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5 Automatic Lead Acid Battery Charger Circuits Explained

5 Automatic Lead Acid Battery Charger Circuits Explained

Lead acid batteries are normally used for heavy duty operations involving many 100s of amps, rated with 100 Ah to 500 Ah or even more. To charge these batteries we specifically need chargers rated to handle high ampere charging levels for long periods of time. Here we discuss one such automatic lead acid battery charger with high Ah charging capacity.

This 5 useful and high power lead acid battery charger circuits presented below can be used for charging large lead acid batteries in the order of 100 to 500 Ah, the design is perfectly automatic and switches of the power to the battery and also itself, once the battery gets fully charged.

UPDATE: You may also want to build these simple 12V battery charger circuits, check them out.

1) Using a Single Opamp

Looking at the first high current diagram for charging large batteries, we can understand the circuit idea through the following simple points:

There are basically three stages in the shown configuration viz: the power supply stage consisting of a transformer and a bridge rectifier network.

A filter capacitor after the bridge network has been ignored for the sake of simplicity, however for better DC output to the battery one can add a 1000uF/25V capacitor across the bridge positive and negative.

The output from the power supply is directly applied to the battery which requires to be charged.

The next stage consists of an opamp 741 IC voltage comparator, which is configured to sense the battery voltage while it is being charged and switch its output at pin #6 with the relevant response.

Pin #3of the IC is rigged with the battery or the supply positive of the circuit via a 10K preset.

The preset is adjusted such that the IC reverts its output at pin #6 when the battery becomes fully charged and reaches about 14 volts which happens to be the transformer voltage at normal conditions.

Pin #2 of the IC is clamped with a fixed reference via a voltage divider network consisting of a 10K resistor and a 6 volt zener diode.

The output from the IC is fed to a relay driver stage where the transistor BC557 forms the main controlling component.

Initially, power to the circuit is initiated by pressing the "start" switch. On doing this, the switch bypasses the contacts of the relay and powers the circuit momentarily.

The IC senses the battery voltage and since it will be low during that stage, the output of the IC responds with a logic low output.

This switches ON the transistor and the relay, the relay instantly latches the power via its relevant contacts such that now even if the "start" switch is released, the circuit remains switched ON and begins charging the connected battery.

Now as the battery charge reaches about 14 volts, the IC senses this and instantly reverts its output to a high logic level.

The transistor BC557 responds to this high pulse and switches OFF the relay which in turn switches of the power to the circuit, breaking the latch.

The circuit gets completely switched OFF until the start button is pressed once again and the connected battery has a charge that's under the set 14 volt mark.

How to set up.

It's very easy.

Do not connect any battery to the circuit.

Switch ON power by pressing the start button and keep it depressed manually, simultaneously adjust the preset such that the relay just trips or switches OFF at the given rated transformer voltage which should be around 14 volts.

The setting is complete, now connect a semi discharged battery to the shown points in the circuit and press the "start" switch.

Due to the discharged battery, now the voltage to the circuit will drop under 14 volts and the circuit will instantly latch, initiating the procedure as explained in the above section.

Circuit Diagram for the proposed battery charger with high ampere capacity is shown below

high current automatic cut off battery charger circuit

2) 12V, 24V / 20 amp Charger Using two opamps:

The second alternative way of achieving battery charging for a lead acid battery with high amperage can be observed in the following diagram, using a couple of opamps:

LM358 Pinout

Op-amp (IC LM358)

For more auto cut-off charger ideas, you can read this article regarding opamp automatic battery charger circuits.

At low battery level, the lower opamp output remains high disabling the upper transistor relay driver, which allows the battery to initiate its charging process through the N/C contact of the relay.

As the charging voltage rises, the potential at (-) input pin of the lower opamp exceeds its (+) input level, which switches OFF the lower BC547, however the upper opamp output being still at logic zero the relay sustains its N/C relay position and carries on charging the battery.

At the full charge level when the potential at (+) input pin of the upper opamps tends to exceed its (-) input pin, the output of the upper opamp goes high, switching ON the relay and toggling its contacts to N/O.

This action switches OFF the charging current for the battery.

In this condition, a positive feed back from the 1K/10K resistive divider is applied to the (+) input pin of the upper opamp, which latches the opamp hard, and this in turn latches the relay in the N/O position.

Now, if any intended load is operated through this battery, and as it goes through a discharging process....after a period of time the battery voltage tends to drop to a point wherein the potential at (-) input of the lower opamp gets lower than its (+) input pin....this instantly causes its output to go high, switching ON the lower BC547.

The BC547 grounds and breaks the feedback latching potential and also the base triggering voltage of the relay driver transistor.

The action switches OFF the upper relay driver transistor allowing the relay contacts to revert to its N/C position, yet again initiating an automatic charging of the attached battery.

Video Clip:

The set up of the above circuit can be visualized in the following video which shows the cut off responses of the circuit to the upper and the lower voltage thresholds, as fixed by the relevant presets of the opamps

3) Using a Single BJT for high Ah Charging

The third circuit explanation below details how a battery may be charged effectively without using any IC or relay, rather simply by using BJTs, let's learn the procedures:

The idea was suggested by Mr. Raja Gilse.

Charging a Battery with a Voltage Regulator IC

I have a 2N6292 . My friend suggest me to make the simple fixed voltage high current DC power supply to charge an SMF battery. He had given the attached rough diagram. I don't know anything about the above transistor. Is it so ? My input is 18 volt 5 Amp transformer. He told me to add 2200 uF 50 Volt capacitor after rectification. Is it works ? If so , is there any heat sink necessary for transistor or/and IC 7815 ? Is it stops automatically after battery reaches 14.5 volt ?
Or any other alteration needed ? Please guide me sir

Charging with an Emitter Follower Configuration

Yes it will work and will stop charging the battery when around 14 V is reached across the battery terminals.

However I am not sure about the 1 ohm base resistor value...it needs to be calculated correctly.

The transistor and the IC both may be mounted on a common heatsink using mica separator kit. This will exploit the thermal protection feature of the IC and will help safeguard both the devices from overheating.

Circuit Diagram

high current Battery charger using 7815

Circuit description

The shown single transistor high current battery charger circuit is a smart way of charging a battery and also achieving an auto shut off when the battery attains a full charge level.

The circuit is actually a simple common collector transistor stage using the shown 2N6292 power device.

The  configuration is also referred as an emitter follower and as the name suggests the emitter follows the base voltage and allows the transistor to conduct only as long as the emitter potential is 0.7V lower that the applied base potential.

In the shown single transistor high current battery charger circuit, the base of the transistor is fed with a regulated 15 V from the IC 7815, which ensures a potential difference of about 15 - 0.7 = 14.3 V across the emitter/ground of the transistor.

The diode is not required and must be removed from the base of the transistor in order to prevent an unnecessary drop of an extra 0.7 V.

The above voltage also becomes the charging voltage for the connected battery across these terminals.

While the battery charges and its terminal voltage continues to be below the 14.3 V mark, the transistor base voltage keeps conducting and supplying the required charging voltage to the battery.

However as soon as the battery begins attaining the full and above 14.3 V charge, the base is inhibited from a 0.7 V drop across its emitter which forces the transistor to stop conducting and the charging voltage is cut off to the battery for the time being, as soon as the battery level begins going below the 14.3 V mark, the transistor is switched ON again...the cycle keeps repeating ensuring a safe charging fr the connected battery.

Base resistor = Hfe x battery internal resistance

4) 12V 100 Ah Lead Acid Battery Charger Circuit

The proposed 12V 100 ah lead acid battery charger circuit was designed by one of the dedicated members of this blog Mr. Ranjan, let's learn more regarding the circuit functioning of the charger and how it could be used as a trickle charger circuit also.

The Circuit Idea

My self Ranjan from Jamshedpur, Jharkhand. Recently while googling I came to know about your blog, and become a regular reader of your blog. I learned a lot of things from your blog. For my personal use I would like to make a battery charger.

I have a 80 AH tubular batery and a 10 Amps 9-0-9 volts transformer. So I can get 10 amps 18-0 volts if I use the two 9volts leads of transformer.(Transfomer is actually obtained from an old 800VA UPS).

I have constructed a circuit diagram based on your blog. Please have a look on it and suggest me.  Please note that,.

1) I am belonging to very rural area hence there is a huge power fluctuation it varies from 50V ~ 250V. Also note that I will draw very less amount of current from the battery( Generally using LED lights during power cuts) approx 15 - 20 Watt.

2) 10amps transformer i think safely charges 80AH Tubular Battery

3) All diodes used for the circuit are 6A4 dides.

4) Two 78h12a used as parallel to get 5+5 = 10 amps output. Although I think Battery must not draw full 10 amps. as it will be in charged condition in day to day use so internal resistance of battery will be high and will draw lesser current.

5) A switch S1 is used thinking that for normal charge it will be kept in off state. and after fully charging the battery it switched to on state to maintain a trickle charge with lower voltage. NOW question is that is this safe for the battery to kept in charge unattended for long time.

Please reply me with your valuable suggestions.

100 Ah battery charger circuit diagram designed by Mr. Ranjan

simple 100 Ah lead acid battery charger circuit

Solving the Circuit Request

Dear Ranjan,

To me your high current lead acid battery charger circuit design looks perfect and should work as expected. Still for guaranteed confirmation it would be advisable to check the voltage and current practically before connecting it with the battery.

Yes, the shown switch can be used in the trickle charge mode and in this mode the battery can be kept permanently connected without attending, however this should be done only after the battery has been fully charged upto around 14.3V.

Please note that the four series diodes attached with the GND terminals of the ICs could be 1N4007 diodes, while the remaining diodes should be rated well over 10amps, this could be implemented by connecting two 6A4 diodes in parallel at each of the shown positions.

Also, it is strongly recommended to put both the ICs over a single large common heatsink for better and uniform thermal sharing and dissipation.

Caution: The shown circuit does not include a full charge cut-off  circuit, therefore the maximum charging voltage should be preferably restricted between 13.8 to 14V. This will ensure that the battery is never able to reach the extreme full charge threshold, and thus remain safe from over charge conditions.

However this would also mean that the lead acid battery would be able to attain only around 75% charge level, nevertheless keeping the battery undercharged will ensure longer life for the battery and allow more charge/discharge cycles.

5) IC 555 Lead Acid Battery Charger Circuit

The fifth concept below explains a simple, versatile automatic lead acid battery charger circuit. The circuit will allow you to charge all types of lead acid battery right from a 1 AH to a 1000 AH battery.

Using IC 555 as the Controller IC

The IC 555 is so versatile, it can be considered the  single chip solution for all circuit application needs. No doubt it's been utilized here too  for yet another useful application.

A single IC 555, a handful of passive component is all that's needed for making this outstanding, fully automatic lead acid battery charger circuit.

The proposed design will automatically sense and keep the attached battery up to date.

The battery which is required to be charged may be kept connected to the circuit permanently, the circuit will continuously monitor the charge level, if the charge level exceeds the upper threshold, the circuit will cut off the charging voltage to it, and in case the charge falls below the lower set threshold, the circuit will connect, and initiate the charging process.

How it Works

The circuit may be understood with the following points:

Here the IC 555 is configured as a comparator for comparing the battery low and high voltage conditions at pin#2 and pin#6 respectively.

As per the internal circuit arrangement, a 555 IC will make its output pin#3 high when the potential at pin#2 goes below 1/3 of supply voltage.

The above position sustains even if the voltage at pin#2 tends to drift a little higher. This happens due to the internal set hysteresis level of the IC.

However if the voltage continues to drift higher, pin#6 gets hold of the situation and the moment it senses a potential difference higher than 2/3rd of supply voltage, it instantly reverts the output from high to low at pin#3.

In the proposed lead acid battery charger circuit design, it simply means that, the presets R2 and R5 should be set such that the relay just deactivates when the battery voltage goes 20% lower than printed value and activates when the battery voltage reaches 20% above printed value.

Nothing can be as simple as this.

The power supply section is an ordinary bridge/capacitor network.

The diode rating will depend on the charging current rate of the battery. As a rule of thumb the diode current rating should be twice that of the battery charging rate, while the battery charging rate should be 1/10th of the battery AH rating.

It implies that TR1 should be around 1/10th of the connected battery AH rating.

The relay contact rating should be also selected as per the ampere rating of  TR1.

How to set the battery cut off threshold

Initially keep the power to the circuit switched OFF.

Connect a variable power supply source across the battery points of the circuit.

Apply a voltage that may be exactly equal to the desired low voltage threshold level of the battery, then adjust R2, such that the relay just deactivates.

Next, slowly increase the voltage up to the desired higher voltage threshold of the battery, adjust R5 such that the relay just activates back.

The setting up of the circuit is now done.

Remove the external variable source, replace it with any battery which needs to be charged, connect the input of TR1 to mains, and switch ON.

Rest will be automatically taken care of, that is now the battery will start charging and will cut off when its fully charged, and also will  get connected to power automatically in case its voltage falls below the set lower voltage threshold.

IC 555 Pinouts

IC 7805 Pinout

How to Set Up the Circuit.

The setting up of the voltage thresholds for the above circuit may be done as explained below:

Initially keep the transformer power supply section at the right hand side of the circuit completely disconnected from the circuit.

Connect an external variable voltage source at the (+)/(-) battery points.

Adjust the voltage to 11.4V, and adjust the preset at pin#2 such that the relay just activates.

The above procedure sets the lower threshold operation of the battery. Seal the preset with some glue.

Now increase the voltage to about 14.4V and adjust the preset at pin#6 to just deactivate the relay from its previous state.

This will set up the higher cut off threshold of the circuit.

The charger is now all set.

You may now remove the adjustable power supply from the battery points and use the charger as explained in the above article.

Do the above procedures with lot of patience and thinking

Feedback from one of the dedicated readers of this blog:

untung suharto January 1, 2017 at 7:46 AM

Hi, you have made a mistake on preset R2 and R5, they should not be 10k but 100k, I just made one and it was a success, thank you.

As per the above suggestion, the previous diagram may be modified as shown below:

Wrapping it up

In the above article we learned 5 great techniques which could be applied for making lead acid battery chargers, right from 7 Ah to 100 Ah, or even 200 Ah to 500 Ah, simply by upgrading the relevant devices or the relays.

If you have specific questions regarding this concepts, please feel free to ask them throug the comment box below.


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!

167 thoughts on “5 Automatic Lead Acid Battery Charger Circuits Explained”

  1. Howdy, Friend! Interested to Learn Circuit Designing? Let's Start Discussing below!
  2. Good day sir! I have question about IC 555 Lead Acid Battery Charger Circuit number 5 in this article. I want to add a 5mm LEDs. 1 red led for charging and 1 green led when its done and what are the resistors in each led. Is it possible sir?

    • Good Day Michael, Add one LED in series with T1 base, add another LED across pin#4/8 and pin#3 through a 1K resistor

  3. Olá amigo.
    Estou tentando fazer um carregador ajustável para baterias de litio, com corte automatico de carga de 1,4V até 4,2V com um relé de 5V, um trimpot 1K e um 2n2222a. É possivel? Você tem um diagrama?

    • Paul, sorry that is not possible because the the two ICs have different pinout arrangement.

      You can try two separate IC741, or use 2 opamps from the IC LM324

  4. sir which rating transformer is used in this circuit.
    in using two opamp ckt what is charging supply input (from transformer?).
    and in two opamp ckt what u use before 8050 thansistor which u show a diode sign with different colour and before bc547

    • Imran, the red color thing is a RED LED.

      The transformer rating should match the battery voltage rating and current should be preferably 10 times less than the Ah rating of the battery if it is a Lead acid battery, or 50% less if it’s a li-ion battery

  5. I thing there is a small mistake on this circuit.
    PIN number 2 is mention next to PIN number 8. I think it should be pin 7.

  6. again sir.
    I am needing this automatic charge cutoff for my solar installations, if it can work well for both solar and windmill it would be great!

    • yes it will work for kinds of sources between 3V and 30V, for higher voltages just make sure that the IC supply is restricted below 30V (for the last circuit)

  7. Thanks for directing me here sir… now i have some questions, can i replace IC LM358 with TL072? can i replace transistor 8050 With any darlington pair or any high voltage transistor such as tip transistor? please i will take your advice from here if i have anymore questions.
    thanks for the help sir.
    Youre amazing teacher for real

    • Thanks Nkwenti, yes you can replace LM358 with TL072, just make sure the pinouts are correctly exchanged.
      The relay driver transistor can be replaced with other types but preferably it should be selected as per the rating of the relay, having said this any transistor such as a TIP122 will also work

  8. dear sir,
    on the second circuit in the link you provided,i am asking if LM338 input is connected to battery input?
    and what formular should i use when calculating resistor in parallel with the LEDS?

    • Jude,

      yes the input is connected to the supply source (+), the output is connected to the LED (+), the negative goes directly to the LEDs. The formula is

      R1 = 1.25 / LED safe current (max current)

  9. Dear Sir,
    is it proper to just connect all my bulbs(dc bulbs) directly to the positive and negative terminal of the battery without any damage to the bulbs? using the bottom charging circuit diagram.

    or advise on how best i can handle this.


  10. Dear Sir,
    i appreciate so much for the kind responses.
    i will be building the charger circuit at the bottom article this weekend.
    i need a simple dc emergency light circuit diagram where i have to go straight and connect my load(i.e DC bulbs) through the charging circuit..
    i just need to power five DC bulbs of 5Watts each.
    Humble regards

  11. Dear Sir,
    i want to build this circuit,but i want to use it to charge 12V, 200AH battery.

    My questions are:
    1-what are needed to be modified in the circuit to be able to charge this battery comfortably?
    2-will the battery discharge after power outage when the battery is not in use,if yes,how do i prevent this?
    3-how can i set up this circuit to power dc bulbs without having to build extra circuit for it.
    4-i want it in such a way where i have to switch to DC for lighting in my home when there is power outage from the utility,how do i achieve this?
    5-how do i wire it so that when the utility restores the light and i switches to utility,my battery starts charging automactically.

    Best regards

      • Dear Sir,
        thanks for your quick response.
        i have checked the link and this questions to ask;
        i am a newbie and i don`t want to make a lot of mistakes while building this circuit…does the circuit in the link has charging capacity?,
        i want to build a kind of DC project where i have to power ten dc bulbs like 4watts each,through my 12V, 200AH battery.
        i need a clearer explanation on how i can achieve this project in such a way that the battery will keep charging automatically when there is AC and won`t give out power except the AC is OFF.
        Please note i don`t want the battery to discharge when it is not in use.

        i appreciate your assistance.

        • Jude, if you are a newbie then I am afraid this circuit may not be suitable for you, it will require a good knowledge of electronics to finish it successfully. If possible I’ll try to find an alternative design which may be easier than this one, and let you know

          • Dear Sir,

            Newbie here does not mean i can`t finish it successfully,what i needed know is if the combination of the charger and the emergency light circuits can achieve the design i want and how to prevent discharging of the battery when not in use.
            The emergency light circuit in the link does not have a charging unit and wanted to be clarified and how best i can integrate the charging circuit with the emergency light circuit to achieve my aim.
            thank you again for the kind responses

            • Jude, yes it will accomplish the required objectives. To stop the battery from self discharging when no load is connected you could add a small value resistor from the charging supply + directly with the battery +, this will enable a permanent trickle charging for the battery and never allow it to self-discharge. The resistor value could around 1K 5 watt

  12. Is the 741 used just as a comparator? Can it be replaced with any other regular comparator like lm393 which has two comparators as opposed to 741 which has just 1?

    • LM393 is a comparator and the output might need a pull up resistor, but can be applied in the above concept, other opamps like LM321 LM358 etc can also be used

  13. I have found one problem in this circuit Sirji.
    Whenever circuit is in high cut mode, I discharged the battery at 11v. But the charging doesn’t start automatically. Then I set preset for low cut so led will not off of low cut led even at 14v. So set high preset so low cut problem starts. I used 42ah battery. So what can I do?
    Please guide me sir.. ..

    • Prashant, which circuit are you referring to?

      the second circuit is tested, you can see it in the video how it correctly responds to the high and low voltage thresholds..

      • Prashant, do one thing, just swap the input supply with battery and battery with input supply….meaning apply supply input from the relay side on the right…and connect the battery at the left side….

        I’ll update this in the second diagram soon….

  14. Dear swag sir,
    I am requesting to you that l want to make 15-20 A charging current circuit. So please give me the components list for that including transformers and bridge circuit data.

    • Dear Prashant, you just have to buy a 20 amp transformer, connect a 30 amp bridge rectifier with the output and a 10,000uF / 50V filter capacitor to get the required output, if you want to have any special design, please specify the details….

      • I have 1.5kva inverter with 24v dc and 5a charging Current circuit but I want to make 15 to 20a charging Current circuit and add it in that inverter and synchronized with it.
        Can you give me details of transformer winding like primary and secondary winding gauge and no. of turns of charging transformer. Dear sir,
        I want a Transformer data for 12v and 24v DC Battery charging with 15 to 20a charging current.

  15. Hello, its not that simple as i thought. I just purchased all the items, 10k preset, 10k resistors, LM358, BC 547, 8050, 12v Relay, 1N4007 diode,1N4148 diodes, 6V Zener, leds. But after 2 days of completely attaching the components together I am not able to use it. “””” I want a Wiring Diagram of the Second Diagram””‘ so that i can remake it again. What happen to that i build? When i attached the input voltage of above 12v relay turns on and no output on the other side and even no led lights glow. On other side I have a 12v battery connected to the circuit, when i removes the input voltage the battery itself powers relay and makes zzzzzzzz noise.

    Please provide me a Wiring diagram in which all the components are connected in simple manner and with pics…. Or u can also provide me a sketch of this too… Its my first time so please help !

    • It is an extremely simple circuit idea but only if you do it with proper understanding, how did you set up the presets? did you use a variable power supply for this?

      Give me two days I’ll make a working video of it and upload it.

      • I will be waiting for that video…. Please make that video as soon as possible… “” Talking about presets I didn’t setup them just soldered them into the board and thought I will rotate the screw at the top of them to set when I need it to turn ON and OFF but …… Problem persists…

        • yes you have set it up after soldering only, so how did you do it….I’ll try to upload the video but can’t guarantee you whether it will be quickly or not because I have other work also, but I will do it for sure, it will be not just for you for but all other readers…
          In the meantime you should try fixing your circuit by correctly setting up the presets

          • Okay at least for now You can Provide a drawing of Second Diagram/Schematic. Just show components instead of their symbols. It will make me clear about the connections. “””” Actually I am not sure about my connections and It is difficult for me to read schematic where LM358 is connected and where Relay is connected. I hope you will provide me a Wiring diagram of mentioned schematic.

            • My breadboard set up is ready and the presets are responding perfectly, now only the actual battery charging procedure is pending which I’ll try to finish by tomorrow…I’ll update the diagram by inserting the pin numbers to the IC soon…

          • Im waiting for drawing or video. Please make me feel better as i haven’t slept from 2 days… Red eyes…. Figuring out solutions… Resolving schematic

            • I have updated the circuit diagram with the IC pinouts, and also I have shifted the upper LED towards the base of the transistor since this showed a better response while testing my prototype

      • Yes, but how can set up this variable value?
        suppose, I want to set high cut at 14v DC. So what should be the value between 2 and 3 lead of preset of both preset.

        • you will have to set it up by piratical procedures, first keep the slider of upper presets to ground level, adn the slider of the lower preset to the POSITIVE level.

          then feed the over charge voltage from the left side, say at 14.3V, now adjust the upper preset until the relay just clicks ON and the upper LED is ON. At this point the upper opamp will be latched.
          Now reduce the input voltage slowly up to the lower discharge value, say at 11V, now adjust the lower preset until the lower LED is ON and the upper LED is OFF and the relay is also OFF.

          That’s all your circuit is setnow…you can stick or glue the presets in that position.

            • I am glad the circuit is working for you…current will need to be adjusted from the input power supply side, the above designs are only for controlling the charging levels of the battery…

  16. I am referring second circuit.
    1. How many lm358 ICs are Used in that circuit ?
    2. How many Supply voltage is providing to lm358?
    3. Can I providing the capacitor in parallel with relay diode? What value?

    • yes BC547 can be used if the relay coil resistance is not less than 200 ohms.

      only one LM358 is used

      LM358 can operate with max 32V supply

      you an put a capacitor in parallel with relay coil, it can be a 100uF/25V

  17. Sir, Im am a newbee… I wanted the simplified/wiring diagram of the Second Schematic diagram. Please provide me that, as i am unable to figure it out how to connect the components.

  18. Hi Swag,

    Hope you’re doing well.

    I have three questions.

    1. Can this circuit be used to charge different amperage batteries? I mean if I want to charge 45 AH, 70 AH and 200 AH?

    2. What is the best way to calculate input amp/transformer power rating for a battery, assume 100 AH?

    3. Do you have any transformerless power supply circuit that can provide that kind of high amperage?

    • Hi Saqib,

      1) the circuit can be used for charging any preferred battery, by upgrading the transformer, diodes, and the relay accordingly.

      2) for lead acid batteries the ideal rate of charging is always 1/10th of its AH rating, the trafo can be selected based on this rule.

      3) sorry I do not have any high current transformeless SMPS circuit at this moment.

    • OK thanks.

      To avoid having multiple chargers for different amperage batteries, can we modify this circuit to automatically detect battery capacity and give out as required like Samsung adaptive charging technology which gives 5v 2amp output when required by cellphone otherwise gives some milliamps for normal cellphones.

  19. Hello Sir,
    I am a fan of your work. I am having a little problem. I need a circuit that will automatically cutoff supply from a charger to 3pieces of 200ah battery charging in parallel. I was thinking of using a comparator but I have not found anything useful. The maximum charge voltage for the batteries must not be above 14volts and I definitely would love one that works with MOSFETs or any other transistor you may prescribe. Thanks a lot sir

  20. please sir i have great challenge i need design a battery in the the battery capacity is 3.7V and overall battery capacity is 20A and a need a rechargeble circuit for that

  21. Hai swagatami want to make automatic battery charger (without switch) with cut off. I have battery 24v with 200AH. Would u like to show me how the circuit diagram? And the components? Thanks


  22. Sir please I'm having serious challenge after making those changes and connect my power supply at the point were we have the battery connected still the led did not come up when varying, and when i measure, the voltage is not in line with the zener diode at pin#3 i have 13.16v, pin#7 i have 16.2v while pin#2 is 12.6v but when i apply the voltage sample in the opposite side i have at pin#3 4.7v, pin#7 15.18v then when i vary the 10k preset at pin#2 such that i have a voltage higher than that at pin#3 there will be no output at pin#6 but when i vary it such that voltage at pin#3 is greater there will be output at pin#6 which i have to 13.2v

    But in all these varying i noticed that the led is on irrespective of fact that the voltage at pin#2 is changing its remain on
    Then when i measure the voltage at the point were the battery will be it's 19.23v

    Please sir what could possibly be the error and the voltage at the point were the battery will be connected i thought it should be my supply voltage and not less

    If this is going to give me little problem please can you recommend another working 24v automatic battery charger please ur advice is seriously needed
    Thanks and best regards

    • Faith, in the previous link of 48V charger which you are trying, you must feed the voltage from the points where it is mentioned "48V"….you cannot apply from the other side for this design because the mosfet would be blocking the voltage….right??

      therefore if you are applying the voltage from the left hand side then it is correct and your adjustment results also look perfectly correct.

      when pin#2 voltage goes above pin#3 voltage then pin#6 must become zero (cut-off)…that's correct.

      Now as for the LED being illuminated all the time, this could be due to the leakage offset voltage from pin#6 of the IC741. Did you connect the shown zener diode at pin#6 of the IC?? make sure you connect that to solve the LED switching problem.

      And what did you use for the power transistor, is it a mosfet or a BJT, remember if it is a BJT then you will require a resistor at its base, and the LED shifted towards the zener side, and not at the base of the BJT.

      this circuit is the easiest one, and you are almost near….if you are having difficulty with such an easy circuit then just imagine what you might have to face with the other relatively complex ones…actually all opamp designs are extremely straightforward it's just about understanding them patiently.

    • I really appreciate your help and fast response please i build and tested the first circuit thishttps://homemade-circuits.com/2012/08/make-this-48v-automatic-battery-charger.html during setting when vary the preset the led never turn up, could it the 22k or any component i will remove from the circuit to make it work properly

    • the pin#2 resistor must vary and become higher or lower than pin#3 reference voltage in order to change the output state, while adjusting the preset.

      yes, you can try replacing the 22K with 1K, and remove the 4k7 attached with the pin#2/ground since your supply is not 48V rather 24V only

  23. Wen i put my digital meter across the transformer and at the same time clap it with my clap meter and power it burn of my digital meter but the clap meter read 68A. My question is can this transformer be use to charge a three 12v @200A connected in parallel?

  24. Please sir how do test the current of my power supply because when i use a clap meter to test the current at the positive line its reading 8. 5A could that be the current of my power supply

    • Faith, you can connect you DMM prods directly with the trafo wires for a few seconds and check the reading which will inform you regarding the current rating of the trafo.

      make sure the DMM is set at AC 10 amp or 20 amp range.

  25. Please sir how do test the current of my power supply because when i use a clap meter to test the current at the positive line its reading 8. 5A could that be the current of my power supply

  26. Again i noticed that if connect it to battery for charging the battery voltage rises to the transformer voltage as if its fully charged so please i want to ask it the charging voltage or the its fully charged once connected

    • your power supply current(amp) rating must be 10 times lower than the battery AH rating….did you follow this rule?…if not then please make sure you do it.

  27. Hi
    i have built this circuit but when i try to set it the relay dose not flikers and it only works if the switch is connected pls what could be problem

  28. Hi dear Swagatam thanks a lot with this good battery charger circuit. This circuit is for 10 to 20 ok. There five diodes 4 diodes are rectifier and 5 diode is used between bridge positive and battery positive and all diodes have the same value. Now if I charge a 50 ah or 70 ah batteries. should I change all diodes amp or only the bridge rectifier. Thank you very much with best regards. Rohullah.

  29. Dear sir, again my transformer with three terminal produce 14V between the last 2terminal and 7V between any of the last terminal and the center terminal. Is this the type of transformers you recommend?

    • yes it will do, you can use the 14V for your 12V battery, and make sure the current of the transformer is 8 to 10 times less than the AH value of the battery

  30. Dear sir,
    Thanks for this circuit. I couldn't get 6V zener but 6.8V and 3V zener but 2.8V. will still give me a good result. my hope is the 10K pot. please put me through

  31. hello swagatam,

    Is this a 3-state battery charger? if no, do you have any circuit that is 3-state and charges 300AH battery. Tnx

  32. Pls sir, can u guide me to upgrade this battery charger to be able to charge my 96vols/400amps car battery, tnx!

  33. Hi…I am confused that how are u producing dc output just after the bridge because u said in a comment that u can remove 100u capacitor.although if we continue with this capacitor I think it will discharge very quickly and produce huge ripples at output

    • Hi, 100% pure DC is not essential for charging a battery, however in the other comment my recommendation about removing the filter capacitor in order to eliminate the relay chattering might not be correct.

      the correct remedy to prevent relay chattering at the cut-off threshold is to add 100K hysteresis resistor across pin#6 and pin#3. This will latch the opamp at the cut off levels and enable a quick and complete switch OFF of the relay and the entire system.

      Filter capacitor need not be removed.

    • hello jade, only the transformer, the relay and the diode rating will need to be upgraded as per the battery charging current and voltage specs.

      the charging current rate of a 300AH batt can be around 30 to 40 amps

  34. hello sir.. still, the relay did not tripped after the battery is fully charged.. i also put and LED in series with the transistor base. there is no changes in the led too after the battery is fully charged. thank you for your response sir…

    • when you connect the discharged battery pin3 voltage is supposed to be lower than the pin2 voltage and when battery is fully charged pin3 should become higher than pin2….please confirm whether this is happening or not

    • sir after i finished making the circuit on the breadboard and tested it… the output voltage of my charger becomes 16~17 vdc even though my battery was discharged…when i pressed the pushbutton, the relay latched but when the battery is fully charged it did not tripped or turn off the charger… i followed your circuit in every detail…by the way sir thank you for your reply on my first question..

    • erwin, connect an LED in series with the base of the transistor…anode to base and cathode 10K…this will give the exact idea how the opamp is behaving.

      I am assuming you have done preset adjustment correctly as per the given instructions

    • sir…will the circuit still function even if my charger's output voltage is 16~17 volts dc? thank you for your quick reply sir..

  35. Hi Swagatam,

    I need to change 50-64 amp car battery. Can I use this circuit? How much amp transformer I would be required. And also is there any other changes with capacitors and diode are need to be done here?

    • Hi Nabeel, yes definitely you can use this circuit for charging a 50AH 0r 60AH battery, the transformer current rating should not be more than 8 amps.

      the bridge rectifier diodes must be rated at 10amps each…or a single 10amp bridge module will also work at the indicated position

  36. I have a modified pc smps that gives 14v and 20A
    so I want a circuit that charges a 12v 100ah battery automatically ie it should turn off the smps when battery is full and turn on the smps when the battery is low

  37. thank you for the replayed sir,no i did not make any modifications but one thing is that i use 1n5399 for the 4 diode showing there and also i used In540(16A4) the one that connect to the battery positive is it 6A or 6.4A? sir i build one of your circuit How to Make a Simplest 200 VA Uninterrupted Power Supply UPS Circuit i build only the charger it work very well but i don't think it can charge 24vDC battery two 12v battery? if it can chard what can i do sir help me with the circuit thank you jah guide you and hop to hear from you soon..

  38. Hi Swagatam,

    Thanks for your prompt and positive response.

    I just read, – Hire me for Designing Circuits – Circuit Ideas, Inventions for Sale – in your blog.

    I have a special requirement for my set up, the details I will send to your email ids you have mentioned, would you be kind enough to reply whether there is any scope for making them and your charges as well?


  39. Hi Swagatam,

    I wish I had seen your blog earlier. In my set up, I need my charger, connected to the 220 Volts AC mains to start automatically to charge the battery when the voltage drops to 11 and stop the charger when the battery is charged to 14 volts.

    Your advise and guidance would be of immense value.


  40. Thanks very much, you can do the following things:

    Connect an LED in series with the transistor base 10k (anode to base, cathode to 10k) for indication.

    Keep the transformer disconnected, and apply an external power supply of 14.4v across IC supply pins.

    Now flick the preset to and fro until you find a point where the LED toggles ON/OFF, adjust this point to just switch ON the LED.

    Your circuit is all set now.

    Now carry on with its normal operations through the trafo.

    While adjusting the preset do not connect the battery.

  41. remove push-button connections, connect the trafo directly to mains, and connect the battery positive to bridge positive via relay N/O and pole contacts.

  42. thank you so much my friend… youre so very kind and very quick when it comes in response.. once youre circuit done and it worked for me, i will then donate something to you. i am very interested with this design to build. thank you so much and more power swagatam! please keep update me soonest possible. thanks again.

  43. Sir,
    Thank you very much for your quick reply. 10 amps would be enough for me , would you be able guide us for using ICs like 338, 196 etc on this circuit ? it would be very useful for you.

    Thank you once again sir

    With best regards

    Maung Pru

  44. Hi Maung,

    can you specify your current requirement, if it's upto 10 amps then it can be simply done using ICs like 338, 196 etc, otherwise it would become sightly complicated.

  45. Sir,
    Thank you once again.
    would it be possible to add a charging current controller?
    would you please kindly explain us what type of component need to be use ?

    Thank you and with best regards

    Maung Pru

  46. Thanks, I appreciate your attempt, here are more precise answers:

    AH signifies the charge or the discharge rate of the battery. ideally it indicates the full charge or full discharge rate of the battery in an hours period.

    Opamps are excellent comparators and they work very well in battery charge cut off applications.

    hysteresis is surely needed so that the relay doesn't keep switching ON/OFF at the thresholds due to the extreme accuracy and sensitivity of the opamp.

    Hysteresis delays the relay actions and makes the response slower so that it doesn't chatter…… without hysteresis the relay would snap rapidly producing rapid switching at the thresholds.

    A hysteresis can be added in the above diagram to avoid the same.

    In the above diagram the position of the relay switches OFF power completely, right from the transformer, so it becomes much safer and reiable.

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