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You are here: Home / Battery Charger / Battery Full Charge Indicator Circuit using Two Transistors
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Filed Under: Battery Charger, Indicator Circuits

Battery Full Charge Indicator Circuit using Two Transistors

Last Updated on February 23, 2018 by Swag 50 Comments

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This little circuit will alert the user regarding a battery reaching its full-charge level (over charge) while it's being charged, by illuminating an LED. The circuit uses just a couple of transistors as the main active components.



Main Feature

The main feature of this design is not only its mini design but also its supply voltage specs which can be as low as 2V, meaning it can be used for all batteries ranging from 2V to probably 60V with minor changes

I have already discussed a similar concept which is designed for exactly the opposite function, that is to indicate the lower discharge threshold of a battery.

Circuit Description

Both the designs are exactly similar, just having opposite polarities and indication specs.

Battery Full Indicator Circuit using Two Transistors

Referring to the proposed battery full charge indicator circuit we can see that it is made using just a couple of transistors and some resistors and a single preset for setting up the desired upper threshold indication.

The LEd is supposed to begin illuminating as soon as the battery reaches approximately close to the set threshold.

The setting up procedure of the preset is actually very simple.

The user must feed a supply voltage that may be equal to desired high charge level of the battery, and then gently adjust the preset with a screw driver to force the LED to illuminate marginally.

For example suppose the indicator circuit is been installed for monitoring a 12V battery over charge level at 14.3V, then the preset may be tweaked to make sure that the LED just begins illuminating dimly at around 14V, that would automatically enable the illumination  to grow at its optimal brightness until the charging level has reached the 14.4 or 14.5V limit.

PCB Design



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Reader Interactions

Comments

  1. Abhishek Sharma says

    Hi sir, I want to use it with 12 v 7.6ah battery but, I want to ask you that during charging the battery connect to this circuit?

    Reply
    • Swagatam says

      Abhishek, yes the above circuit will need to be connected to the battery which needs to be monitored…

      Reply
    • Mbutho Dlamini Motsa says

      hi sir my name is mbutho i am also an electronics hobbyist too….so i need your help…..i need a circuit that can switch any single phase appliance using a cellphone…..i hope you will be interested in helping me. i first design the circuit using motors but now i need something more electronical or digi related ……use nanamotsa@gmail.com

      Reply
    • Swagatam says

      mbutho, you can try the following circuit

      https://homemade-circuits.com/2012/01/how-to-build-gsm-based-cell-phone.html

      Reply
  2. Davis Kakumba says

    hi SWAGATAM where exactly is the battery connected to the circuit, i see (+) and (-) which a dc supply input

    Reply
    • Swagatam says

      Hi Davis, the +/- points are the battery points, the supply from the battery powers the circuit and simultaneously helps the circuit to detect its level

      Reply
  3. prabhudevan thalwar says

    What is threshold…?

    Reply
    • Swagatam says

      threshold refers to the voltage level

      Reply
  4. Anonymous says

    Mr.Swagatam Majumdar, I would like to thank you for all the time and work you have dedicated to helping others. You are an amazingly, talented, caring person. Not only do you provide incredible examples you reply to comments. If you have a book etc. please let me know I would definitely buy it. Thank you again Sir!

    Reply
    • Swagatam says

      Thank you very much! I appreciate your thoughts!

      I have not yet written any book, however i plan to write one, when i do so I'll surely let you know.

      Reply
  5. Brock Wood says

    Hi, Swagatam! I love your circuits and blog. Thank you! I am trying to make this circuit indicate a full charge of 14.25 volts (for charging a 12 volt NiCad battery). The circuit seems to come on properly at 14.25 volts. When I then lower the voltage, however, the LED stays illuminated. That is strange. It is as though the circuit has "latched" into the "on" state and will not go back to the "off" state. Any help is appreciated!

    – Brock

    Reply
    • Swagatam says

      Thanks Brock, yes it will show some level of hysteresis since the design is too basic.

      for more sharpness you can perhaps include a BC547 stage at the right side, and connect the LED across its collector arm, that would probably increase the efficiency of the circuit to a much higher level.

      remove the existing LED and join the points with a link (don't remove the 10k resistor)

      connect the base of the BC547 with the associated BC557 collector…
      connect the emitter of the BC547 with the ground line…and finally connect the LED across its collector and the positive line.

      make sure to connect a limiting resistor with the LED, any value between 4k7 and 10k will do.

      Reply
    • Brock Wood says

      Thanks so much! I will add a BC547 as suggested and let you know how it works for me. You are the best, Swagatam! Per the other commenter, you should write a book! I'd buy it. In hardback. Full retail price. Better make it an e-book. I am running out of shelf space for hobby electronics books. – Brock

      Reply
    • Swagatam says

      Thanks so much Brock for the motivation, I appreciate it a lot, I will surely start writing an ebook soon and let you know as soon as it's finished.

      Reply
  6. Chad Lee says

    Hi Swagatam, I have been collecting electronic junk and pieces that I want to use in projects and learn. I have plenty of Random transistors with normal numbers and most are "house"? numbered. Sadly I have no 357 or 557 transistors. Can I substitute others? I do have pn2222a, pn2907a, c945 and plenty of unknowns that Google can't find..

    Reply
    • Swagatam says

      Hi Chad,

      you can use 2n2907 instead.

      Reply
  7. Rohit Singh says

    i have tried the above circuit, but the led keeps glowing even when the battery powee level goes down.. it keeps on glowing ??

    battery low circuit using bc547 is working well but this circuit not…

    Reply
    • Swagatam says

      It should work as mentioned in the article because it's exactly similar to the low batt indicator design except the polarity…

      Reply
  8. Rohit Singh says

    can you explain this battery full circuit working on Bread board??

    Reply
    • Swagatam says

      at the preferred high voltage threshold, the left transistor is just forced to stop conducting, which allows the right side transistor to just start conducting via the negative feed from its base 10K resistor.

      when the voltage begins dropping below the threshold, the left transistor slowly begins conducting, inhibiting the negative base bias of the right transistor which eventually stops conducting and the LED stops glowing.

      Reply
  9. Success Ola says

    Sir,

    I would like to know how to build a very cheap and simple electric speed controller (ESC) that can work for brushed and brushless DC motor.

    Pls sir, I would need a detailed explanation because am new to circuit

    Reply
    • Swagatam says

      Success,

      I have explained both the types in this website, please use search box on top right to find them:

      just type for

      DC motor speed, and
      BLDC

      Reply
  10. Rohit Singh says

    at last it worked,
    what will happen if i replace two 10k resistances with 33k, but not replacing the 10k resistance that is connected with led.

    Reply
    • Swagatam says

      OK,

      with 33K also It will work with a different preset setting.

      LED resistor is only for limiting LED current, it is not related to voltage level detection…..you can use any value from 2.2K to 10K

      Reply
  11. Soumen Bhowmik says

    hi sir,
    I am only a 12th pass student…… please tell me what is the meaning of the arrow to the 47k resistance….. thanks sir………Soumen

    Reply
    • Swagatam says

      the 47K is a preset or a trimpot, the arrow head represents the center terminal of the preset

      Reply
  12. Josh Sinday says

    Hi sir, i want to add an indicator while it's still charging. Any suggestions? 🙂

    Reply
  13. Sachin says

    Hi Swagatam,

    I have 4 batteries connected in series which is needed for electric bike.

    Is it possible to charge them individually without disconnecting the series connection?

    Thanks
    Sachin

    Reply
  14. Swagatam says

    Hi Sachin,

    yes that's possible, you can join the (+)(-) of the supply with the relevant terminals of each of the batteries one by one, as each one gets fully charged

    Reply
    • Sachin says

      Swagatam,

      I need to charge all 4 batteries at same time with four individual 12v chargers without disconnecting the series connection (not one-by-one). Possible?

      Thanks
      Sachin

      Reply
    • Swagatam says

      that's also possible as long as the units are not attached in any manner except the battery under charger.

      Reply
  15. miss simple says

    Hai sir
    I currently do the project of Fast Charger For Lithium-ion Mobile Phone Charging that involves raspberry pi. And now I’m still searching the circuit for this project. Can u give me a tips in buid the suitable circuit for this project?

    Reply
    • Swagatam says

      Hi Miss, Presently I do not have a Raspberry pi based design, possibly I'll try to get one soon and post it for you in this website!!

      Reply
    • miss simple says

      thanks sir. i will wait for it. thanks for help me 🙂

      Reply
  16. Unknown says

    Sir i created the above circuit and working fine ,but it cut off while volts go up from particular fixed with preset 47k. But i did the little bit changes in circuit i replaced second transistor to bc547 relevant chances to work it. now its ok. thanks for your circuit and test it and if it is true change it and publish.

    Reply
    • Swagatam says

      I appreciate your inputs, thanks very much for updating it…

      Reply
  17. Guitar.mod says

    Dear Swagatam, very nice little circuit! It could be also used as over voltage protection – just to add a "cut-off" relay to right transistor (in parallel to 10k resistor and red LED). Would be much better than circuit using IC 741 opamp. Why?
    1. I do not like opamps if they are not realy necessary;
    2. Much simpler to make, more versatile to supply voltages and etc.
    Regards, Guitar.mod

    Reply
    • Swagatam says

      Thanks Guitar.mode,

      yes definitely it could be used as a battery overcharge cut off simply by adding an additional transistor and relay stage.

      I'll try to update the design soon here.

      please keep reading…thanks

      Reply
  18. sheen sheen says

    Hi sir,
    I need a battery indicator for indicating upto 15v. Also having leds for indicating some different ranges. My email id is;
    aamirele@gmail.com

    Reply
    • Swagatam says

      Hi Sheen, you can use the above simple indicator circuit with any voltage level upto 60V…you just have to set up the preset accordingly

      Reply
  19. khdr hamed says

    thanks for your help to all of us, I have a fbattery of 12 volts and 120 Ah and I need a circuit to show me when it fully ofcharged , please help me to that.
    Thank to you

    Reply
    • Swagatam says

      You are most welcome!

      you can try the circuit which is mentioned in the above article, it will faithfully provide you with the required results.

      Reply
  20. Taneem says

    hello sir,

    i am a newbie. i want to monitor 3.7v 18650 battery. i have completed this circuit but i can't understand where i have to put the multi-meter to adjust the desired upper threshold. please help me detail with this.

    thank you.

    Reply
    • Swagatam says

      Hello Taneem, a meter will not be required for this, you will need a 4.2V source, so get a precise 4.2V supply to the circuit, and then start adjusting the preset until the RED LED just begins glowing.

      if you find the the LED glowing initially, in that case adjust the preset until the LED is completely shut off, and then readjust the preset until the LED just begins getting illuminated. that's all is required.

      Reply
  21. Abubakar says

    Hello sir,.
    I wonder if you could help me with a circuit to charge 4lithium iron cell of 4v 3/2A connected in series,using 18.1v 100w panel or any other Ac source with automatic quote up or full charge indicator.
    Am so greatful for considering me.

    Reply
    • Swag says

      Hello Abubakar, you can use the second circuit from the following article

      https://homemade-circuits.com/2012/01/how-to-make-simple-low-battery-voltage.html

      Reply
  22. Abubakar says

    Hello swagtam.
    Thank uu for recommending the second ckt.
    But,what I would like you to shade more light is my batteries were connected in series giving 16v.the issue is that the 2nd ckt has 4.7v zener diode,could it be possible to charge the batteries or my first question was not clear.am doubt pls!come again with another solution
    God bless you.

    Reply
    • Swag says

      Hello Abubakar,

      the zener diode has nothing to do with the battery voltage, you can use any zener between 3 and 9V.

      The zener is only for providing a base reference point to the pin#3 so that pin#3 is able to detect the battery thresholds as per the preset adjustment, with reference to pin#2 zener fixed reference voltage.

      Reply
  23. Abubakar says

    Thanks a million

    Reply
    • Swag says

      you are welcome!

      Reply

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Swagatam is an ardent electronic researcher, inventor, schematic/PCB designer, manufacturer, and an avid publisher. He is the founder of https://homemade-circuits.com/where visitors get the opportunity to read many of his innovative electronic circuit ideas, and also solve crucial circuit related problems through comment discussion.
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