adjusted the Pin Outs of the circuit for the LM741. Example: Output on Pin 6, Ground on Pin 4 etc. Any help would be appreciated.

  12. How much output current this circuit will give??? Since i’m using 12V 7A battery to charge it……..

  13. It’s actually a good cct but i noticed sometimes that the relay was getting off and on intermittently. Like when power is connected the charging voltage increased above the battery voltage, so the relay disconnected and the battery voltage goes low and the relay connected again and the happen continuously thereby damaging he relay contact.

    what do you think could be the problem.


  14. you seem to have used a 5Amps transformer.
    I want to charge a 12Volts, 7Amps battery for now at least. what kind of transformer and resistors do I need?
    I also want to find out if the lm324 is a charging & regulating circuit together?

  15. when i vary the pot the battery get charging….but no switching off automatically

    how to set the exact threshold value…

    give me link or video demo for setting the value

  16. hello and how are you.i have a self built mosfet based power inverter of 2KVA capacity.with CD4047 cct is ok and charging as well through the relay switiching in place but i want to add your LM324 based charge controller to my cct.kindly put me through on the ff.,hope the charging option of your cct will not affect mine.Thank you while waiting for your response

  17. I have same problem the relay turns on and off repeatedly. How can I solve this problem i have tried to vary the potentiometer. please help 🙂

  18. hello sir my inverter is not charging it gives voltage to the battery but not amps I reset the pot but there is always 0 amps sir what to do
    my battery is 12v 150 amp
    plz do reply

  19. please can you tell me the detail function of 100 k resistor which is connected with the transistor..what is the relation of 100k resistor with UTP and LTP.if i dont use this part of circuit,then what will happen? wont the ckt operate?plz reply asap..

  20. If you are to use BC558, then you have to reverse the 2 inputs the other way.that is, the 5.1v of the zener diode will be connected to the inverting input (-ve of the LM324N) while the other input will be connected to the non-inverting terminal (+ve of the lm324N). Reason, BC558 is the directly opposite of BC548, it connects the emitter to the base weneva there is no output frm the lm324 (that is when the -ve terminal is greater than the +ve terminal)

  21. I have 4 batt each 2 conected in series and each group(2 series batt)conected in parrellel each batt is 12v 110amp and my source 115 ac volt 60hz how can i use this circiut to charge my batteries

  22. Dear Sir,
    I’m trying to accomplish the above old circuit of yours.
    The problem that i’m facing is that the once the switching transistor turn on the relay after the output of the compartor goes high i.e almost 10.4 volts, but after that when the proportional voltage from variable resistor goes high that reference voltage and output of comaparator goes low i.e almost to 0.6 volts, the transistor still keeps on passing collector current and the relay does not turns off.

    i have use same value of resistors as told in your circuit.

    kindly guide me.

    best regards
    M. Salman Ayub

  23. very interesting circuit you have of the charge controller.
    I’m going to try it as a charger for nicad batteries.( 12 volt units mostly)
    thank you ‘
    Carl S.

  24. How can i charge a 4.7 V battery from an emergency lamp with an adaptor giving an output voltage of 7 V ? Can you please suggest a better way in which anything like transistors can replace the role of Relay in this circuit.. I have tried using a NPN transistor to make the 7 V power available to the battery when a sufficient base signal is fed.. but the problem is the battery is not getting sufficient charging current.. the transistor is limiting the current… when i tested the battery connecting directly to the 7 V supply the amount of current drawn by the battery in discharged condition was about 390 mA… when i have used transistor coupling with the opamps output stage that current reduced even below 60 mA… please help me in this issue

  25. please sir you made mention of 8.2ohm 10 resistor in series with the circuit. so I want to know that where actually in the circuit will this 8.2ohm 10watt resistor be connected is it connected to the output positive terminal or negative terminal of the circuit. and how much current will this circuit produce .

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