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.

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.

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.

36 Replies to “Battery Full Charge Indicator Circuit using Two Transistors”

    1. Mbutho Dlamini Motsa

      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
    1. Swagatam Post author

      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
  1. Anonymous

    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
    1. Swagatam Post author

      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
  2. Brock Wood

    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
    1. Swagatam Post author

      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
    2. Brock Wood

      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
    3. Swagatam Post author

      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
  3. Chad Lee

    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
  4. Rohit Singh

    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
    1. Swagatam Post author

      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
  5. Success Ola

    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
    1. Swagatam Post author

      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
    1. Swagatam Post author

      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
  6. Sachin

    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
  7. Swagatam Post author

    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
    1. Sachin

      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
  8. miss simple

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

    Reply
  9. Unknown

    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

Leave a Reply

Your email address will not be published. Required fields are marked *