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12V Smart Battery Charger

The following automatic 12V smart battery charger circuit was exclusively designed by me in response to requests from two keen readers of this blog, Mr. Vinod and Mr.Sandy.

Let's hear what Mr.Vinod discussed with me through emails regarding the making of a smart battery charger circuit:

Discussing a Personal 12V Battery Charger The Design

"Hi Swagatam, My name is vinod chandran. Proffessionaly i am a dubbing artist in malayalam film industry but i am an electronic enthusiast too. I am a regular visitor of your blog. Now i need your help.

I just built an automatic SLA battery charger but there is some problems with that. I am attaching the circuit with this mail.

The red LED in circuit is supposed to glow when battery is full but it glows all the time .(my battery shows only 12.6v).

Another problem is with 10k pot. there is no difference when i turn the pot left and right. . So i request you to either correct these problems or help me to find an automatic charger circuit which gives me a visual or audio alert when battery is full and low .

As a hobbyist i used to make things from old electronic appliances. For the battery charger i have some components. 1. Transformer from an old vcd player. out put of 22v, 12v,3.3v.

And i don't know how to measure ampere. My DMM has only the ability to check 200mA. It has a 10A port but i can't measure any ampere with that.(meter shows "1") So i assumed that the transformer is above 1A and below 2A with the size and requirments of the vcd player. 2. Another transformer -12-0-12 5A 3.

Another transformer - 12v 1A 4. Transformer from my old ups(Numeric 600exv). Is this tranformer's input is regulated AC ? 5. couple of LM 317's 6. SLA battery from old ups- 12v 7Ah. (Now it has a 12.8v charge) 7. SLA battery from old 40w inverter - 12v 7Ah. ( the charge is 3.1v) One thing i forgot to tell you. After the first charger circuit, i made another one (i'll attach this too). This is not an automatic one but it is working. And i need to measure the ampere of this charger.

For that purpose i googled for an animated circuit simulation software but didn't get one yet. But i can't draw my circuit in that tool. there is no parts like LM317 and LM431(variable shunt regulator). not even a potetiometer or led.

So i request you to help me to find a visual circuit simulation tool. I hope you will help me. regards

vinod chandran"


12V Smart Battery Charger Circuit

Hi Vinod,  The red LED should not glow all the time and turning the pot should change > the output voltage, without the battery connected.

You can do the following things: > > Remove the 1K resistor in series with the 10K pot and connect the pot's  relevant terminal directly to ground.

Connect a 1K pot across the base of the transistor and ground (use center and any one of the other terminals of the pot).

Remove everything that's presented at the right side of the battery in the diagram, I mean the relay and all..... Hopefully with the above changes, you should be able to adjust the voltage and also adjust the base transistor pot for making the LED glow only after the battery is fully charged, at around 14V.

I don't trust and use simulators, I believe in practical tests, which is the best method of verifying. For 12v 7.5 ah battery, use a 0-24V 2amp transformer, adjust the output voltage of the above circuit to 14.2 vollts.

Adjust the base transistor pot such that the LED just starts to glow at 14V. Do these adjustents without the battery connected at the output. The second circuit is also good but is not current controlled, though. Let me know your thoughts.  Thanks, Swagatam

Hi Swagatam,
First of all let me say thank you for your fast replying. I will try your suggestions. before that i need to confirm the changes you mentioned. I will attach an image consisting your suggestions. So please confirm the changes in the circuit. -vinod chandran


Hi Vinod,

That's perfect.

Adjust the transistor base preset until the LED just starts glowing dimly at around 14 volts, with no battery connected.


Hi Swagatam Your Idea is graet. The charger is working and now one LED is glowing to indicate the charging is in progress. but how can i configure the charge full indicator LED. When i turn the pot to ground side (means lower resistance) LED starts glowing.

when resistance goes high LED will be off. After 4 hours of charging my battery shows 13.00v. But that charge full LED is off now. Plz help me.

I am sorry disturb you again. The last email was a mistake. i didn't see your suggestion correctly. So please ignore that mail.

Now i adust the 10k pot to 14.3v(it's quite difficult to adjust the pot, because a slight variation will result a bigger voltage output. ). And i adjust the 1k pot to glow a little. Is this charger supposed to indicate a 14v battery?. After all let me know the danger level full charge of the battery.

As you suggested, everything was alright when i test the circuit from breadboard. But after solder into PCB thing are happening strangely.

The red LED is not working. charging voltage is ok. Anyway i am attaching the image that shows the present condition of the circuit. plz help me. After all let me ask you one thing. Could you please give me an automatic charger circuit with a battery full indicator. ?.

Hi swagatam, Actually i am in the middle of your automatic charger with hysteresis feature. I just added a few modifications . i will attach the circuit with this mail. plz check this out. If this circuit is not ok then i can wait for you to tomorrow .

regards vinod chandran



I forgot to ask one thing. My transformer is about 1 - 2 A. Idon't know what is the correct. how can i test with my multimeter?.
Besides if it is a 1A or 2A transformer, how can i reduce the current
to 700mA.

Hi Vinod, The circuit is OK, but won't be accurate, will give you a lot of trouble > while adjusting.

A 1 amp transformer would provide 1amp when short circuited (check by connecting the meter prods to the supply wires at 10amp range and set to either DC or AC depending upon the output).

Meaning the maximum power of is 1amp at zero volts. You may use it freely with a 7.5 ah battery, it won't do any harm, as the voltage would drop to the battery voltage level at 700ma current and the battery would get safely charged. But remember to disconnect the battery when the voltage reaches 14 volts.

Anyway, a current control facility would be added in the circuit that I would be providing you, so there's nothing to worry


I'll provide you with a perfect and easy automatic circuit, please wait  until tomorrow.

Hi swagatam,
I hope you will help me to find a better solution. Thank you.
vinod chandran

In the meantime, another keen follower of this blog Mr.Sandy also requested a similar 12V smart battery charger circuit through comments......

So finally I designed the circuit which will hopefully satisfy the needs of Mr.Vinod and Mr.Sandy for the intended purpose.

The following figure shows an automatic 3 to 18 volts, voltage controlled, current controlled, double stage battery charger circuit with standby charging feature.

For circuit explanation please read the comments.....



Need Help? Please leave a comment, I'll get back soon with a reply!


  1. Hi Dedy,

    2K2 is an incorrect, i don't know why the datasheet itself shows the wrong values on many occasions, I practically found that it should be a 10K pot for getting a range of 1.25v to 24V variable option. Please replace the 2k2 with a 10K preset or pot, you will be able to fix the desired 14.4V mark with it

  2. Please do not connect the battery while setting the preset, also the 10k feedback resistor from pin6 should be kept disconnected while adjusting the preset, once its adjusted this resistor should be connected back into position.

    For the charging operation, the battery should be first connected to the circuit and then power should be switched ON.

    The transformer current must be rated at 1/10th or not more than 1/5th of the battery AH value.

  3. Did you set the 10k preset at the opamp pin#3 for 14V relay activation?
    After setting R2 to get 14V at the output, you must set the opamp 10k preset to just activate the relay.
    Do this without connecting any battery and the 10k link disconnected.
    After the above setting is done, adjust P2 to vary the voltage from 12 to 15V, and confirm the relay activation at 14V by repeating many times.

    For 5ah battery the chaging current must be around 600ma or 0.6amps select BC547 emitter resistor as 0.6/0.6 = 1ohm.

    The above settings will produce a perfectly smooth charging and over voltage cutoff, without any relay vibration

  4. I could not follow what you are suggesting, the diagram is correct, as long as the upper threshold is not reached the battery charges via the N/C contact, once the over threshold is reached the relay activates and breaks its N/C contact cutting off supply to battery.
    The two diodes at battery positive ensures that the battery level never falls below 13V mark.

  5. Can this circuit be used for a 60 Ah 12 V deep cycle lead accid battery( it is a bank of three 12V 20 Ah batteries in parallel ) ? If it can be, what are the modifications needed?
    What will be R value for the battery rated here?
    Another doubt too. Can this circuit be used for all low ang high Ah batteries?

  6. Hellow sir have a good day
    Hoping that you will reply to ma previous comment. Now i am back with a question that ,whether this circuit can be used with the single transformer inverter/charger/changeover circuit ( which you posted one year before in this blog ) as an automatic charger section ?
    I am using a higher watt transformer ( about 300 W ) which is supposed to draw current more than 20 A

  7. Dear sir,
    Is this charger a completely automatic one ? Means will it be able to stop charging after a higher threshold and also able to start charging when the battery voltage reduces down to a MINIMAM THRESHOLD ?
    How this circuit can be caliberated for these two THRESHOLD LEVELS ? Can u plz explain it. You didn't even mention about it in this article

  8. This circuit cannot be used for 60AH battery, it can be used for charging all types of batteries rated at below 50AH

  9. The single transformer inverter/charger can be added with a current limit feature just by adding a calculated high watt resistor in series with D6, the above circuit is not necessary.

  10. Dear Ramsree,

    yes the above circuit is completely automatic, the 10k preset at pin2 determines the upper cut off while the feedback resistor value across pin6 and 10k preset center lead decides the lower restoration point. This feed back 10k resistor should be replaced with a 100k preset for enabling the lower threshold setting-up option

  11. Hello, mr swagatam
    i have been following your blog just of recent. about two days ago. amd i have been following your comment with keen interest. i need your help/assistant concerning the circuit for battery charger that i want to construct( Automatic battery charger for acid lead battery be it smf/ deep circle etc.) that is as high as 200AH. pls is this cct okay for it if not is there any midification or is there any one that that i can use for that project. and i want it to be smart battery charger. if none smart charger i will prefer anyone that has been tested and constructed by one of the blogger and is working fine.
    sorry for asking this too much question as i have read through your comment on one of your Automatic Lead acid battery charger using NE555 you said the modify circuit is below the page of the blog but i could not find it . i need your assistant concerning that.

  12. hello sir,
    i will be expecting your reply as soon as possible. keep it up with the good work.


  13. Hello Mr. Day,

    I will recommend you the IC 741 circuit that's been discussed in the above article, because it's extremely accurate and is a tested design, the one using 555 IC is still a bit controversial and hasn't been tested by me yet.

  14. hii mr swagatam, i just built last schematic on this article, i want to know is ..that's N/C contact is in default state when in charging mode ?or when charging mode, the relay will activate..??? because i really confuse, what's condition the relay will activate , in charging mode will activate or in default state will charging?? and one thing.. when reaching voltage/near supply voltage, the relay will fluctuating ...

  15. Hi hui,

    yes N/C means Normally Closed or closed at the indicated N/C contact in the deactivated or default state.
    when switched ON, the relay will not activate and the battery will get charged from N/C default state of the relay, once fully charged the relay will activate and switch from N/C to N/O disconnecting the battery from charging.

  16. the 10k feedback resistor from pin6 of the IC is introduced to prevent fluctuations at the threshold cut off points, did you connect it properly?

  17. i've checked the resistor 10k resistor to make sure connected to the pin 6 of IC but still fluctuating, and i already checked on the internet what is normally closed means on the relay pin, the 87a called the N/C and 87 is N/C so i connected as schematic for N/C and the other.., i ve forgot to say on my last post, i used TL071, because i dont have LM741, did this ic caused the relay fluctuating?? transistor i used was D438 .when charging state, red led will off and red led will glow if charging is stopped.. correct me if im wrong...

  18. TL071 is fine, but the circuit shouldn't fluctuate if everything's done correctly.
    Your power supply amps should be rated at 1/10th of the battery AH, or should be much lower than the ah of the battery.

    remove the battery and check the response by flicking the preset to and fro. if the relay clicks would indicate your circuit is good.

  19. Hi everyone
    A little late, but a common mistake in charging lead acid batteries is too low a charge voltage. LA batteries (12 volts ones) require a charge of 15.2 volts to get fully charged. This is difficult to achieve without boiling the batteries though. Only going to 14.2 volts will shorten the life of the battery because it does not return sulphates to the electrolyte and eventually the plates will increase the sulphate buildup- This is accumulative (it is never completely removed at the standard 14.2 volt charge). HOWEVER, getting to 14.2 volts must be done very carefully and slowly - high current, low voltage. This is usually only accomplished with some sort of a "pulsed" current so the batteries get a short rest between pulses and do not get a chance to boil.
    Having said that, getting to 14.2 V is pretty standard for most chargers (meaning that most are not very good for your battery). Even if you just do the standard 14.2V most of the time and once every 10 cycles or so give them a 15.2 volt boost it should be OK. Just keep in mind that 1) 14.2 volts will not allow your battery to last as long as it should AND, 2) be careful not to boil the battery when trying to get to 15.2 - that is also bad for the battery and if it cannot be achieved, just go as high as you can without boiling.
    Things are generally not what they seem in the world of "standard".
    I wish I could give you a circuit that would do the 15.2 volts reliably but I am not an electrical design engineer.

  20. Thanks Joan! however I don't agree with you.

    The standard spces are always the correct and safe.

  21. sir this ckt work for 12v 40ah batry ?

  22. yes, it can used for charging it.

  23. can i use 12v 3A transformer ........... and 0.6/ .6A= 1 ohm 1/4 w resistor ???

  24. Hello, you don't have to break anything

    it indicates the relay coil connection points....your relay's coil will have two leads, just connect those leads across the points marked as "relay coil"

  25. the voltage rating of the relay coil should be equal to the supply voltage fed to the circuit

  26. Hi swagatam, thanks for your tutorial...
    I wonder that if you have any idea about to design 3 stage battery charge circuit for 12 lead acid battery? or any suggestion for it? hope you can help me, thanks

  27. Hi Basil,

    you can check out the following post:

  28. Thanks for your reply.
    Yeah, i think it will match my application
    But can i use step down transformer with 15v/2A for a 12v 7.2ah lead acid battery? Then can you describes what resistance value of R1,R2 &R3 will match my battery, I am using 12V 7.2ah sealed lead acid, initial current 1.08A and control voltage 13.6V, how can i modify the circuit to fit with my requirements?
    Thanks for your attention

  29. The formula for calculating R1, R2, R3 is furnished in the article, just replace the AH values appropriately in the slots for getting the results

  30. So for my battery, 7.2Ah=3.6Ah
    R1 = 0.6/3.6 = 0.167 i supposed to use 0.1 or 0.2ohm?
    R2 = 0.6/1.8 = 0.34 ohm
    R3 = 0.6/0.9 = 0.67 ohm, it's right?
    I am using a 2 amp rating transformer, do i need to change to 10 amp rating?

  31. R1 calculation is OK, the other two are not.

    R2 = 0.6 X 5/7.2 = 0.41 ohms

    R3 = 0.6 x 50/7.2 = 4.1 ohms

    2 amp trafo will be sufficient.

  32. Pls sir at the op amp stage which pin of the ic is the 10k at the center pin of the pot connected is it pin 3 or pin2

  33. the center of the preset will go to pin#3

  34. Is this circuit tested cos my relay activates despite the status of the pot and the red led glows a little could it be that the op amp is damaged

  35. connect a 4.7V or a 3V zener diode between pin6 and the LED joint, this will stop the leakage voltage from activating the transistor all the time.

  36. Do u mean connecting the negative side of the zener to pin6 and positive to all components in pin 6 pls tell me how the connection will be

  37. yes that's correct, see this article diagrams for the details:

  38. So I have studied the whole circuit and it all makes sense except the relay part. I feel like an idiot because everyone else seems to get it... so the n/c switch in the circuit is part of the relay? I get the part that says relay coils but what is connected to the actually switching part of the relay?

  39. The N/C section shows the switching contacts associated with the relay coil. you can get more info regarding the same from the following article:

  40. Ok I just needed to make sure. Thanks!

  41. Hi Swagatam,

    I would like to use this circuit in conjunction with this inverter circuit:

    When the battery is drained, I would like to provide a cut off so that no more power is drawn from the battery. I'm planning to incorporate another comparator circuit to do that. My query is how I should cut off the battery. I believe these are the options:

    1. Cut off power to the inverter circuit using a transistor.
    2. Cut off the battery positive terminal to the transformer using a MOSFET like IRF540 or better.
    3. Both.

    As I don't want to run a relay on a battery that's already drained out, what would you suggest?

    If this is a duplicate post, please ignore it as the previous one didn't get posted properly.


  42. Hi Swagatam,
    I would like to get some help if possible in puting together a smart NiMH battery balancing charger 120V AC Input 24V DC output 19.5 AH
    let me know your thoughts!
    Thank you very much!

  43. Hi Eddie, Lipo charger circuits are a little complicated than ordinary chargers since it has to detect the levels of the different cells and charge them separately and also cut off separately when charged fully.

    I have tried to design one similar circuit which you can find here:

  44. But resistor is connected in series with the Battery. So it consumes the entire 14 volts that delivers the power supply.

  45. Sir, this circuit has a little mistake: You need to send the Vout to the NO pin of the relay so the 741 can trigger it without falling off every time. Everything else works perfectly.

  46. Ezequil, the positive supply should go to N/C as shown in the figure, so that the relay activates only at the full charge level, and deactivates at the lower charge level. If the opposite is done then the relay will activate at the lower battery level which means if the input supply is not connected the relay will keep discharging the battery even further below its lower threshold.

  47. at N/C the battery charging is cut off with the relay switched ON, and at N/O battery charging is switched ON with relay switched OFF, this makes sure that the relay is always OFF at battery low, and not drawing power from the battery as soon as reaches the low level and with no input supply.

  48. hola señor me gustan mucho sus proyectos. he estado bus cando el circuito de cargador de baterias que usted diseño que solo utiliza un transistor bc y 2 diodos 1n4007 y no puedo encontrarlo. creo que tambien lo incluyo en un inversor automatico tambien. por favor me podria dar el linck de este circuito gracias

  49. Thanks Jesus,

    here's the link you are looking for

  50. muchas gracias amigo felicitaciones por su trabajo

  51. Hello engineer, pls is there any 3 step automatic charger for single transformer charger/inverter/changeover. also, i made a charger- 13.9v,15amps using only transformer and rectifier to charge a 12v, 400ah deep cycle battery but it gets very hot in 10mins, why and what can i do.

  52. thank you and wish you the same!!

  53. Hello Atinuke....a 400AH will require at least 40 amp current, that means your charger should be equipped to handle minimum 50/60 amp current....otherwise it will get hot and even get damaged.

    I already have a 3 step charger circuit posted in this website but I don't think it's absolutely essential to use a 3 step charger for charging a battery optimally, can be done with a 14 hour low current charging....

  54. thanks for all your response. sir, how can i integrate this circuit into single transformer charger/ inverter/ changeover to automatically trip off at 14.4v and automatically come up at 12.5v.

  55. which circuit? can you provide the specific link of the inverter??

  56. sir, how can i integrate this circuit into single transformer charger/ inverter/ changeover to automatically trip off at 14.4v and automatically come up at 12.5v.

  57. sorry that doesn't look feasible...but actually an automatic cut off is not required for this inverter because the transformer will never produce more than 14V for a 12V battery and therefore the battery will never get overcharged.

  58. sure! You can try the following concept

    make sure to adjust the relevant pots and presets for the intended 18V battery....the charging voltage need to be be around 20V for a 18V battery.

  59. Hello Swagatam. From Vinod's corrected circuit, which diodes should be used for rectifier if 0-24V 2A trafo is used. And also which capacitors and their voltage ratings.
    Thank you

  60. Hi Tshepiso, you can use 6A4 diodes for the bridge, 1N5408 can be also tried, but they might get quite warm at 2amps, so 6A4 is the better choice.

  61. Thank you, Swag. And what do you think about the filtering capacitor sizes? Should they even be a couple or will one work just fine?

  62. My pleasure Tshepiso, just one will do, a 1000uF/25V will be good enough!


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