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Battery Full Charge Indicator Circuit using Two Transistors

Battery Full Charge Indicator Circuit using Two Transistors

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.

Now let's see how the persent circuit is designed to function and how it can be set to perform the required battery warning indication.

We will study two simple designs, the first one will switch ON an LED at the full charge level of the battery while the second one can be used to do just the opposite, that is switch it OFF at the set preset value.

LED Switching ON When Battery becomes Full

The circuit diagram shown below is intended to illuminate the LED indicator as soon as the connected battery reaches its full charge level.

How to Fix the Presets

To set up the circuit the user has to feed the desired upper charge level to the circuit, and adjust the preset such that the LED just begins to illuminate brightly at that level.


simple 2 transistor LED battery over charge indicator circuit


Video Clip:

LED Switching OFF at Full Battery

The following circuit is configured to force or to turn off the LED when the battery reaches its upper charge level.

Battery Full Charge Indicator Circuit using Two Transistors

For users who wish to see the LED switch OFF at the upper threshold can use the above shown design, the working may be understood wit the following points:

As per the requirement the LED illumination is supposed to begin diminishing as soon as the battery reaches approximately close to the set full charge 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 just shut down at the desired level..

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 shutting down at around 14V.

PCB Design


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!

67 thoughts on “Battery Full Charge Indicator Circuit using Two Transistors”

      • Sir, thanks for the reply.
        your advice works in 12v lead acid battery, and buzzer beeping in 14,3 volt, it’s perfect for it. i want to use circuit (LED switching On when the battery becomes full) for 6v lead acid battery and buzzer beeping around 7.3v and up. which component change for my purposes. Your help would be appreciated.

        • That’s great Hakan, you can use the same circuit for your 6V application also, no need to change anything in the design.

  1. Hi Swag,
    I modified the third circuit as per your revised edition and now it works as described. I would suggest the you could change your text to show both options that either indicate that both circuits could be used but one will have the LED on until the set threshold is met and then turn off and the other circuit the LED will be at full brightness when the set threshold is met.
    Thanks for your efforts.

  2. Hello Swagatam
    I built two of these circuits with a hope to use them in my Lithium-ion replacement for my Ni-Cad battery packs in a cordless drill. The drill pack is 16.8 volts fully charged so I wanted to set the LED to come on at 16.5 volts. Unfortunately I have found two problems. The first was I used a 50k pre-set as 47k was not available. I now find that the LED is on until I get to the 16.5 volts then it turns off (back to front). The second problem is that the pre-set is nearly fully wound clockwise. Your help would be appreciated.

    • Hi Eddie,

      the preset value is not crucial, you can use any value within the range 4k7 and 47K.
      Regarding your issue, that may not be possible unless there’s some problem in your circuit.

      As per your setting, let’s assume 16.8V is the cut off threshold, in that case any voltage level below 16V would allow the left side transistor to conduct and switch ON, this will in turn cause the supply voltage to appear at the collector this transistor and subsequently to the base of the right side transistor inhibiting its conduction and LeD illumination….meaning the LED should switch ON only when 16.8V is reached..

      • Thank you for your quick reply. I meant to so anti-clockwise in my post not clockwise. Both circuits I have built are doing the same thing which I find strange. I have built them both on vero board so they were easy to construct. I have checked them over and over again and can not find a fault. I am using T-092 case BC557 transistors and both are correct in orientation so I am at a loss why a simple circuit like this is not working.

        • The right way to check the response is to first apply the maximum voltage at which cut off is expected to happen.

          then adjust the preset such that it just illuminates the LED at that voltage.

          After this the input voltage may be changed to check the response. A lower voltage than the set value should shut down the LEd and vice versa.

          • Hi Swag,
            I have tried your instructions but still get the same effect. I changed the presets to 4.7k which allows setting the trip voltage at the center of the preset. Frustrated I built a third circuit and get the same effect. However, I can make the LED glow at lower voltages and slowly increase to full glow when the trip voltage is reached. If I increase the voltage past the trip point the LED turns off. I can also make the other two circuits I built do the same. I think I prefer that the LED turns off to indicate the lithium batteries are charged. one question, is there a way to make the preset less sensitive to adjustment.
            Thanks for your help. Eddie

            • Hi Eddie, I will build and confirm the circuit tomorrow and update the results for you.

              By the way if you prefer the LED turns off at the threshold level, in that case your existing situation seems to be fulfilling the requirement?

              to decrease the sensitivity of the preset, you can remove the preset series resistor from the shown position and bring it in series with the base of the transistor., this might help to increase the adjustment dial of the preset.

            • H Eddie, you were right, the circuit indeed did not work the way it is expected to be, but never mind I have modified it appropriately and now it should be working as intended.

    • Nkwenti, yes that’s possible, just add a 100uF/25V capacitor across the relay to avoid relay chattering at cut off thresholds.

  3. HI.. please due to curiosity, i wish to ask . what is the voltage at a battery’s terminal when it is on a charging process.

  4. 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.

    • 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.

  5. 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.

  6. 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.

    • 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.

  7. 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

    • 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.

    • 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

  8. 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

    • 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

  9. 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.

  10. 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?

  11. 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

    • 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?


  12. 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?


  13. 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

  14. 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.

    • 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

  15. 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

    • 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

    • 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.

  16. 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…

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

  17. 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..

  18. 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

    • 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.

    • 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

    • 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.

  19. 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.

    • 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

  20. 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?

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