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One Transistor Automatic Battery Charger Circuit

One Transistor Automatic Battery Charger Circuit

A very simple single transistor automatic battery charger circuit is described in this article, which uses just a single transistor for the voltage detection as well as for automatically disconnecting the battery from the supply when it gets fully charged.

Circuit Operation

As shown in the diagram we can see a straightforward configuration where a single transistor is connected in it’s standard operating mode. The circuit functioning may be understood with the help of the following points:

Considering the battery to be charged is a 12 volt battery, we know that it is advised to charge the battery until it reaches between 13.9V to 14.3 volts.

The transistor base voltage is adjusted using the preset P1, such that the transistor just conducts and operates the relay at around 14 volts.

How to Adjust the Cut of Thresholds

This adjustment becomes the high voltage trip point of the circuit and is used to switch OFF the charging voltage to the battery when it gets fully charged or its voltage reaches around 14 volts.

The lower trip point of the circuit cannot be adjusted as this circuit is too simple and does not incorporate the low voltage detection feature.

However the transistor is itself equipped with a switch OFF feature in case its base voltage becomes too low.

Typically a general purpose transistor like the one which is shown (BC547) when adjusted to switch ON at 14 volts may have the lower threshold of around 10 volts, when it might get just switched OFF.

This wide voltage difference between the high set threshold and the lower natural threshold is because of the involved big hysteresis with the design. This acts like a natural hysteresis in the design.

The lower threshold of 10 volts is dangerously low and we cannot wait for the circuit to restart the charging process until the battery voltage falls to this dangerous 10 volts level.

Allowing the battery to discharge down to 10 volts can make the battery flat permanently and reduce its life. . Therefore to eliminate this issue the circuit needed to somehow reduce the hysteresis level. This is done by introducing a couple of diodes at the emitter of the transistor.

We know that normally a 1N4007 diodes would drop around 0.7 volts across it and two if them would make a total of 1.4 volts. By inserting the two diodes in series with the emitter of the transistor, we force the transistor to switch off 1.4 V earlier than its normal specified limit of 10 volts.

Therefore now the lower operating threshold of the circuit becomes 10 + 1.4 = 11.4 volts, which may be considered just OK for the battery and for the automatic restart of the charging process.

Having both the thresholds updated as per the standard charging requirements, we now have an automatic automotive battery charger that’s not only cheap to build but also smart enough to take care of the battery charge conditions very efficiently.

Circuit Diagram

Battery charger using a single transistor and relay

Parts List for the proposed one transistor automatic battery charger circuit

R1 = 4K7
P1 = 10K preset,
T1 = BC547B,
Relay = 12V, 400 Ohms, SPDT,
TR1 = 0 - 14V, current 1/10th of the battery AH
Bridge diodes = Equal to the current rating of the
Emitter diodes = 1N4007,
C1 = 100uF/25V

PCB Design

single transistor battery charger PCB layout

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!

98 thoughts on “One Transistor Automatic Battery Charger Circuit”

  1. Hello sir( happy republic day) … My. Question is that sir. I have 13-0-13 4a transformer and i make battery charger from this transformer by using 4 IN 5408 DIDODE, 4700uf/35v and the dc voltage come from that is 18v how to simply adjust 14v 1A from given cricuit diagram with their autocutt function… Please suggest me sir…

    • Happy Republic Day Apurva.
      For setting the circuit, apply 14V to the circuit from an external power supply, and then adjust the preset such that the relay just clicks. That’s all, your circuit is all set now..
      After this you can connect the discharged battery and switch ON the transformer, once the battery reaches 14V the relay will cut off.

      Remember you will have to connect the battery first and then switch ON the transformer.

      And you must NOT use 4 amp for a 7Ah battery. If your battery is a 7Ah, use a 1 amp or 500mA transformer.

      Or alternatively, you can connect a 12V motorcycle side indicator bulb in series with your battery positive for your existing 18V 4 amp output. The lamp will take care of the current.

  2. Hey I added the transformer of greater rating and circuit worked but now I see that my transistor has blown aka Base emitter is open… If I were to place a resistor in series with the Base would it prevent damage to the transitor? also the relay buzzes even with a flywheel diode

    Best regards Adrian Sassin

    • Hi, the transistor can never get damaged because R1 is already present. R1 will restrict the current in case the preset adjustment is shorted accidentally.

      The transistor can possibly burn if the flywheel diode is not good. You can try changing the diode with a good quality 1N4007 diode.
      To stop buzzing you could try adding a 10uF capacitor across base/emitter of the transistor and and also a 100uF capacitor parallel to the relay coil.

  3. Greetings I used a 15V 1A transformer to charge the battery for the circuit but when I charge the circuit noting I get 0.4 mA on the Dc side and my battery starts to make hissing noise… It cuts off at 13.7 but after and hour or two it goes to 13.4V what is the reason for this hissing and the discharge and what can I do to rectify the problem? I’m using this circuit in conjunction with and emergency light… my battery is a 12V 7ah

    • Hi Adrian, that looks quite strange, because at 1 amp a 7Ah battery will never hiss. Try adding a current control stage using the following circuit and set the current to 500mA:


      If still you hear the weird sound then the problem could be in your battery.

      the one transistor cut off circuit explained above may not be very accurate since the preset adjustment could deviate slightly overtime, giving rise to errors, you can try using fixed resistors instead of the perset to get reasonably consistent cut off response from the circuit

  4. Hey I wanted to ask I’m getting 0.45mA when the battery chargers.. How can I increase current flow to 0.7A to change lead acid 12V battery 7ah… Transformer I’m using is 15V 1A and is quite warm after a few hours

    Regards Adrian

    • Hey Adrian, your transformer may not be actually rated at 1 amp, because most transformers usually print slightly higher rating than its actual capacity. Try some other slightly higher rated transformer, may be upto 2 amps and see the response.

      If still the current does not increase then your could be having problems.

  5. Circuito não esta funcionando….tem um erro no R-1 e tambem no C-1 negativo, …. estão sem coneção ( Design do PCB ) só funcionou depois de fazer mudanças

    • Obrigado por apontar o erro no design do PCB, ele foi projetado por uma pessoa externa, não eu, mas de qualquer maneira, estou feliz que você possa corrigi-lo e fazer com que ele funcione para você

  6. Greetings if I use a 12V 7ah lead acid battery… What will the charging current be that will charge the battery?

  7. Hi I wanted to ask.. The Base voltage I measured when it starts conducting (when charger cuts off) is 1.45V? why is this voltage so high? Regards Adrian Sassin

  8. sir can I use this circuit to charge my 60ah battery.. if yes. what are the changes required in the circuit

  9. Sir,
    I built a battry charger using a step down tranfo and this circuit. The challanges am facing is that, If our Main AC is above or below 220v the DC output is varying either above 13v or below 11v and this may damage my battery.
    So an Idea came to my mind and I changed transfo with a UPS tranfo that has 175v, 195v, 220v and 250v primary and 7v-0-7v secodary and am using outer taps that gave 14v to charge battery.
    I used four (4) switches, 1st switch From Main to 250v of tranfo, 2nd swtch from main to 220v of transfo, 3rd swtch from main to 195v of transfo and the last one to 175v, If the Main AC power drop below 220 am switching either 195 or 175 switch so that the output will be okey for charging the battery and if the Main is 220, I off all the switch and On 220v switch. I am doing the same, to others depending on the Main AC voltage reads on the analong metre. At that moment, I realised that, since our Main power is either above 220v for many hours and sometime dropped below 220v for many hours, then I have to introduce a circuit that can do this changes of Main AC input to my tranfo automatically so that my battry will charge and remain safe. I need your help.

    Thank you Sir.

    • Aminu, if your step down trafo is 0-12V rated, and if you use a bridge rectifier with a high value filter capacitor, that will be enough to raise its output 14.5V at 220V.

      Or alternatively you can use a 0-24V trafo and use an LM338 regulator to regulate the output at 14V

      If you are thinking of employing a voltage stabilizer circuit that will be an overkill..

  10. Sir I did it, and it's working fine.
    The transfor is 12v 5A
    The bridge rectifier is 10A
    The Diode i.e +recitifier to battery+ is 6A 400V
    And the Caps is 4700uf 25v.
    The output I got is around 14v to 16v.
    Now, How can I set the above circuit for autor cut-up?

    Thank you very much for your kind help.

    • Aminu, first adjust the preset such that the slider completely touches the ground side of the preset.

      now apply the supply voltage to the circuit….now very carefully and skillfully adjust the preset upwards until the relay just switches ON.

      that's all your circuit is set.

    • …while practically charging the battery make sure the discharged battery is connected first and then power is switched On

      the power supply current must be 1/10th of the lead acid battery AH

    • thanks for the reply sir.
      So, do you means, I cannot charge other battery type than lead asid (as you have suggested in last reply) with is circuit?

    • you can use any form of battery, lead acid is not compulsory, but the charging current can be different for different types of battery, for example if a Li-ion batt is used then the charging current could be as high as its AH value, but for lead acid it is recommended to be 1/10th of its AH

    • Good Day Sir,
      I am done with this battery charging circuit. But the diode you suggested me to use between rectified positive power from transfo to the positive to battery is getting hot. And it is a 6A diode. I tried finding higher current in the market but I didn't get one.
      So, can I use MBR20100CT, which is 20A 100v three pin diode in form of transistor?
      If yes, I need your help on how to coonect it.

      Thanks very much Sir.

    • Aminu, you can clamp the diode with an aluminum heatsink for cooling it. you can innovate the clamping method by some thinking

    • Okey, thank you very much sir.
      I will try this.

      that means MBR20100CT cannot be use, which is very simple to clamp on the heatsink?

    • Aminu, connect a 2200uF/25V filter capacitor and then check the response.
      also check whether the bridge is getting warm or not, without load….if it is then your connections could be wrong.

  11. Thank you Sir.
    If I got you right, connecting a diode from a 12v tranfor to a positive of 12v battery can charge the battery to 14v?

    • Aminu, the diode needs to be connected after the bridge/capacitor network, if you use only the diode without the bridge then the output voltage will not reach even 13V

    • Okey Sir.
      Sir, Can I use KBL406 diode bridge and 12v 5A transfo with this circuit to charge a car battery. i.e 12v 50A battery?

      Again, I had once connected KBPC2504 to a 14.2v transfo and the voltage at + and – output of the tranfo at diode dropped to around 11v. What mades the voltage dropped to 11v?

      Thanks for the help you are offering us Sir.

  12. Okey Sir.
    Sir, could you help me on how to rectify 12v to 14v? And, How/Where do I connect the 60ohm resistor and of what watts?

    Thanks for the help.

  13. Thank you Sir.
    Sir, If am using 12v tranfor, is there any harm to battery if it is fully charged?
    So that, no need to use battery charge circuit for battery full cut off.

    thanks one again.

    • 12V after rectification/filtration will become almost 14.5V…although it won't harm the battery much, it is not recommended to keep the battery connected with a 14.5V supply after it is fully charged. use a low ohm resistor around 60 ohm in series this will allow you to keep the supply connected permanently.

  14. Sir,
    Using the above circuit, can I replace the relay with DTDP, so that, I can configure it to off the charger from main power when the battery is fully charged.
    And, If I apply transfo with any volts above 14 (as specify) will it damage the battery? let say the transfro volt is 18 to 20 while the current remain 1/10th Ah of the battery.

    Thankd you Sir.

    • Aminu, you can do it but you may to incorporate an ON/OFF push-switch in parallel with the contacts which is wired with the mains, this push-switch will be used to switch ON the circuit initially.

      18V will damage the battery once it's fully charged even if the current is 1/10th…so make sure the voltage is not above 14.5V

    • Hi Raja, yes you can connect the mentioned battery by suitably upgrading the relay to a 20amp relay and by replacing the BC547 with a 2N2222

  15. hello, try adding a few more diodes with the existing emitter diodes D5, D6 and see if it makes any difference..

  16. Hi, replace the relay coil with a 1K resistor…connect the gate of the mosfet with the collector of the BC547….connect the source with the ground, and connect the battery across the positive supply and the drain of the mosfet

  17. Can you design a circuit to automatically charger 3 batteries at a times assuming that not all the batteries have thebsane amount of charge so they should be able to shut off individually when they reach full charge

    • simply make 3 separate chargers and connect them separately to the batteries…I have discussed many automatic 741 battery charger circuits which can be replicated 3 times for the desired results.

  18. sir
    this circuit is work very well for me i have tried a lot of 12v charger circuit but if i conpair this is the best for me thank you ok,but sir i want to put led where it should be put to indicate the battery is full charge? thank you.

    • OK thanks! for a full charge indication you can connect a red LED across the relay N/O contact and the negative of the circuit via a 1K resistor.

  19. sir please i don't really know the AH i asked the seller man he said it American gell battery it don't have AH but it a 21plate battery.

    • your battery could be drawing large current beyond the capacity of the diodes, increase the diode rating to say 10amps each or more.

  20. sir
    please i build this circuit but when i it start charging the rectifier diode got hot almost damage if i didn't switch off,i used 1n5399diode same problem sir i have used three type of rectifier diode but not succeeded sir please any help? thank you.

  21. sir please i have two 12v batt 200AH and i want to use 12v charger circuit to charge the two of them but sir i want ask you that if series the two 12v batt 200AH which is 24v batt i have automatic 12v charger so i want to use it charge this 12v 200AH,i have series them 24v so sir im going to send you that 12v automatic circuit please publish it for me so that i can use it to charge my 24v batt 400AH thank you

    • you will need to make a boost circuit using IC555 to generate 28V and use it to charge the series 24V battery.
      you cannot use a 12v charger to charge the series connection

  22. sir please to set btt cut off using variable power supply it plenty difficult for me so i want you to explain it simply for me to understanding if i use variable power supply do i have to remove the transformer from the bridge rectifier and connect variable power supply to the bridge rectifier and remove after set and connect the former or not that way? thank you.

    • you can keep the transformer connected but do not plug it in to mains.
      now you can use a variable power supply whose output must be connected to the circuit's positive and negative terminals directly.
      After this the required voltage may be fed to the circuit and the preset adjusted for setting the desired cut off thresholds.

  23. ok sir thank you for accepted my request my prayer is that god will give you strewth than you once again im looking forward.

  24. hi sir please i need something i been searching everywhere but not avail so please im appealing to you if you can do it for me, this is special request,that thing is in my country we have 3phace meter and single phase meter so if you use single phase meter you will get one power and negative from the electricity pole but sometime one phase go off and also low current so sir what i burgin you to design for me is 3phase change over i will bring all the 3 power from the electricity pole for single phase meter so that when one power go off or low current then it change to another phase,i have use this circuit for changing 2phase but when one phase go off it change but if it come back again while the other one is not off. so i will be glad if you can design me one 3phase change over thank you sir looking forward to hear from you.

  25. hi sir this circuit Simplest SMF, Automotive Battery Charger Circuit Diagram Explained can i use 15v transformer? so the relay going to switch off when the batt reach full? thank you looking forward to hear from you.

  26. What can. Be added to the circuit for an led indicator?
    Can I change any one of the diode and add an 1watt LED?
    Please instruct me sir.
    thanks in advance

    • you can remove both the diodes and replace the position with a 1 watt LED, but the indication will not be instant rather gradual, rather you can use the N/O contact of the relay and connect an LED across it and ground via a 1k resistor.

  27. sir i want this circuit in such a way thaT relay should active at 14.4 and deactive at 13.5. so I connect total 5 diode to the emmitter of the BC 547 . But the relay won`t active in any position of wiper of preset. could it be possible to do so ? how it could be done please help me

  28. hi swagatham
    this is srinivas naidu
    you did a great job and ihave a problem
    ihad a two transformers one is 12-0-12 and another one is 0-24 volts .how can i use with this circuit can u sugest me please.
    great job
    thanks & regards

    • Hi Srinivas, thanks very much!

      It depends on the battery voltage that you are using, if it's a 12V then you may use the 0-12V tap as the input, if you are using a 24V battery then the 0-24V would become suitable.

      However for 24V make sure the relay is also upgraded to 24 rating and R1 increased to 10k.

    • hi swagatham
      this is Krishna
      I've a 1kw inverter (18-0-18 primary,0-230 v secondary & 24 V battery)how can i use with this circuit and which current rating transformer are using.plz help me.

    • Hi Krishna,

      What is the ampere of the transformer? and what is the AH of the battery? Pls specify them so that I can provide my suggestions.

    • yeah sir
      i'am using 40 A Transformer and 100 Ah battery, and that inverter circuit using 8 IGBTs (IRF 250N).

    • sir
      i want one more help, i also use 100 W,24 v solar PV with this ckt,how could interface with it.

    • You will need to add a voltage regulator circuit using the IC LM196 for limiting the current upto 10amps. Connect this circuit before after the bridge network in the above design.
      The bridge diodes must be rated at 15amps each.
      The input can be then either from the mentioned trafo or solar panel.
      That's all, this is enable to charge your 100 ah battery safely.

    • sir,
      Thank you very much sir, now i try to this circuit. sir and than wich current ratting transformer i used in above circuit sir.

    • for charging the battery you can use the transformer that you have with you presently(40amp), the IC will restrict it to 10amps.

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