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6V, 12V, 24V Battery Charger Circuit

A simple yet accurate automatic, regulated 6v, 12v 24v lead acid battery charger circuit is explained in this article. The circuit switches off the current to the battery as soon as the battery reaches full charge. An illuminated LED at the output indiacates the fully charged condition of the battery.

The circuit diagram may be understood from the following points:

Fundamentally the voltage control  and regulation is done by the versatile, work horse IC LM 338.

An input DC supply volt in the range of 30 is applied to the input of the IC. The voltage may be derived from a transformer, bridge and capacitor network.

The value of the 4k7 preset is set to get the required output voltage, depending upon the battery voltage to be charged.

If a 6 volt battery needs to charged, R2 is selected to produce a voltage of around 7 volts at the output, for a 12 volt battery it becomes 14 volts and for a 24 volt battery, the setting is done at around 28 volts.

The above settings take care of the voltage that needs to be applied to the battery under charge, however the tripping voltage or the voltage at which the circuit should cut off is set by adjusting the 10 K pot or preset.

The 10K preset is associated with the circuit involving the IC LM324(1/4) which is basically configured as a comparator.

The inverting input of the IC 324 is clamped at a fixed reference voltage of 6 via a 10K resistor.

With reference to this voltage the tripping point is set via the 10 K preset connected across the non inverting input of the IC.

The output supply from the IC LM 338 goes to the battery positive for charging it. This voltage also acts as the sensing as well as the operating voltage for the IC 324.

As per the setting of the 10 K preset when the battery voltage during the charging process reaches or crosses the threshold, the output of the IC 324 goes high.

The voltage passes through the LED and reaches the base of the transistor which in turn conducts and switches off the IC LM 338.

The supply to the battery is immediately cut off.

However the moment voltage is switched OFF across the battery terminals, the battery voltage tends to fall rapidly to some lower value, normally this would trigger the opamp back into its original position and the power would be restored to the battery giving rise to a rapid switching of the output at the threshold levels. The introduction of the 470K resistor eliminates the above possibility and latches the system to a switch OFF position even with the fall in the battery voltage level….however this position is sustained only for some fixed lower level of the battery depending upon the value of the resistor (470k), increasing the value will make the difference less and vice versa.

(presently the 470K is replaced with a 1n4148/1K diode resistor, making the circuit a one-shot latch up device….so it won’t respond to battery low level threshold, and will need to be manually reset in case the battery voltage falls below the lower threshold.)

The illuminated LED indicates the charged condition of the connected battery.

This 6V, 12V, 24V battery charger circuit can be used for automatic charging all lead acid or SMF batteries having voltages in between 3 and 24 volts.

6v, 12v, 24v battery charger circuit with automatic cut off and shut off using LM338 IC


  1. Hi!

    I'm interested at your automatic battery charger. But I'm confused with LM388. I couldn't find it in my circuit wizard's voltage regulators. I saw it on google as 14pin IC and what does 2 in R2k2 and 7 in 4k7 means?

    Thank you.

  2. Hi Swagatam.
    I have gone thru all the battery charger circuits posted in your blog spot.. It great.. but most of the charger are related to lead acid battery chargers.. can you get Lion (lithium ion)battery charger circuits..

  3. hi Swagatam.
    This is when the circuit can be used directly in 12V battery?
    when the input voltage would I use 21V DC from the solar panels ..

    Thanks.. šŸ™‚

    • Swagatam, I want to ask again.

      why the LED indicator is not lit when the voltage of the battery has exceeded the threshold .. and when the light is on, it also as an indicator when the voltage used to charge the battery disconnected as well?
      I also checked the condition of the LED can still be used.

      do you think is the problem I face now

    • Swagatam i'm a filifino sir thanks for your circuit very useful.i ask also can i use to solar panel directly and can be use as automatic charge the battery and reach the voltage and stop the charge get from solar panel and not backing voltage from battery to solar and also can i use this for substitute to solar charging control, thanks much for the answers.. Godbless

  4. If a 6 volt battery needs to charged, R2 is selected to produce a voltage of around 7 volts at the output, for a 12 volt battery it becomes 14 volts and for a 24 volt battery, the setting is done at around 28 volts.
    Hi Swagatam,
    How can I made possible to use this circuit by immediately selecting needed voltage say; if the battery to be charged is 6v, then after that, there is a need to charge a 12V battery, and lastly, if the need arises to charge a 24volts battery?
    Can you possibly elaborate the connections on how to do it? maybe my question would be also a great help to others who wanted to have this circuit simply and clearly modified for convenient use.
    More power to your new site
    e-mail address : edgardodiolola@yahoo.com

    • Thanks Edgardo,

      With R2 = 2k5, gives 14 volts,

      with R2 = 1k2 gives 7.5V

      and with R2 = 5k1 gives 28 volts.

      So the above resistors may be suitably arranged and selected using a selector switch for getting the desired output voltages.

      Thanks and Regards.

  5. Hi Swagatam, need to be clear on one thing, the output of charging circuit (i.e output of 338) is connected to the 741 comparator and keeps a constant voltage of 14 voltage around 14 volt right? I know that the voltage across the battery increases with charge but if the constant output is connected to the comparator how can the non-inverting end of 741 get a increasing voltage with charging? I'm little confused. Please need guidance.

    • Hi ahnurbd,

      The voltage at the output of 338 will immediately drop to the level of the discharged battery voltage and will gradually increase as the battery gets charged.

  6. sir , i have 10W solar panel delivering 21V , 0.57A and i want to charge 12V 7Ah SMF battery. can you please replace the 10K trimpot with a specific rated resistor because it is difficult for me to adjust. i have to construct 31 peices like this for a rural electrification project. charge controllers casts me 800 rupees and itr is not so ecnomic. please reply as soon as possible. i reqiure very badly. and how much is 104 50V capacitor rating. waiting for ur reply.

    • sir please instruct me how to set the preset , and where shud i take the output whether just before pin 3 or at pin 6.

    • Temporarily disconnect the 4k7 resistor from the base of the transistor and connect it to ground, then without connecting any battery, adjust the preset until the led just lights up.

      Now reconnect the 4k7 to the base of the transistor.

      The charger is ready.

      When using the circuit for charging Always connect the battery first before switching ON power.

      Before setting the preset change R2 to 2.5K value…..

    • thank you very much sir. will try it out and let you know. if i have doubts i would surely ask you. thank you once again.

  7. Hi Sir i am impressed from u and ur knowledge! Sir i want to take a start in Circuts and their uses! i want to Charge 6 volt 4.5 Am Dry bettry and i have a Transformer of 6 Volts and 5 Am will u plz guide me and give me a circuit according to my need… thanks i m w8ing!

  8. Will you plz explain the ratio between
    Battery Voltage:Transformer Voltage
    Battery Am : Transformer Am
    and shy we cant attach the positive of transformer with the Positive of battery and Negative respectively. to charge a battery i mean why we need a Circuit šŸ™ plz explain Bro i am just taking start in this field

  9. hi mr. swagatam.if i wanted to insert another led in this circuit to show that the battery is still charging,where must i put the led?is it before the potentiometer or after it?or is there anyplace else that i should put it?

  10. Hi and thanks for sharing this great circuit! I need some clarifications though.. I am totally new with electronics (besides soldering and desoldering stuff) and would like to know the following:

    (all mentioned components are all available through a local store and they have a bunch of stuff to pick so here goes…)

    1) when u say LM338 do you mean LM338M or LM338T or any other?

    2) when u say IC741 do you mean LM741CN 8Ī”, UA741CH Ī¤ĪŸ5, UA741, ĪœĪ‘741Ī, LM741CH (METAL) or any other?

    3) when u say IC 7812 do you mean 7812CV (LM340T12), L7812CT, or I.C.7812 (SKU: 800003003002)?

    4) R1 240 means a 240 ohm resistor?

    5) 104 50V means 104 ceramic 50V capacitor?

    6) C1 you say 0,1uF… how many volts is this? is it electrolytic?

    7) the capacitor after the 741 IC (100uF/25V) is electrolytic?

    8a) The variable 15-24 VDC in; shouldn't it have a possitive and negative?
    8b) if yes, where are they in the diagram?
    8c) and how do they connect to the cicuit?

    9a) does it make a difference if you feed 15 VDC or 18,5 VDC or 24 VDC or any other value in between 15-24 with respect to the R2 resisor so we can charge 6V,12V,24V ?
    9b) I mean if you input 15VDC could you charge a 24V lead acid battery with selection of proper R2?
    9c) and if yes without modifying R2, if you input 20VDC could you still charge the same battery?
    9d) whould any input voltage between 15-24VDC make a difference for the same battery with the same R2?

    10) in your second diagram you have substituted 6V 400mW thingy for a 10K

    11) 10K means 10.000 ohm resistor?

    12) lastly, how could I add another led (ie yellow) to indicate that the circuit is currenlty charging the battery?

    I told you I am totally new so if you could be so kind as to enlighten me with your answers, I would gain a better understanding of things…
    Hope I wasn't too much of a hasle and thanks in advance šŸ™‚

    • Hi, here are the required specs

      1)LM 338 in TO-3 package, any type will do.

      2) IC 741 in DIL (dual in line) 8-pin package, any type will do.

      3) IC 7812, in TO-220 package, any type will do but it should have the 7812 print.
      4) 240 means 240 Ohms, 1/4 watt will do.

      5)104 means 0.1uF/50V, any type will do as long it's 0.1uF

      6) see point 5)

      7)100uF/25V is obviously electrolytic

      8)Obviously the positive will go to the input of the IC and negative to the ground marked line.

      9) When it's written 15-24V it means all voltages between them will work. All voltages in the given range will produce almost the same voltage at the output.

      11)10K means 10,000 Ohms

      12) connect it across 100uF/25V, with a series 1K resistor, anode of the LED will connect with the ground or the negative line.


  11. Hi Swagatam, Thanks for sharing this circuit. I have a doubt about the IC 741, is this a General Purpose Operational Amplifier ??? (LM741 or UA741)…..Thank's in advance.

  12. Mr Swagatam, I want to use as power supply of this circuit an old switching power supply of 30v 2A and convert it on an automatic charger for 24V. can I use a LM338T (TO-220) and this will require a dissipator ???. Thank's in advance

    • Yes Mr.Lklunder, you may proceed with your plans, LM338 is the best option you have, TO-220 will do since the current is moderate… a heatsink will be required though, for managing better efficiency.


  13. Hi Mr. Swagatam, I need your help.
    I have finished the circuit and tested but don't work well, the input voltage is 30.2v, I have used a resistor on R2 a 5.6k to give a 28.8 volts output, but the led from pin 6 don't lit , I could see the voltage polarity are inverted (-1.5v) therefore the led don't lit , neither cut the voltage charge.
    When I try to adjust the trimpot, the voltage output vary from 28,8v to down, I guess that's normal when trimpot polarizes the voltage to GDN, but the LED never lit:. ( tested with battery lit).
    I have reviewed the circuit and I don't find the error, all connections is ok.
    Could you help me please,

    • Hi Lklunder,

      I am sorry, I think there are some mistakes in the circuit.

      First of all the IC741 will get damaged above 20 volts, so you will have to use a LM324 IC (1 opamp out of the 4)

      I'll correct the circuit so that you can build it again.

  14. Hi Swagatam, I have reviewed the circuit and now works with a 12v battery. If I vary the trimpot the output voltage goes from 7v to 28.8v … I have adjusted the trimpot reading 12,6v at output, but when the battery has reach 12,8v the voltage output is don't cutted, is this normally ?…The led indicator of the battery is charged lit when I set the trimpot to a voltage minor to the battery. I have installed the LED charging indicator, (R 1K to pin 6 and + to output, the voltage output drops to 6V. (??)
    Now I have adjusted the trimpot to 26,0v voltage output and I connected two battery 12v on serie, but the R1 (240) was burn.
    What's changes needs this circuit in order to charge 24V batteries ???
    Thank's in advance

    • Hi Lklunder,

      Please make the circuit as in the above new diagram.

      After you make this then I can tell you how to proceed, please see my previous comment also.


  15. Hi Mr. Swagatam,
    Sorry for so many questions but I am very interested in this circuit.

    In your new diagram, I can see that the LM324 pin connections are different to the datasheet of LM324 pins, eg. pin 7 in your diagram is connected directly to Vout of LM338, and on the datasheet the pin 7 corresponding to Output 2, same with pin 4, on datasheet correponding to Vcc+ and pin 11 to Vcc-.I think it would not work if LM324 is connected in this manner.

    When I made the above circuit, I put the 7812 regulator to power the IC741, so has not been damaged.
    Thanks and best regards.

    • Hi MrLklunder,

      when i relaced the diagram with LM324 circuit, i just forgot to change the previous 741 pin numbers for the 324 IC.

      Now I have corrected it please check it.

  16. Hi againg Mr. Swagatam,
    Having got the new components, I have finished the new circuit, but before to test it, I want to know how to set the preset of 10K, I understand I have to adjust at 28V the preset of 4K7 for output voltage.
    Could you help with this setup and how to include a charging indicator led ?.
    Thank's in advance.

  17. Hi Mr. Swagatam,
    I just have done the LM338 section (the connections are :Vin 30v, Vin C1 to gdn, Vout to R240 to adj 4K7 to gdn) without other connections and only I got 26.5v max on the output (the 4K7 preset regulates from 1.5v up to 26.5v) inclusive I have increased to 32V the input but the output voltage will not was increased. I think 26.5v is a bit low to charge 24v batteries. The 4K7 preset works fine and it have a resistence of 4K7 ohms. What's you think about this ???. What changes will be needed ???.
    Thank's in advance.

  18. Hi Mr. Swagatam,
    I have changed the preset 4K7 to 10K and works fine, now I have 28V. Following was connect the LM338 to LM324 and setup the 10K preset without R470K, but when I try to adjusting the preset and the center connection of the preset reaches 6V-6.1v, the red led is lit but very very little, almost invisible (is a 3V led), in this moment the output voltage drops to 26V and, if I continue turning clockwise, the output voltage continues to fall, but the led don't shines more.
    I can see that the voltage power supplied to the red led is just of .5v max.
    Is normal that the output voltage drops when the led was turned on ??…. The use 3v led is correct. ?? .
    What's you think about this ???. What other changes will be needed ? ..
    Thank's in advance.

    • Hi Lklunder,

      Disconnect the 4k7 from the transistor base initially and connect it to ground.

      Connect a 3V zener in series with the LED…. band side will go to the IC output and the other side to LED anode.

      Now repeat the adjustment until the LED just lights up.

      Once the setting is done, reconnect the 4k7 to the base of the transistor.

  19. Hi Swagatam,
    Following your instructions I done the setting using the zener diode, the LED brightness was fine during the setting, and the output voltage remained at 28V, but when is reconnected the R4K7 and is removed the zener, the voltage drops to 26V and again, the LED brightness is very very little.
    After adjustment, the output voltage is fixed only of 26V, without connect any battery . Is this normal ???.
    Thank's in advance.

    • Hi Lklunder,

      Yes this is perfect…..once you set the circuit as per the suggestions…. it is done.

      Now the only way to test it is by connecting a discharged battery and then wait until it reaches the set 14.3V…. when the LED will light up and the circuit will latch stopping the charging process.

  20. Hi Swagatam, OK.
    As you remember, I need a charger for 24V (for two 12V batteries connected on serie)
    About this, I have a doubt about the danger of reverse voltage towards the circuit while the charger is off and the batteries are being connected, on this moment there will be 24v from the batteries at the circuit and could be damage it. The last time I remember that the R240 was burned. What's think about this ??..Could be protected with a diode or no is necessary..
    Thank's in advance.

  21. Hi agaim Swagatam,

    Finally I have finished the circuit obtained the following results.

    1.- I set the 10K preset until red LED light up at this point the output voltage was 25.2V. (very very poor shine)

    2.- I connected two 12V batteries on serie at charger (previously discharged), and then switch ON of the charger; at this moment the charging voltage it was increasing at interval from 24v until 26.2V during 8 hrs approx., the initial current was 700ma and was down until 120ma during the hours. Eight hours. after, the red LED never lit, and the voltage of charge never was stopped..

    3.- I proceeded to disconnect the batteries and their voltage was 25.9V. (higher to the setup of output voltage of 25.2V but the red LED never lit).

    4.- At first when I connected the LED charge indicator, the output voltage as well as the current began to fall rapidly until 23V; at this voltage the charger no working. When disconnecting this LED was that kept the voltage at 25.2V and could charge the batteries.

    5.- Definitely the voltage for red LED is very very limited for lit, it's only 0.5V and no shine, almost invisible, is possible improve this ??.

    What do you think about these points ??..The charger still does not work well .Any other adjust or recommendation ??
    Thank's in advance.

    • Hi Lklunder,

      I have made some changes in the circuit please do them and check, you would understand why these changes have been made, without which the circuit wouldn't work.

  22. hi swagatam, i have some doubts on this circuit so, please clear it.
    1)will that led glow after the battery is fully charged?
    (i'm going to charge 12v/7amps battery with this circuit)
    2) is that 10K is 10K POT? which is connected with LM324 positive side?
    3)instead of 6V/400mW zener diode can i use a normal diode, if yes which one can i use?
    4)there are 14 pins coming out from the IC=LM324, in which pin you have connected the LED's negative side and you have connected two 10K to this IC, please let me know the pin numbers of this IC.
    5)can i take the battery's output to the following circuit directly?

    • Hi Deepan,

      Here are the answers:

      LED will glow and cut off supply to the battery when its fully charged.

      2) 10K can be a pot or a preset depends on the user what he prefers, a preset is better, though.

      3)You may use 4 diodes in series, cathode down, in place of the zener.

      4)324 contains 4 opamps, you may use any one of them and connect the pins as per the diagram.

      5)Yes you can connect the battery directly to the link circuit.

  23. hi swagatam, today purchased this circuits parts but some parts are not available in my locale store so please advice me about that.
    1)LM338 is not available
    2)240 ohms is not available
    3)INSTEAD OF 1N4148 I BOUGHT PH4148 OF ZENER Diode, is it ok?

    • Swagatam,

      The schematic still indicates a 470K resistor in series with the base of the transistor (output of op amp through a LED). Is this correct?

    • Swagatam,

      Please ignore my earlier question regarding the 470K resistor. 50 milliseconds after I hit "return," I realized the base resistor is 4.7K. Thanks.

  24. Hi again Mr Swagatam,
    With the last changes on the circuit, now works fine, now I have a 24v automatic charger from a recycled line adapter, thanks for all your help and patient.
    Thank's again.

  25. I've just put together a prototype of this using the same components as you specify. I am attempting to set it up to charge a 12v SLA.

    I hooked up a battery that is fairly well charged and turned on the charger, then checked voltage at battery terminals (it was up around 16.4 volts) so I shut it down and decided to check with you.

    I'm guessing that the 4.7k trimmer is not set quite right? I wasn't sure exactly how to adjust it, so I just measured 4.7k ohms before I put it into the circuit. I'm supplying the 338 with 30vdc.

    What I'm wondering is the procedure to adjust the output voltage of the 338 chip. Reading through all your replies I believe I understand the procedure for adjusting the 10k pot for the op-amp, just wasn't sure if I got the other correct.

    • If you are charging a 12V battery then it's recommended not to use an input supply of more than 24V, this is also required to keep the ICs well away from their threshold tolerable voltage limits.

      The adjustments are very simple.

      The charging voltage which is the output voltage of the IC LM338 is set by adjusting the 4k7 preset. Use a digital meter while doing this.

      This must be done without connecting a battery.

      For a 12V battery it should be set to about 14.3V.

      Next the 10K preset should be adjusted such that at the above voltage the LED just comes ON. This must be done by keeping the feedback loop at pin#4 of IC324 disconnected. Once the LED just lights up, the feedback 1K/1N4148 loop may be reconnected.

      The adjustments are over.

      Switch off the circuit.

      Connect a discharged battery to the shown points……and now switch ON power, the battery will start charging and will get disconnected as per the settings discussed above…..

      The input current should be 1/10 or 1/5 of the battery AH

  26. Can you draw here a low battery indicator using the unused part of this LM324? so we can incorporate it with the circuit. please..

    • It can be done through a simpler way, just add a 220K variable resistor in series with the 1K resistor at pin#4. This can be adjusted for fixing the low voltage restoration point.

  27. OK, I have some 7ah 12v batteries and with the charger operating, the current on the secondary of my transformer is about 1.2 amps…which I assume would be about right. Unfortunately, it's not rated for that kind of load.

    So, what I'm wondering (please forgive me if it's a silly question) is if there would be a way to incorporate some form of current limitation into the circuit in order to be able to reduce the size of the transformer. (Space being the main consideration in my project)

    Oh, and thank you for your ongoing participation in these conversations!

    • Oh sorry, I have 4 batteries but I'm only trying to charge one at a time. I'm going to build 4 copies of this circuit and incorporate each one into a battery powered led lamp. After starting this project I saw you have several other designs for this type of project, but since I already started with this one, I was hoping to get it to work.

      So to summarize, one 12v 7ah battery. Charging time is not really a concern. Transformer is 120v .14a primary-24v 10va secondary.

      And one other thing I just wanted to double check with you. For adjusting the 10k pot for the lm324, I need to disconnect one end of the 1n4148/1K feedback loop, then adjust till the led just turns on, then reconnect the feedback loop is that right?? I'm guessing I have missed a short or made a mistake somewhere, because I have smoked a few 324's during this step.

    • Your transformer secondary current should not be more than about AH/5 of the battery.

      After rectification if the voltage exceeds 32V, your IC will fry, also with wrong supply polarity.

      Yes the setting up procedure that you've mentioned is correct.

    • That is about what the secondary current is, but what I'm wondering is, if there's a way to clamp the current at a lower value. Or in other words, is it possible to control the current regardless of size of battery.

    • OK, I added the current limiting resistor and that worked like a charm. I wanted to get the load on the input transformer under .4 amps so I put in a 2.2Ohm 5watt resistor (closest they had to 2ohm) per the .6/current calculation and 14.2x.3=4.26 for wattage. Thanks bigtime for your help there.

      Now last night when I put it into action, it seemed to be working perfectly, nothing got too hot and the battery was charging nicely. It wasn't fully charged before I went to bed, so I decided to unplug it just in case while I slept,I disconnected transformer and battery. This morning I plugged it back in and was intending to monitor the rest of the charge cycle.

      I plugged it in, and while I was checking the voltage at the battery terminals with a DMM something on the board smoked. I disconnected the opamp siide of the circuit again and the regulator is still reading 14.2v output, but when I connect the rest of the circuit and check voltage at battery, I'm seeing around 15.5 volts. I can't see where the smoke came from, but I'm assuming I've burnt the 324 again, even though there is no outward sign of damage.

      So, what I'm wondering is what could be causing this? Could connecting the meter to check voltage at the battery be the culprit? This time I used a 16v source, so I'm pretty sure there is no way the voltage across the 324 power inputs could have exceeded the 32v max. And after re-checking the 338 output and finding it correct, I'm assuming the higher voltage is perhaps due to the source backfeeding through the 324 due to some internal damage.

      It will be several days before I get the next shipment of 324's in, luckily they are only about 40 cents apiece, heh. So, until then I can't do much further testing. I'm just curious to get your thoughts on what might be happening here.

    • At 16V nothing should go in smoke because the range is well under the tolerable limits of the devices, so it's very difficult to asses the fault without practically checking it.

      Checking the battery voltage can never cause anything to burn, so it has no connection with the fault.

      Is the IC supply pin of 324 connected at the output of IC338 or directly to the 16V supply? It should be connected to the output of 338.

      Are you using a relay at the output? how did you wire the relay contacts with the battery, because 15.5V is weird and cannot happen if it's fed with 14.2V, from where is the excess voltage generating??

      Probably the answers to the above questions would help you to trace out the fault.

    • I was using the schematic as shown above(How to Build an Automatic 6 volt, 12 volt, 24 volt Lead Acid Battery Charger Circuit / http://homemadecircuitsandschematics.blogspot.com/2012/02/how-to-build-automatic-6-volt-12-volt.html)but with the addition of the power resistor inserted into the ground between the 104 50v capacitor and the transistor emitter (which limited the current on my transformer secondary nicely so it's operating within it's rated output). So yes, the Vcc to the 324 was fed with the dc source, but I can change that easily. And no, I have no relay on the output since there is no relay in the circuit per that drawing that I could see.

    • OK if you re using the above circuit then it has to be exactly as shown in the diagrm, the supply to the Vdd and to the reference pin cannot be taken from the output of 338 that would cause the operations to malfunction…..so now the mystery still remains unsolved because everything's right in the above circuit.

    • Hmm, I'm almost stumped. Is there any possibility that the transformer inrush could be playing a part in this? I'm really just making wild guesses at this point. This is the specific chip I have in there:


      Best I can tell from the data sheet it is what you had in mind, 32v max. The power section I have built includes a transformer with 16v ac secondary which I have running through a 4amp bridge rectifier to supply the above circuit. I don't know if it's likely, but at the moment the transformer is energized might there be a spike that could damage the chip after repeated on/off cycles?

      It's probably something very simple, I'd love to bring the card by your shop to look at, but alas Mumbai is a bit of a long trip from Indiana, haha.

    • Transformer power suppllies are normally very gentle, so I don't think a surge could have caused the frying of the IC, moreover 16V is far below the rated spec of the IC.

      The image shows an SMD type IC, such ICs are difficult to solder on general purpose boards and requires skillfull hands for fixing them correctly, I am doubting a short or a drysolder across the pins, may be….just a guess though.

      I would suggest you to use an ordinary type of IC,as shown here:


      Yeah…Indiana is too far away, no chance of a practical assessment haha..

    • Ok, I got some more 324 chips and so far they seem to be performing well. They are through hole, not SMD, the webpage just had the wrong picture.

      If you don't mind could you just go over the 10k pot adjustment for the above circuit (6,12,24v automatic, no relay)one more time in detail, becuase something is not going right for me there. With the feedback loop disconnected, there is a point where the LED is just barely on, but at that point with the circuit completed and battery slightly discharged and connected to the charger board, the LED comes on as soon as I plug it in and the regulator shuts down. During the adjustment, the LED never really just turns on to full bright, in fact I can turn the pot almost it's full range without that happening. It acts more like a dimmer, just gradually getting brighter as I adjust it.

    • I assume you have connected the zener resistor 10K upper end and pin#4 to the positive source that's connected to the input of 338, this is very important otherwise the circuit just won't function.

      The 10K preset should be set without connecting a battery.

      First adjust the 338 output to the required battery full charge level, and then set 10K such that the LED just comes ON brightly at this voltage.

      After this switch off the supply, connect the discharged battery and switch ON the supply back.

      The transformer rating should be much lower than the battery AH so that the 338 output voltage drops in response to the connected batteries discharged state.

      Now as the battery charges up, the IC324 waits until the full charge level is reached when the intended functions happen, that is the o/p of 324 toggles, switching off the lm338 output and subsequently the charging voltage to the battery……the LED illuminates brightly.

  28. Hi again Mr Swagatam,
    I have a question about your suggestion of add a low voltage restoration point to the charger.
    How it works ??…..It's posible use it for monitoring one battery which after that was charged, and while the battery continues connected to the charger and unused loses some charge and when this happen the charger may start a new charging cycle (type flotation charge) ???.
    I would like add this functionality to the charger …It's possible ???
    Thank's in advance.

    • Hi Mr.Lklunder,

      yes the circuit will automatically sense the low voltage mark and restart the charging process.

      The feedback resistor needs to be adjusted correctly for this to happen.

  29. OK, can I use a mini trimpot 220K ??.
    I guess that the 1K resistor goes connected to pin4, then to the trimpot 220K, and the other pin of the trimpot goes to ground …….is this correct ???

    • Disconnect the 1K from the output of the IC,

      connect the center pin of the preset to the output.

      Next connect any one of the other pin of the preset to the disconnected of 1K.

      Leave the other end of the preset free (unconnected)

  30. hye swagatam.
    can you please tell me the specification of the lead acid battery?because there'll be 12v with different current when i was searcing for them at the store.can you tell me 12v with what range of current should i buy so that it'll make the circuit works?

    • Hi, Afiq,

      The shown circuit will not support batteries above 50 AH, so anything below 50 ah can be charged on this. The input current needs to be 5 times less than the connected battery AH rating.


  31. hi swagatam, WISH YOU HAPPY NEW YEAR

    in my local, some electrical parts are not available, so can you buy it and send it to me by postal or by courier, i'll pay for it.
    i need the following parts for making several circuits(from this blog).
    2)R1 10ohms/2watt :25No
    3)IC1 TIP122: 2No
    4)IC2 LM338: 2No
    5)104 disc: 3No
    6)0.1uF: 3NO
    7)0.01uF: 3No
    8)R2 240ohms :4No
    9)common board: 2No (in low cost)
    10)4k7 pot :2 No
    11)LDR : 5No

    and this is my address.
    1/39 pillaiyar kovil street,

    thank you in advance,

    • Hi Deepan,

      Wish you a Happy New Year, too!

      Providing small quantities of parts will be difficult for me, and moreover it will be too expensive for you….the actual cost of the above parts may be around Rs.400/- but the forwarding cost will be around Rs.500/-

      So it isn't a feasible idea.


  32. Hi Swagatam

    I was trying your circuit above but it seems that i can't make it to work properly. The output voltage that i'm getting from the diode 1n5408 is limited to 5-7v only and i can't get a higher value. I am using a 3Amps 12-0-12 transformer. hope you can help me, i really need this for my project. thanks in advance.

  33. hello sir.. im just confused with the voltage going to the non-inverting part of the comparator.
    why is it connected to the Vout of the 338 regulator and not on the + battery?

    • Hello Reg, you are right, ideally it should connected at the positive of the battery but it won't matter much with the shown connection, just the preset will need to be set at 0.6 V higher, meaning at 15V instead of 14.4V.

  34. hello sir.. im just curious what will happen if i change the connection of 10K preset connected on the anode of 1N5408 and move it to the cathode part?
    thanks in advance…

  35. Sir, to charge the battery needs power battery input or can i use AC transformers to create input power 15-24 volts? if ever what components can i change? thanks in advance!

  36. Thanks a lot for your effort and congratulate, I hope much more to come out day by day, I also have a query regarding this "Automatic 6 volt, 12 volt, 24 volt Lead Acid Battery Charger" Circuit, I'm using 2 x 12v/7Ah SMF Batteries in series to get 24 volts, I want to use your circuit as a charger, can I substitute LM338 with LM317 with proper Heat Sink, I don't need to boost charge the batteries and so no heavy current will flow through regulator IC, I think LM338 will be the wastage of resources an money for such a little charging current, please suggest. also can you suggest the modifications so that current for charging the batteries can be controlled for better life of the batteries, as you have already included the over charge protection and cut-off in your circuit but can you please provide the add-on circuit for "Over Discharge Protection" or "Low Voltage Cut-Off" to have more functionality from this circuit, another query is regarding the Op-Amp LM324 is used as Voltage Comparator in this circuit, would you suggest using LM339 instead of LM324 because it is meant for comparator use only…..please suggest….please email me on my address also if possible…shishirbhatnagar@gmail.com….thanks

    • You are welcome Shishir!

      yes definitely LM317 can be replaced here, since the required current is less than 1.5amps.

      LM339 would also do, however I would suggest Lm324 because it has better overall spec ratings.

  37. Hi Sir Would you kindly Post the circuitry for 12 volt automobile Rectifier regulator unit for two wheeler motorcycle like hero honda splendor, etc

  38. I found a similar circuit using a LM338 with a LM741 that switches off the regulator with a 2N2219 transistor.

    The first thing I noticed was that there was no diode on the output of the LM338. That allows battery current to go through the 240 ohm resistor when the transistor turns on.

    In your circuit above the diode goes to the battery only, but it also isolates the comparator from the battery voltage and only allows it to read whatever is on the regulator output, which may have been turned off. The comparator needs to read that battery voltage to function correctly!

    Now if you move the diode to the output of the regulator and allow the battery voltage to go to the comparator you may find that you don't need the comparator at all. When the battery voltage goes as high or higher than the regulator output voltage, the diode is reverse biased and shuts off current to the battery on its own.

    I found this out when I started my car and the alternator voltage shut off the regulator current from a solar cell. The comparator never turned it off as I set that higher than 14 volts to see what would happen.

    So all we have to do is set the regulator voltage to the voltage that the battery should reach when it is fully charged. Use the comparator to indicate the voltage status and set it accordingly. Your hysteresis feedback resistor needs to be on the pin side of the comparator also. Use larger resistor values for a better voltage trip spread.

    With a 741 I used 2M and 47K on the pot to allow a half volt drop before the comparator went low. That is better than using a 100 mf capacitor like you used in the other circuit too. It also allows the red LED to be placed straight from the comparator output to ground through a 680 ohm resistor.

    • Thanks for the observations!

      Here the position of the diode doesn't matter because the above circuit is a "one shot" kind of design meaning the whole circuit would get latched once the battery voltage reaches the full charge threshold.

      Even with the shown position of the diode the output voltage from the source would drop appropriately in response to the connected discharged battery lower potential, provided the input current is rated at 1/10th or at least at 1/5th of the battery AH.

      A battery can never attain higher voltage than the source voltage, it would tend to show the full charge level but it's only because the source voltage keeps pushing the charge into it…..which can be dangerous for the battery if it's not switched OFF in time….that's why it becomes imperative to use automatic chargers.

    • How is it latched? It might be oscillating. When the regulator is shut off, the only voltage on the comparator input is whatever the regulator gives it. That would make the comparator switch low and turn it back on, no? Why not read the battery voltage directly?

      The diode in that position only keeps the battery from draining through the cutoff transistor. That's fine but why not read the actual battery voltage directly?

      Also if the hysteresis is set up correctly then you don't need the LED in series on the base to allow the transistor to turn off properly.

    • when the battery gets fully charged, voltage at pin#3 would become higher than pin#2 which would prompt the output to go high, this high would reach pin#3 via the 1N4148 diode latching it up immediately into a permanent high position.

      Since the supply to pin#4 of the IC is taken from outside, it would remain operative even after the output from LM338 gets inhibited.

      you can check it practically, you would find that the position of the diode is not critical here, though technically it should be placed just after the 240 ohms resistor.

      The LED has no relation to hysteresis, it's just for indication purpose….it's a one shot latch up circuit so hysteresis may be an irrelevant issue here.

    • OK, so the 1K feedback latches the comparator output. I was reading your description and it mentions a 470K resistor twice, but it was nowhere to be found.

      My friend suspected that the LM741 needed hysteresis to keep it from oscillating in the other circuit where a 100 uf capacitor was on the output.. He then reasoned that the LED was in series because the output voltage was too high to allow it to shut the transistor completely off. Why not just run run the LED separately?

      I see what you mean about the battery voltage dropping rapidly after cutoff. It does that after the alternator charges it too. Latching it is a good idea! I've never seen a diode on a comparator like that.

      I tried your other circuit with a solar panel. Setting the regulator to 13.3 volts still gave the same 140 ma current as when it sent the full 18 volts directly. The battery charged slowly, but it never made it close to 13 volts unless I ran the engine. What would you set the comparator voltage to so that it doesn't overcharge it?

      Thanks for your quick response!

    • the feedback network has been specifically introduced for eiminating the hysteresis issue, if we remove the 1K/1N4148 network, the circuit would start oscillating.

      the diode provides better isolation, although the circuit would still work without it.

      I guess the solar design includes a current control feature, you may have to calculate the relevant resistor values appropriately for generating more current at the output.

  39. On the 104, 50V capacitor: 104 = 10,000 picofarads

    .000.000.010,000 = .01 uf

    I'm going to see how it works both ways. It depends on how fast it hits that bottom threshold voltage.

    The solar cell really does not need regulating. The battery voltage never gets to 13 volts. I just use it as a second power supply for circuit tests. Normally I just run it through a Shottkey diode. I hope to use one of these circuits for charging tools without a shutoff relay.


  40. Hi dear.Your circuit and explaination is great. It really helps me a lot. Thank you, friend.

    But Iā€™m quite new on this electronics field and still have some question to ask.

    1) You said the inverting input of the IC 324 is clamped at a fixed reference voltage of 6 via a 10K resistor, is the voltage value =6V because of the zener diode?

    2) If yes, the IC 324 only become saturated (output become high) when inverting (-ve) and non-inverting(+ve) value are different. The non-inverting(+ve) pin is connected with 10k potentiometer/variable resistor, and that potentiometer is connected with positive terminal of battery, as well as their negative terminal. I just wonder how could the voltage value that come to the non-inverting(+ve) pin could change while the battery is fully charged? Can you explain that?

    3) The last one, how could the high output of IC 324 that connect to base of the BC547 transistor can switches off the IC LM 338? Is switch of LM 338 means the whole circuit is switched off?

    Sorry for asking too many questions. Thank you again

    • Hi Hazwan,

      1)yes the camping voltage is determined by the zenr value.
      2)The output of any opamp would go high (saturated) when the non-inverting input gets a higher potential than the inverting input and vice versa.
      3)As the battery voltage rises, the center pin of the preset is also subjected with a proportional rise in potential which ultimately crosses the set limit in response to battery full charge level, this results in the output going high.

  41. Hello Swagatam,
    First of all, I want to thank you for all the effort you put in your designs and in answering all our questions.

    Now, after looking through the comments and with the help of the scheme I finally have this circuit working. I am using a 19v laptop power supply. Following your instructions I set the 4k7 pot so as to have 14.3v at the output of the lm338. After that I disconnected the 1n4148 loop of the lm324 and set the 10k pot for the red led to barely light up.

    My problem is that when I reconnect everything and turn the power on I don't get 14.3v at the ouput, I get less, like 13.5v at the most.

    So I have a couple of questions.

    1. First of all, how can I calculate the treshold voltage in advance?
    2. Why, after connecting the second stage of the circuit (the one with the lm324), I don't get the full 14.3v that the lm338 was providing before?

    One thing that may be responsible for this difference is that my zener diode is 6.3v and so I have 6.25v or so at the inverting input of the lm324. Should I just set the 4k7 pot so I get full 14.3v at the lm338 with all the circuit assembled and no battery connected?

    If you need any other info just ask me please.
    Thank again for everything,

    • Helo Migue,

      Thank you very much!

      You can eliminate the issue by doing the following couple of things:

      replace the 4k7 preset with a fixed 2.5K resister and connect a 10uF/25V capacitor across the junction of 1K/1n4148 and ground.

      Hopefully these two mods will help to produce the desired results accurately.

    • Hello again Swagatam,
      I will try your modifications asap (now I'm at work). Btw, what is the capacitor for?

      Moreover, I've been giving it some thinking and looking at the diagram I thought of a plausible explanation for this behaviour: if I have the complete circuit assembled and there is no battery connected, the voltage at the non-inverted input will be higher than that of the inverted input (since there is no battery to lower the output voltage). Then the lm324 will go high and the bc547 will start conducting and therefore adjusting the lm338 (if adj is shorted to ground the lm338 shuts off?).

      So with the complete circuit on and no battery one sees the upper voltage threshold -the one at which the charger will stop- and though it cannot be measured, when the battery is connected the lm338 output will be the one set with the 4k7 pot, but lowered to the actual battery voltage.

      Is this correct? Please correct me if I am wrong. I want to understand the insides of this circuit rather than just build it.


    • Hello Miguel,

      Yes that's why the 1N4148 link needs to be removed while setting up the circuit, and I have recommended somewhere in the article to switch ON the charger only once the battery is connected.

      Without any battery connected, the opamp would respond with a high output, switching ON the transistor and the LED. This would result in a complete switch OFF of the LM338 output, meaning when the LED's ON the output of LM338 would show a zero voltage. This is how it should work.

  42. Hello mate,

    Wonder if there is a schematic in your collection that does the following:

    Monitors a 24V bank (therefore a range of >18 to <30 volts) and cuts off at low levels (i.e 22V). Maybe it could also do a high voltage cut off, but not necessary for my application.

    No external power supply would be much preferred.

    Thank you in advance,

  43. Thanks for the fast assistance! I think this is exactly what I need.

    Just a small clarification, can it also be adjusted for an upper threshold with the circuit as it is?

    Thanks again!

  44. hello swagatam, is there any process to utilse this circuit for a 48v system. i go thru your post on 48v charger.which did'nt work for full-battery disconnect. how about adding some mosfets and schottkey diodes into this circuit?


    • Hello James,

      the 48V circuit is perfect and will cut off for both under/over voltage conditions, you will need to set i properly as per the instructions.

  45. Hello Swagatam Majumdar,

    Can I use yours' (this) circuit for charging SUNCA RB640CS 6V 4.5A rechargeable Lead Acid battery from 220 V line voltage with 9V 1.5A Transformer in the input section.


  46. Hi,Swagatam :
    I have a little knowledge in electronics and trying to repair an Old 24 Volts battery charger 20A,and looking in the web I

    found this web page of how to build batteries charger.

    A friend of mine that repairs CB radios check the charger and told me that

    everything is OK(the timer, the diodes,the capacitor)and found that the transformer

    is bad (it's a big transformer) and have no information sticker on it.
    I know it's 120V AC primary and dont know the secondary amount output.

    Also know it's a center tap transformer and the center tap cable goes out of the

    charger to the Negative post of the 24V battery bank.

    And each one of the other two live wires goes to a diode each one that's mounted in

    a non grounded plate and from there goes a cable that goes to the Positive of the

    battery bank.
    Also the transformer output side have two little cables that goes to a capacitor.

    My friend told me to buy a 120AC primary/24AC secondary center tap transformer to

    repair it.
    Is this is the correct output for this charger??
    I see the pictures of the center tap transformer and they have only 3 cables on the

    secondary (the center tap ground cable and the two live wires).
    my question is how to connect the capacitor ???
    Can someone explain to me??
    Help is apreciated and Thanks You In Advance for your help…
    Att Carlos

    • Hi Carlos,

      From your explanation it looks like a center tap bridge rectifier configuration that's employed with your trafo.

      For confirming the trafo simply switch ON power and measure the voltages across the three taps by keeping one prod at the center and the oother to the outer taps, you should get a reading of 24V AC across them.

      No voltage or voltage only across one tap will ensure a faulty trafo.

      Remember to keep your meter on AC volt range while doing the above.

      The capacitors may be connected by joining their negatives together and connecting the joint to the center tap, and the remaining two positives to the other two taps of the trafo.

  47. Hi Swagatam:
    Thanks you for your Help and fast reply.
    My friend measure the output of the transformer between all the wires and it reads 1 ac volt,for that reason he told me the transformer is bad..
    Also measures the output cables that goes to the battery bank and also measures 1 volt DC.
    i will check the capacitor to verify which of the two connectors is the positive one.
    The Capacitor info is:
    AEROVOX 250P440 4M
    4UF 440V 50/60HZ 70C
    SH9848 W.O. 3178 M3448

    Thanks You Again and we will be in touch

  48. Hi Swagatam,

    For 12V Battery
    Q1. If input voltage of 14V to charge a 12V battery, why select 6V zener diode as reference. Can explain the principle and how the circuit work ?

    Q2. Vin=15V, Vout=14V by adjusting R2=4k7 (potentialmeter)
    What is tripping voltage (How many volt ?) at which the circuit should cut off by adjusting the 10k potentialmeter ?

    For 24V Battery
    Q1. Vin=30V, Vout=28V by adjusting R2=4k7 (potentialmeter)
    What is tripping voltage (How many volt ?) at which the circuit should cut off by adjusting the 10k potentialmeter ?


    • Hi Nicholas,

      zener voltages are not critical, you can use any value below the battery voltage rating, because the trip point is ultimately set with respect to the zener voltage.
      for 12V batt full charge tripping voltage is 14.3V
      for 24V, it's the double of the above = 28.6V

      so the preset should be adjusted by supplying the above mentioned upper thresholds and the preset setting should be sealed with some glue.

  49. hello swagatnam i could not found 324 in store and library.could i replace it with lm741? if yes plz guide me the pin connections. thanks

  50. i test it on multisim using lm741 instead of 324, it was not working. suggest me what i should do? i used 317 instead of 338, this section was working. i am using it for 12v battery

  51. i have a 12v 7.2Ah SMF Battery, so i want to good automatic charger for this battery. Above circuit diagram can work for this ?
    Is this above diagram can prevent battery from Discharge when circuit not in use..
    I think there must we a relay for protecting the battery or auto cut- off..
    pls reply.. because i want good charger for my battery and fully automatic.. plz help..

  52. so basically this circuit automatically adjust to suit the battery that you want to charger the only thing you set is the cut off point?
    and can I replace lm324 with anything else I want this design for 12v batteries only

    • it works as per the setting of the preset done by the user.

      the max charging current needs to be limited from the source, the circuit is not equipped for that.

  53. I built this circuit and i have a mutimeter hooked up across the output and i set my circuit to 12v and ny battery is at 12.5 and but the light doesnt come on .

    If i adjust the 10k pot it will come on but at that time there is no voltage going out

    • yes it's OK, for easy understanding and assembly you can even try IC 741

      do the set up in the following manner:

      initially connect LED's 4k7 end to ground

      keep the 1N4148 feedback disconnected from pin3

      now apply 14.3 vlts from a variable power supply.
      adjust the preset to just illuminate the LED.

      once this done, restore the 4k7 and the 1N4148 ends to their original points.

      your circuit set now and will cut off the battery charging as soon it reaches 14.3V

      always switch ON power to the circuit after the battery is connected to the respective charging points, otherwise the circuit will fail to work correctly.

  54. sir ,
    can i use this circuit to charge 2 no of 12 volt 15 AH batteries in series i.e. 24 volt,15 AH battery
    if i go with this same circuit.
    plz tell me about i/p power section of this circuit
    actually i am having 1.2 to 31 volt dcc power supply circuit already made, where i have used 1 amp 24 volt ac o/p and 230 volt i/p transformer ,
    what if i use this circuit to power above circuit ,
    i think there i must have to increase transformer o/p current rating only or any other modification should be there ,that power supply is simply made using transformer and bridge rectifier.

    • Nikhil,

      you can use the above circuit for your 15AH battery.

      The input should be not more than 2 or 3 amps

      use IC 741 instead of the LM324 because LM324 has 3 more opamps inside which must be correctly terminated, so it's better to use IC 741

      but for IC 741 make sure its pin4 is supplied through a 7812 IC otherwise it'll blow of.

  55. Hello, my name is Nunes'm from Brazil, I wonder, if possible, how do I load my 12v battery 240 A with solar panel Solar Panel 230W Monocrystalline model chn230-96m with Pmp 47,6v voltage. I have 4 panels of 230W and two 12v batteries. Thanks in advance.

    • by doing series parallel connections if you are able to generate 15V, 450 watt or 30 amps from the panel, it would allow you to charge the battery within 6 hours.

  56. (1) Hi MR swagatam, good day Sir, please i want to confirm the following component from you

    What is the value of the resistor that is connected in between the Vout of the ic LM 338 and BC547, the one withe the No like 240, is it 240k or 240 ohms?

    (2) The diode with red colour that is connected to the 4K7 resistor is it a LED or just a diode?

    (3) the capacitor c1 and 104 50 volts i will like to know if they are voltage capacitor or not, please explain to me what kind of capacitor the are please.

    My regard. i so much appreciate your designs thanks very much as i look foward to see your reply.

  57. hi mr swagatam please i want to confirm from you:
    What is the value of the resistor, the one which is connected at in between the Vout of IC LM338 and BC547, the one with the No 240 please is it 240k or 240ohms?.

    The diode that is painted Red colour that is connected 4k7 resistor, is it LED or just resistor?

    the capacitor C1 and 104, are they voltage capacitor or not please i will like to confirm all these from you Thank u
    you for your designs.
    am hoping to see your reply as soon.

  58. Sir.please reply what all are the modification I need to do to charge 6v battery. Whether this circuit prevent the battery from over discharge. I need this circuit.please reply me sir. I need a circuit having following specifications 1) constantvoltage 2)autocutoff when fully charged 3)auto cut off when fully discharged. Whether the above circuit work like this for 6v battery charging

    • Vijay, this circuit is also featured to prevent the battery from over-charging and self-discharging (through the circuit).
      you just have to adjust the LM338 pot to get a precise 7V across the points where the battery is supposed to be connected…

      do the above without connecting a battery.

      Next adjust the 10K preset until the red LED just lights up.

      do the above with the 1N4148 feedback disconnected…you can connect it back once the setting up is complete.

  59. hi swagatam,
    I want to use your circuit for my project, i have some questions:
    1. the LM 338 is not available on local market,
    what replacement can I use? the input is 12 volts dc, 5 amps, and will charge a 12 volts battery,
    2. the input voltage and ampere will came from Thermoelectric generator, (series-parallel connections in order to obtain the voltages and amperes) will it work with your circuit?
    3. What type of battery I can use will you advise? how many Ah?

    I need this for my final year project, I'm an Electronics and Communication Engineering Student, please help me, thanks!

    • Hi YCA,

      LM338 is a very popular regulator IC, if it's not available then I guess the less popular will not be either, but anyway I do not know about any other equivalent of LM338,so you'll have to find it anyhow.

      all voltages are same in nature so it doesn't matter from where it's derived from….it'll work.

      use a battery whose AH may be around 5 or 10 more less than your input current rating.

    • hi sir, i have an adjustment for the input current, from 5 amps, it will be 3 amps, does the circuit will work?
      is there any possible replacement for lm338? based on the input of 3amps? thank you so much!

  60. Hi Swagatam,
    I think u know I am fan of ur work, How are you today? hope u doing great, I am in big trouble now days with 4v 1ah and 4v 1.5ah rechargeable batteries, i just bought from market to make led emergency light, and i want to make simple charger for those batteries, please help me out, again i want to tell you that i want to make simple circuit for those batteries. I hope you will solve for me.
    Thx again.

    • Thanks Kamlesh, I think you can use any standard cell phone charger for charging the battery, just make sure to connect a small 6V flashlight lamp in series with the positive wire….as long as this lamp stays illuminated, you can assume the battery to be charging and when it shuts off will indicate the battery is fully charged.

    • Thank you for quick reply, you mean i can connect any standard cell phone charger to charge my 4v 1/1.5ah battery? great… i have 5v 700ma charger, but again confuse with 6v flashlight lamp ( u said in series) u mean i have to connect 2nos 3v led(i have red color small led) in series (3+3=6v) to the positive side of charger?

      5v 700ma–(+)–3vLED+3vLED—(+)BATTERY
      5v 700ma–(-)—————————(-)BATTERY

      This is ok???

    • Thx Friend,
      I got it. I think i havd 2nos of this Incandescent bulb ( from old torch light).. I will do put 2nos bulb in series and will connect.
      and let you know if any issue..
      Thanks again šŸ™‚

  61. hi sir, i have an adjustment for the input current, from 5 amps, it will be 3 amps, does the circuit will work?
    is there any possible replacement for lm338? based on the input of 3amps? thank you so much!

    • you can use IC 741, LM324 will also work but it has 4 four opamps, implies that the remaining 3 opamps will need to be correctly configured which can become a headache…. so 741 looks better, but do not exceed 20V for IC 741

  62. Sorry for asking a lot bro cz I just want to make sure for my project !!

    1. 741 means LM741 ? Same for this also, I can only see LM741CN (DIP 8 pins). Can i just use this ?

    2. If so, will the pin connection still the same with the diagram u have provided ?

    3. For 4k7 and 10k variable resistor, I use trimmers for them. There is no difference between potentiometre and trimmer right ?

    With regards,

    • Hi Chaw,
      all 741 ICs will work, the prefix can be ignored.

      Pin connections will be same for all 741 ICs

      By trimmers I assume you are referring to presets, yes it will work

      pots are large and facilitate knob control, while presets need screwdriver for the adjustments

  63. Noted with thanks, sir. Btw you have mentioned not to exceed 20V if I use 741 IC. Does this voltage mean the Vcc power supply or comparable voltage between V+ and V- ? I want to clear about it. If this is the case, let's say I want to charge 24V battery, I set my power supply as 27V due to LM338. Then my Vcc pin of 741 IC must connect separately to give 0-20 V and Vee goes to ground. Am I on the right track ? Plus, the two 0.1uF capacitors are ceramic and 10uF used is electrolytic ? Thanks sir, I found electronics quite interesting now and helping out for my internship project.


  64. For 741 IC chip, I order the 741CN model to use in this circuit from the market. When I check for the datasheet, it is mentioned that the supply voltage is from 10V to 36V. So, means that I could be able to use. But then, why do you said that cannot exceed more than 12V Vcc power supply ?

    • If you have found a 36V IC then you are lucky you can use it with your high input voltage, the earlier versions were restricted to 22V that's why I recommended not to exceed this limit.

  65. Hi Swagatam,
    2 Things:
    Does this circuit has a auto-cut-off when the battery is fully charged?
    The LED? It's stays ON during charging, going OFF when the battery is charged or it light's up when charging is complete?

    Best Regards.

    • Hi Nelio,

      yes it has an automatic cut off feature, the LED lights up when the full charge cut off is triggered.

      you can add another LED across positive and pin6 of the IC with a series 4k7 resistor for the "charging ON" indication, this will shut off when the charging is over.

    • Hi Nelio, initially I had designed the circuit using 1 opamp from the 4 opamps of the LM324, but later on I updated the diagram with LM321, so presently what you are seeing is using an LM321 opamp IC.

      I recommend LM321 because it is a single opamp while for LM324 you will need to wire its 3 unused opamps also through suitable configurations to keep the IC stable

    • yes you can use the circuit with the recommended inverter, but make sure to first convert the AC from the inverter to DC using a bridge rectifier before feeding the ballast circuit.

      the bridge could be made using 4nos of 1N5408 diodes

  66. hello sir, Chaw here again !! After i bought 1N5408 diode from online and start to prototype the circuit, the pins of diode are too big that it couldnt fit into the breadboard holes. Can you suggest how should I proceed or is there any other type of this diode which can fit into breaboard holes ?

  67. Btw sir, instead of LM321 i used LM741 Ic chip. Is the pin connection still the same as the diagram provided ?
    For example like pin4 of LM741 Ic connect to 3V zenor diode ?

  68. Yes noted, I have followed ur instructions to make a setting before charging. I have mentioned that I will made use of a 7.2V battery to test. In this case, may i know the purpose of why u advice me to get a 8.5V at the battery terminal ? Like for example, if I were to charge another battery (12V,24V,etc…) with different voltage, how do I know the voltage that I should achieve ? Additionally, adjusting 4k7 trimmer to get 8.5V at the battery terminals (cathode of 1N5408 and ground), do I also need to get this voltage at the Vout pin of LM338?

    Since my battery is 400mAh, I try to use 40mA for my input current because I got read from some of the comments that u suggested to use the input current which is five or ten times lesser than the battery mAh. Thus, the time taken will be up to 4 hours to charge my battery, am I correct ? Is there any way to shorten my time taken like increase the input current in the other way ?

    One more thing to clarify is that if I test with my multimetre to the battery terminals while charging, the voltage will keep on increasing after some time and when it reaches 8.5V, it will auto cut off the current and said to have a full charging ? I do apologize for so many questions every time sir. Since I am still doing an internship, I want to understand in depth about all this project. I really do appreciate your patience to me šŸ˜€
    With regards,

    • For a 12V battery the recommended full charge level is 14.3V, so with this yardstick we can easily calculate the full charge levels of other batteries using the following simple cross multiplication formula:

      12/V(Batt) = 14.3/V(full)

      Here V(batt) is the normal batt voltage of any battery, and V(full) is the required full charge result.

      the charging voltage should be measured and set after the diode cathode so that the diode FWD drop is countered appropriately, meaning the LM338 terminal voltage will be 0.6V higher than the above level.

      For a lead acid batt the 1/10th charging current becomes a crucial factor and is strongly recommended but for Li-Ion this may not be the case, these batts can be charged even at their full AH rates for achieving a quick 1 hour or 2 hour charging period (temperature of the batt may go significantly high and must be monitored in such cases).

      Yes the battery voltage will gradually keep rising while it gets charged, and as soon as it reaches 8.5V, will be auto cut-off by the opamp circuit.

      Wish you all the best!

  69. I have found out that when I start to charge my battery, I give an input power supply to it. Firstly I connect the mulimeter at the battery terminals to check the voltage, the voltage rises rapidly and after some time, it keeps on dropping. So, it means my circuit is not working properly. But I have already checked all the connections also. Please suggest me how do I know if my circuit is working properly or not ?

    • you should connect the battery first and then switch ON the input supply.

      once the full charge cut-off is initiated the battery voltage will drop to some lower level and settle at that point.

      for example a 12V batt will drop back to 12.8V from its full charge level of 14.3V after it's cut off by the opamp.

      If you don't want this function, you can remove the 1N4148 feedback from pin#6 to the 10K preset, this will not allow the opamp to latch and will keep switching the supply ON/OFF to the battery in response to the rise and fall of its terminal voltage

  70. Hello sir, I have followed all the settings u have mentioned and switch on the power supply after connecting the battery also. The batt I am using is 7.2V and 400 mAh. So, I set my power supply as 10.8 V and 400mA. U told me that when 7.2V reaches 8.5V(the setting voltage using trimmers), the circuit will do auto cut off right. How do I witness this ? In order to check, I connect the battery terminals with my multimeter. When I switch on my power supply, the voltage at the batt terminals rises but after some times, it always stop around 6.3V . Thus, how do I check for 8.5V and cut off ? I have alr check my connections and all seems to be right. The charging status LED is working also. So, my circuit is not working properly ? I am sorry to ask u a lot of questions because I need to finish my intern project šŸ™

    • Helo Chaw, it seems something is not right in your circuit because with 10V input the battery voltage can never settle to 6.3V.

      do this, connect the 10V source directly with the battery and not through my circuit….and monitor the charging continuously…if it charges normally then the LM338 could be the culprit…and you may have to change the design to eliminate this IC and use only the cut-off section for the operations without the LM338, because if the input is 10V then anyway the IC is not so important.

  71. Hello sir, I want to ask u about the cut off. When i am using with ammeter, i see the battery charging at maximum 50mA only. My main power supply is giving 1500mA. If this is the way, i need to charge batt for long hours. How do i adjust the higher ampere output?

    • Hello Chaw, No that should not happen, if you are applying higher voltage than the battery normal voltage rating then the battery must accept the full input current otherwise your battery could be faulty.

  72. Hello sir , I wanted to apply this circuit as a variable 0-24V power supply for set up procedures.I won't charge battery with it , so need of auto cutoff or level sencing , what modifications do I need to do to achieve that. If u have a better circuit for that purpose please surjest .
    Also I tend to replace the 4k7 preset with a volume control so that I can readily select the voltage I want any time, will that work, and if I don't find 4k7 volume control what other range will work?

  73. Hi

    I'm building this for a 24V battery with a rectified 24VAC supply voltage. So about 30VDC is what I'm actually getting out of the bridge rectifier.

    This is meant to charge between 4-5 amps if possible.


    Please let me know of any modifications that are necessary and any advice is appreciated.

    Thank you!

    • Hi, the opamp input pins are wrongly configured in your diagram…reverse it as per my diagram.

      use a single opamp such as 741 or LM321 instead of using one opamp from LM324

  74. I have a simple battery charger with six volt output, manufacturer says it half bridge but it work on my car battery to charge. My question is that can I use my charger to power this circuit. If yes than can it works properly.

    • yes you can use wit the above design, just make sure to add a large value of around 3300uF/25V at the output terminals of your power supply, this will make it a 12V from the 6V


    I need to integrate 12V/7A sealed lead acid battery to my equipment .

    it consumes maximum of 60W.

    The system has AC-DC power supply with output of 26V/6A.
    and DC_DC power supply with 26V input and multiple step down outputs.

    May i use step up converter(12V to 24V) between battery and DC-DC board.
    please suggest me how to step down the 26V AC-DC board to charge the battery and to terminate the battery supply when power source available and how to again switch the battery when power source is not avaialable

  76. Sir I am supposed to buy an ac to dc power adaptor for my 5.1 home theatre system ( F&D ). The DC output voltage written on its power board is 26V. The specification of my speaker system is
    57W(subwoofer) + 14W(centre speaker) + 13W*4(surround speakers)
    the value of wattage given is the RMS one. so total of 123W RMS. So what type of adaptor should I have to chose to replace the existing one ?

    (1). I have found one adaptor whose specification is 24V, 120W ( 24V, 5A ).. Can I select this one for the intended purpose ?

    (2). What power is written on the commercial adaptors like that ( 24V 120 W ). Is it PEAK POWER or RMS POWER ?

    (3). And if I am opting for the above adaptor will it affect the functionality of my speaker system too much since it has a voltage (24V) 2V below than required ?

    please help me to take a better decision..

  77. Hi Mr. Swagatam,
    I was wondering if you could help me adapt this circuit to work with an input voltage of 36VDC (rectified 24VAC). I noticed that the LM321 max input would not suffice for my application.

    Thank You for your help

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